/ Language: English / Genre:comp_programming

Advanced PIC Microcontroller Projects in C

Ibrahim Dogan

• The only project book on the PIC 18 series using the C programming language • Features 20 complete, tried and test projects • Includes a CD-ROM of all the programs, hex listings, diagrams, and data sheets

Advanced PIC Microcontroller Projects in C

From USB to RTOS with the PIC18F Series

Dogan Ibrahim

Preface

A microcontroller is a microprocessor system which contains data and program memory, serial and parallel I/O, timers, and external and internal interrupts — all integrated into a single chip that can be purchased for as little as two dollars. About 40 percent of all microcontroller applications are found in office equipment, such as PCs, laser printers, fax machines, and intelligent telephones. About one third of all microcontrollers are found in consumer electronic goods. Products like CD players, hi-fi equipment, video games, washing machines, and cookers fall into this category. The communications market, the automotive market, and the military share the rest of the applications.

This book is written for advanced students, for practicing engineers, and for hobbyists who want to learn more about the programming and applications of PIC18F-series microcontrollers. The book assumes the reader has taken a course on digital logic design and been exposed to writing programs using at least one high-level programming language. Knowledge of the C programming language will be useful, and familiarity with at least one member of the PIC16F series of microcontrollers will be an advantage. Knowledge of assembly language programming is not required since all the projects in the book are based on the C language.

Chapter 1 presents the basic features of microcontrollers, discusses the important topic of numbering systems, and describes how to convert between number bases.

Chapter 2 reviews the PIC18F series of microcontrollers and describes various features of these microcontrollers in detail.

Chapter 3 provides a short tutorial on the C language and then examines the features of the mikroC compiler.

Chapter 4 covers advanced features of the mikroC language. Topics such as built-in functions and libraries are discussed in this chapter with examples.

Chapter 5 explores the various software and hardware development tools for the PIC18F series of microcontrollers. Various commercially available development kits as well as development tools such as simulators, emulators, and in-circuit debuggers are described with examples.

Chapter 6 provides some simple projects using the PIC18F series of microcontrollers and the mikroC compiler. All the projects are based on the PIC18F452 microcontroller, and all of them have been tested. This chapter should be useful for those who are new to PIC microcontrollers as well as for those who want to extend their knowledge of programming PIC18F microcontrollers using the mikroC language.

Chapter 7 covers the use of SD memory cards in PIC18F microcontroller projects. The theory of these cards is given with real working examples.

Chapter 8 reviews the popular USB bus, discussing the basic theory of this bus system with real working projects that illustrate how to design PIC18F-based projects communicating with a PC over the USB bus.

The CAN bus is currently used in many automotive applications. Chapter 9 presents a brief theory of this bus and also discusses the design of PIC18F microcontroller-based projects with CAN bus interface.

Chapter 10 is about real-time operating systems (RTOS) and multi-tasking. The basic theory of RTOS systems is described and simple multi-tasking applications are given.

The CD-ROM that accompanies this book contains all the program source files and HEX files for the projects described in the book. In addition, a 2K size limited version of the mikroC compiler is included on the CD-ROM.

Dogan Ibrahim London, 2007

Acknowledgments

The following material is reproduced in this book with the kind permission of the respective copyright holders and may not be reprinted, or reproduced in any other way, without their prior consent.

Figures 2.1–2.10, 2.22–2.36, 2.37, 2.38, 2.41–2.55, 5.2–5.4, 5.17, 5.20, 8.8, and 9.13, and Table 2.2 are taken from Microchip Technology Inc. data sheets PIC18FXX2 (DS39564C) and PIC18F2455/2550/4455/4550 (DS39632D).

Figure 5.5 is taken from the web site of BAJI Labs.

Figures 5.6–5.8 are taken from the web site of Shuan Shizu Ent. Co., Ltd. Figures 5.9, 5.13, 5.18 are taken from the web site of Custom Computer Services Inc. Figures 5.10, 5.19, and 6.43 are taken from the web site of mikroElektronika Ltd. Figure 5.11 is taken from the web site of Futurlec.

Figure 5.21 is taken from the web site of Smart Communications Ltd. Figure 5.22 is taken from the web site of RF Solutions.

Figure 5.23 is taken from the web site of Phyton.

Figures 5.1 and 5.14 are taken from the web site of microEngineering Labs Inc. Figure 5.16 is taken from the web site of Kanda Systems.

Thanks is due to mikroElektronika Ltd. for their technical support and for permission to include a limited size mikroC compiler on the CD-ROM that accompanies this book. PIC®, PICSTART®, and MPLAB® are all registered trademarks of Microchip Technology. 

CHAPTER 1

Microcomputer Systems 

1.1 Introduction

The term microcomputer is used to describe a system that includes at minimum a microprocessor, program memory, data memory, and an input-output (I/O) device. Some microcomputer systems include additional components such as timers, counters, and analog-to-digital converters. Thus, a microcomputer system can be anything from a large computer having hard disks, floppy disks, and printers to a single-chip embedded controller.

In this book we are going to consider only the type of microcomputers that consist of a single silicon chip. Such microcomputer systems are also called microcontrollers, and they are used in many household goods such as microwave ovens, TV remote control units, cookers, hi-fi equipment, CD players, personal computers, and refrigerators. Many different microcontrollers are available on the market. In this book we shall be looking at programming and system design for the PIC (programmable interface controller) series of microcontrollers manufactured by Microchip Technology Inc.

1.2 Microcontroller Systems

A microcontroller is a single-chip computer. Micro suggests that the device is small, and controller suggests that it is used in control applications. Another term for microcontroller is embedded controller, since most of the microcontrollers are built into (or embedded in) the devices they control.

A microprocessor differs from a microcontroller in a number of ways. The main distinction is that a microprocessor requires several other components for its operation, such as program memory and data memory, input-output devices, and an external clock circuit. A microcontroller, on the other hand, has all the support chips incorporated inside its single chip. All microcontrollers operate on a set of instructions (or the user program) stored in their memory. A microcontroller fetches the instructions from its program memory one by one, decodes these instructions, and then carries out the required operations.

Microcontrollers have traditionally been programmed using the assembly language of the target device. Although the assembly language is fast, it has several disadvantages. An assembly program consists of mnemonics, which makes learning and maintaining a program written using the assembly language difficult. Also, microcontrollers manufactured by different firms have different assembly languages, so the user must learn a new language with every new microcontroller he or she uses.

Microcontrollers can also be programmed using a high-level language, such as BASIC, PASCAL, or C. High-level languages are much easier to learn than assembly languages. They also facilitate the development of large and complex programs. In this book we shall be learning the programming of PIC microcontrollers using the popular C language known as mikroC, developed by mikroElektronika.

In theory, a single chip is sufficient to have a running microcontroller system. In practical applications, however, additional components may be required so the microcomputer can interface with its environment. With the advent of the PIC family of microcontrollers the development time of an electronic project has been reduced to several hours.

Basically, a microcomputer executes a user program which is loaded in its program memory. Under the control of this program, data is received from external devices (inputs), manipulated, and then sent to external devices (outputs). For example, in a microcontroller-based oven temperature control system the microcomputer reads the temperature using a temperature sensor and then operates a heater or a fan to keep the temperature at the required value. Figure 1.1 shows a block diagram of a simple oven temperature control system.

The system shown in Figure 1.1 is very simple. A more sophisticated system may include a keypad to set the temperature and an LCD to display it. Figure 1.2 shows a block diagram of this more sophisticated temperature control system.

Figure 1.1: Microcontroller-based oven temperature control system LCD

Figure 1.2: Temperature control system with a keypad and LCD

We can make the design even more sophisticated (see Figure 1.3) by adding an alarm that activates if the temperature goes outside the desired range. Also, the temperature readings can be sent to a PC every second for archiving and further processing. For example, a graph of the daily temperature can be plotted on the PC. As you can see, because microcontrollers are programmable the final system can be as simple or as complicated as we like.

Figure 1.3: A more sophisticated temperature controller

A microcontroller is a very powerful tool that allows a designer to create sophisticated input-output data manipulation under program control. Microcontrollers are classified by the number of bits they process. Microcontrollers with 8 bits are the most popular and are used in most microcontroller-based applications. Microcontrollers with 16 and 32 bits are much more powerful, but are usually more expensive and not required in most small-or medium-size general purpose applications that call for microcontrollers. The simplest microcontroller architecture consists of a microprocessor, memory, and input-output. The microprocessor consists of a central processing unit (CPU) and a LCD control unit (CU). The CPU is the brain of the microcontroller; this is where all the arithmetic and logic operations are performed. The CU controls the internal operations of the microprocessor and sends signals to other parts of the microcontroller to carry out the required instructions.

Memory, an important part of a microcontroller system, can be classified into two types: program memory and data memory. Program memory stores the program written by the programmer and is usually nonvolatile (i.e., data is not lost after the power is turned off). Data memory stores the temporary data used in a program and is usually volatile (i.e., data is lost after the power is turned off).

There are basically six types of memories, summarized as follows:

1.2.1 RAM

RAM, random access memory, is a general purpose memory that usually stores the user data in a program. RAM memory is volatile in the sense that it cannot retain data in the absence of power (i.e., data is lost after the power is turned off). Most microcontrollers have some amount of internal RAM, 256 bytes being a common amount, although some microcontrollers have more, some less. The PIC18F452 microcontroller, for example, has 1536 bytes of RAM. Memory can usually be extended by adding external memory chips.

1.2.2 ROM

ROM, read only memory, usually holds program or fixed user data. ROM is nonvolatile. If power is removed from ROM and then reapplied, the original data will still be there. ROM memory is programmed during the manufacturing process, and the user cannot change its contents. ROM memory is only useful if you have developed a program and wish to create several thousand copies of it.

1.2.3 PROM

PROM, programmable read only memory, is a type of ROM that can be programmed in the field, often by the end user, using a device called a PROM programmer. Once a PROM has been programmed, its contents cannot be changed. PROMs are usually used in low production applications where only a few such memories are required.

1.2.4 EPROM

EPROM, erasable programmable read only memory, is similar to ROM, but EPROM can be programmed using a suitable programming device. An EPROM memory has a small clear-glass window on top of the chip where the data can be erased under strong ultraviolet light. Once the memory is programmed, the window can be covered with dark tape to prevent accidental erasure of the data. An EPROM memory must be erased before it can be reprogrammed. Many developmental versions of microcontrollers are manufactured with EPROM memories where the user program can be stored. These memories are erased and reprogrammed until the user is satisfied with the program. Some versions of EPROMs, known as OTP (one time programmable), can be programmed using a suitable programmer device but cannot be erased. OTP memories cost much less than EPROMs. OTP is useful after a project has been developed completely and many copies of the program memory must be made.

1.2.5 EEPROM

EEPROM, electrically erasable programmable read only memory, is a nonvolatile memory that can be erased and reprogrammed using a suitable programming device. EEPROMs are used to save configuration information, maximum and minimum values, identification data, etc. Some microcontrollers have built-in EEPROM memories. For instance, the PIC18F452 contains a 256-byte EEPROM memory where each byte can be programmed and erased directly by applications software. EEPROM memories are usually very slow. An EEPROM chip is much costlier than an EPROM chip.

1.2.6 Flash EEPROM

Flash EEPROM, a version of EEPROM memory, has become popular in microcontroller applications and is used to store the user program. Flash EEPROM is nonvolatile and usually very fast. The data can be erased and then reprogrammed using a suitable programming device. Some microcontrollers have only 1K flash EEPROM while others have 32K or more. The PIC18F452 microcontroller has 32K bytes of flash memory.

1.3 Microcontroller Features

Microcontrollers from different manufacturers have different architectures and different capabilities. Some may suit a particular application while others may be totally unsuitable for the same application. The hardware features common to most microcontrollers are described in this section.

1.3.1 Supply Voltage

Most microcontrollers operate with the standard logic voltage of +5V. Some microcontrollers can operate at as low as +2.7V, and some will tolerate +6V without any problem. The manufacturer’s data sheet will have information about the allowed limits of the power supply voltage. PIC18F452 microcontrollers can operate with a power supply of +2V to +5.5V.

Usually, a voltage regulator circuit is used to obtain the required power supply voltage when the device is operated from a mains adapter or batteries. For example, a 5V regulator is required if the microcontroller is operated from a 5V supply using a 9V battery.

1.3.2 The Clock

All microcontrollers require a clock (or an oscillator) to operate, usually provided by external timing devices connected to the microcontroller. In most cases, these external timing devices are a crystal plus two small capacitors. In some cases they are resonators or an external resistor-capacitor pair. Some microcontrollers have built-in timing circuits and do not require external timing components. If an application is not time-sensitive, external or internal (if available) resistor-capacitor timing components are the best option for their simplicity and low cost.

An instruction is executed by fetching it from the memory and then decoding it. This usually takes several clock cycles and is known as the instruction cycle. In PIC microcontrollers, an instruction cycle takes four clock periods. Thus the microcontroller operates at a clock rate that is one-quarter of the actual oscillator frequency. The PIC18F series of microcontrollers can operate with clock frequencies up to 40MHz.

1.3.3 Timers

Timers are important parts of any microcontroller. A timer is basically a counter which is driven from either an external clock pulse or the microcontroller’s internal oscillator. A timer can be 8 bits or 16 bits wide. Data can be loaded into a timer under program control, and the timer can be stopped or started by program control. Most timers can be configured to generate an interrupt when they reach a certain count (usually when they overflow). The user program can use an interrupt to carry out accurate timing-related operations inside the microcontroller. Microcontrollers in the PIC18F series have at least three timers. For example, the PIC18F452 microcontroller has three built-in timers.

Some microcontrollers offer capture and compare facilities, where a timer value can be read when an external event occurs, or the timer value can be compared to a preset value, and an interrupt is generated when this value is reached. Most PIC18F microcontrollers have at least two capture and compare modules.

1.3.4 Watchdog

Most microcontrollers have at least one watchdog facility. The watchdog is basically a timer that is refreshed by the user program. Whenever the program fails to refresh the watchdog, a reset occurs. The watchdog timer is used to detect a system problem, such as the program being in an endless loop. This safety feature prevents runaway software and stops the microcontroller from executing meaningless and unwanted code. Watchdog facilities are commonly used in real-time systems where the successful termination of one or more activities must be checked regularly.

1.3.5 Reset Input

A reset input is used to reset a microcontroller externally. Resetting puts the microcontroller into a known state such that the program execution starts from address 0 of the program memory. An external reset action is usually achieved by connecting a push-button switch to the reset input. When the switch is pressed, the microcontroller is reset.

1.3.6 Interrupts

Interrupts are an important concept in microcontrollers. An interrupt causes the microcontroller to respond to external and internal (e.g., a timer) events very quickly. When an interrupt occurs, the microcontroller leaves its normal flow of program execution and jumps to a special part of the program known as the interrupt service routine (ISR). The program code inside the ISR is executed, and upon return from the ISR the program resumes its normal flow of execution.

The ISR starts from a fixed address of the program memory sometimes known as the interrupt vector address. Some microcontrollers with multi-interrupt features have just one interrupt vector address, while others have unique interrupt vector addresses, one for each interrupt source. Interrupts can be nested such that a new interrupt can suspend the execution of another interrupt. Another important feature of multi-interrupt capability is that different interrupt sources can be assigned different levels of priority. For example, the PIC18F series of microcontrollers has both low-priority and high-priority interrupt levels.

1.3.7 Brown-out Detector

Brown-out detectors, which are common in many microcontrollers, reset the microcontroller if the supply voltage falls below a nominal value. These safety features can be employed to prevent unpredictable operation at low voltages, especially to protect the contents of EEPROM-type memories.

1.3.8 Analog-to-Digital Converter

An analog-to-digital converter (A/D) is used to convert an analog signal, such as voltage, to digital form so a microcontroller can read and process it. Some microcontrollers have built-in A/D converters. External A/D converter can also be connected to any type of microcontroller. A/D converters are usually 8 to 10 bits, having 256 to 1024 quantization levels. Most PIC microcontrollers with A/D features have multiplexed A/D converters which provide more than one analog input channel. For example, the PIC18F452 microcontroller has 10-bit 8-channel A/D converters. The A/D conversion process must be started by the user program and may take several hundred microseconds to complete. A/D converters usually generate interrupts when a conversion is complete so the user program can read the converted data quickly. A/D converters are especially useful in control and monitoring applications, since most sensors (e.g., temperature sensors, pressure sensors, force sensors, etc.) produce analog output voltages.

1.3.9 Serial Input-Output

Serial communication (also called RS232 communication) enables a microcontroller to be connected to another microcontroller or to a PC using a serial cable. Some microcontrollers have built-in hardware called USART (universal synchronous-asynchronous receiver-transmitter) to implement a serial communication interface. The user program can usually select the baud rate and data format. If no serial input-output hardware is provided, it is easy to develop software to implement serial data communication using any I/O pin of a microcontroller. The PIC18F series of microcontrollers has built-in USART modules. We shall see in Chapter 6 how to write mikroC programs to implement serial communication with and without a USART module. Some microcontrollers (e.g., the PIC18F series) incorporate SPI (serial peripheral interface) or I²C (integrated interconnect) hardware bus interfaces. These enable a microcontroller to interface with other compatible devices easily.

1.3.10 EEPROM Data Memory

EEPROM-type data memory is also very common in many microcontrollers. The advantage of an EEPROM memory is that the programmer can store nonvolatile data there and change this data whenever required. For example, in a temperature monitoring application, the maximum and minimum temperature readings can be stored in an EEPROM memory. If the power supply is removed for any reason, the values of the latest readings are available in the EEPROM memory. The PIC18F452 microcontroller has 256 bytes of EEPROM memory. Other members of the PIC18F family have more EEPROM memory (e.g., the PIC18F6680 has 1024 bytes). The mikroC language provides special instructions for reading and writing to the EEPROM memory of a PIC microcontroller.

1.3.11 LCD Drivers

LCD drivers enable a microcontroller to be connected to an external LCD display directly. These drivers are not common since most of the functions they provide can be implemented in software. For example, the PIC18F6490 microcontroller has a built-in LCD driver module.

1.3.12 Analog Comparator

Analog comparators are used where two analog voltages need to be compared. Although these circuits are implemented in most high-end PIC microcontrollers, they are not common in other microcontrollers. The PIC18F series of microcontrollers has built-in analog comparator modules.

1.3.13 Real-time Clock

A real-time clock enables a microcontroller to receive absolute date and time information continuously. Built-in real-time clocks are not common in most microcontrollers, since the same function can easily be implemented by either a dedicated real-time clock chip or a program written for this purpose.

1.3.14 Sleep Mode

Some microcontrollers (e.g., PICs) offer built-in sleep modes, where executing this instruction stops the internal oscillator and reduces power consumption to an extremely low level. The sleep mode’s main purpose is to conserve battery power when the microcontroller is not doing anything useful. The microcontroller is usually woken up from sleep mode by an external reset or a watchdog time-out.

1.3.15 Power-on Reset

Some microcontrollers (e.g., PICs) have built-in power-on reset circuits which keep the microcontroller in the reset state until all the internal circuitry has been initialized. This feature is very useful, as it starts the microcontroller from a known state on power-up. An external reset can also be provided, where the microcontroller is reset when an external button is pressed.

1.3.16 Low-Power Operation

Low-power operation is especially important in portable applications where microcontroller-based equipment is operated from batteries. Some microcontrollers (e.g., PICs) can operate with less than 2mA with a 5V supply, and around 15mA at a 3V supply. Other microcontrollers, especially microprocessor-based systems with several chips, may consume several hundred milliamperes or even more.

1.3.17 Current Sink/Source Capability

Current sink/source capability is important if the microcontroller is to be connected to an external device that might draw a large amount of current to operate. PIC microcontrollers can source and sink 25mA of current from each output port pin. This current is usually sufficient to drive LEDs, small lamps, buzzers, small relays, etc. The current capability can be increased by connecting external transistor switching circuits or relays to the output port pins.

1.3.18 USB Interface

USB is currently a very popular computer interface specification used to connect various peripheral devices to computers and microcontrollers. Some PIC microcontrollers provide built-in USB modules. The PIC18F2x50, for example, has built-in USB interface capabilities.

1.3.19 Motor Control Interface

Some PIC microcontrollers, for example the PIC18F2x31, provide motor control interface capability.

1.3.20 CAN Interface

CAN bus is a very popular bus system used mainly in automation applications. Some PIC18F-series microcontrollers (e.g., the PIC18F4680) provide CAN interface capability.

1.3.21 Ethernet Interface

Some PIC microcontrollers (e.g., the PIC18F97J60) provide Ethernet interface capabilities and thus are easily used in network-based applications.

1.3.22 ZigBee Interface

ZigBee, an interface similar to Bluetooth, is used in low-cost wireless home automation applications. Some PIC18F-series microcontrollers provide ZigBee interface capabilities, making the design of such wireless systems very easy.

1.4 Microcontroller Architectures

Two types of architectures are conventional in microcontrollers (see Figure 1.4). Von Neumann architecture, used by a large percentage of microcontrollers, places all memory space on the same bus; instruction and data also use the same bus.

Figure 1.4: Von Neumann and Harvard architectures

In Harvard architecture (used by PIC microcontrollers), code and data are on separate buses, which allows them to be fetched simultaneously, resulting in an improved performance.

1.4.1 RISC and CISC

RISC (reduced instruction set computer) and CISC (complex instruction computer) refer to the instruction set of a microcontroller. In an 8-bit RISC microcontroller, data is 8 bits wide but the instruction words are more than 8 bits wide (usually 12, 14, or 16 bits) and the instructions occupy one word in the program memory. Thus the instructions are fetched and executed in one cycle, which improves performance.

In a CISC microcontroller, both data and instructions are 8 bits wide. CISC microcontrollers usually have over two hundred instructions. Data and code are on the same bus and cannot be fetched simultaneously.

1.5 Number Systems

To use a microprocessor or microcontroller efficiently requires a working knowledge of binary, decimal, and hexadecimal numbering systems. This section provides background information about these numbering systems for readers who are unfamiliar with them or do not know how to convert from one number system to another. Number systems are classified according to their bases. The numbering system used in everyday life is base 10, or the decimal number system. The numbering system most commonly used in microprocessor and microcontroller applications is base 16, or hexadecimal. Base 2, or binary, and base 8, or octal, number systems are also used.

1.5.1 Decimal Number System

The numbers in the decimal number system, of course, are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. The subscript 10 indicates that a number is in decimal format. For example, the decimal number 235 is shown as 23510.

In general, a decimal number is represented as follows:

an × 10n + an–1 × 10n–1 + an-2 × 10n-2 + ……… + a0 × 100

For example, decimal number 82510 can be shown as:

82510 = 8 × 102 + 2 × 101 + 5 × 100

Similarly, decimal number 2610 can be shown as:

2610 = 2 × 101 + 6 × 100

or

335910 = 3 × 103 + 3 × 102 + 5 × 101 + 9 × 100

1.5.2 Binary Number System

The binary number system consists of two numbers: 0 and 1. A subscript 2 indicates that a number is in binary format. For example, the binary number 1011 would be 10112. In general, a binary number is represented as follows:

an × 2n + an–1 × 2n–1 + an–2 × 2n–2 + ……… + a0 × 20

For example, binary number 11102 can be shown as:

11102 = 1 × 23 + 1 × 22 + 1 × 21 + 0 × 20

Similarly, binary number 100011102 can be shown as:

100011102 = 1 × 27 + 0 × 26 + 0 × 25 + 0 × 24 + 1 × 2+ 1 × 22 + 1 × 21 + 0 × 20

1.5.3 Octal Number System

In the octal number system, the valid numbers are 0, 1, 2, 3, 4, 5, 6, 7. A subscript 8 indicates that a number is in octal format. For example, the octal number 23 appears as 238.

In general, an octal number is represented as:

an × 8n + an–1 × 8n–1 + an–2 × 8n–2 + ……… + a0 × 80

For example, octal number 2378 can be shown as:

2378 = 2 × 82 + 3 × 81 + 7 × 80

Similarly, octal number 17778 can be shown as:

17778 = 1 × 83 + 7 × 82 + 7 × 81 + 7 × 80

1.5.4 Hexadecimal Number System

In the hexadecimal number system, the valid numbers are: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. A subscript 16 or subscript H indicates that a number is in hexadecimal format. For example, hexadecimal number 1F can be written as 1F16 or as 1FH. In general, a hexadecimal number is represented as:

an × 16n + an–1 × 16n–1 + an–2 × 16n–2 + ……… + a0 × 160

For example, hexadecimal number 2AC16 can be shown as:

2AC16 = 2 × 162 + 10 × 161 + 12 × 160

Similarly, hexadecimal number 3FFE16 can be shown as:

3FFE16 = 3 × 163 + 15 × 162 + 15 × 161 + 14 × 160

1.6 Converting Binary Numbers into Decimal

To convert a binary number into decimal, write the number as the sum of the powers of 2.

Example 1.1

Convert binary number 10112 into decimal.

Solution 1.1

Write the number as the sum of the powers of 2:

10112 = 1 × 23 + 0 × 22 + 1 × 21 + 1 × 20

     = 8 + 0 + 2 = 1

     = 11

or, 10112 = 1110

Example 1.2

Convert binary number 110011102 into decimal.

Solution 1.2

Write the number as the sum of the powers of 2:

110011102 = 1 × 27 + 1 × 26 + 0 × 25 + 0 × 2+ 1 × 23 + 1 × 22 + 1 × 21 + 0 × 20

     = 128 + 64 + 0 + 0 + 8 + 4 + 2 + 0

     = 206

or, 110011102 = 20610

Table 1.1 shows the decimal equivalent of numbers from 0 to 31.

Table 1.1: Decimal equivalent of binary numbers

Binary Decimal Binary Decimal
00000000 0 00010000 16
00000001 1 00010001 17
00000010 2 00010010 18
00000011 3 00010011 19
00000100 4 00010100 20
00000101 5 00010101 21
00000110 6 00010110 22
00000111 7 00010111 23
00001000 8 00011000 24
00001001 9 00011001 25
00001010 10 00011010 26
00001011 11 00011011 27
00001100 12 00011100 28
00001101 13 00011101 29
00001110 14 00011110 30
00001111 15 00011111 31

1.7 Converting Decimal Numbers into Binary

To convert a decimal number into binary, divide the number repeatedly by 2 and take the remainders. The first remainder is the least significant digit (LSD), and the last remainder is the most significant digit (MSD).

Example 1.3

Convert decimal number 2810 into binary.

Solution 1.3

Divide the number into 2 repeatedly and take the remainders:

28/2 → 14 Remainder 0 (LSD)

14/2 → 7  Remainder 0

7/2  → 3  Remainder 1

3/2  → 1  Remainder 1

1/2  → 0  Remainder 1 (MSD)

The binary number is 111002.

Example 1.4

Convert decimal number 6510 into binary.

Solution 1.4

Divide the number into 2 repeatedly and take the remainders:

65/2 → 32 Remainder 1 (LSD)

32/2 → 16 Remainder 0

16/2 → 8  Remainder 0

8/2  → 4  Remainder 0

4/2  → 2  Remainder 0

2/2  → 1  Remainder 0

1/2  → 0  Remainder 1 (MSD)

The binary number is 10000012.

Example 1.5

Convert decimal number 12210 into binary.

Solution 1.5

Divide the number into 2 repeatedly and take the remainders:

122/2 → 61 Remainder 0 (LSD)

61/2  → 30 Remainder 1

30/2  → 15 Remainder 0

15/2  → 7  Remainder 1

7/2   → 3  Remainder 1

3/2   → 1  Remainder 1

1/2   → 0  Remainder 1 (MSD)

The binary number is 11110102.

1.8 Converting Binary Numbers into Hexadecimal

To convert a binary number into hexadecimal, arrange the number in groups of four and find the hexadecimal equivalent of each group. If the number cannot be divided exactly into groups of four, insert zeros to the left of the number as needed so the number of digits are divisible by four.

Example 1.6

Convert binary number 100111112 into hexadecimal.

Solution 1.6

First, divide the number into groups of four, then find the hexadecimal equivalent of each group:

10011111 = 1001 1111

             9    F

The hexadecimal number is 9F16.

Example 1.7

Convert binary number 11101111000011102 into hexadecimal.

Solution 1.7

First, divide the number into groups of four, then find the hexadecimal equivalent of each group:

1110111100001110 = 1110 1111 0000 1110

                     E    F    0    E

The hexadecimal number is EF0E16.

Example 1.8

Convert binary number 1111102 into hexadecimal.

Solution 1.8

Since the number cannot be divided exactly into groups of four, we have to insert, in this case, two zeros to the left of the number so the number of digits is divisible by four:

111110 = 0011 1110

           3    E

The hexadecimal number is 3E16.

Table 1.2 shows the hexadecimal equivalent of numbers 0 to 31.

Table 1.2: Hexadecimal equivalent of decimal numbers

Decimal Hexadecimal Decimal Hexadecimal
0 0 16 10
1 1 17 11
2 2 18 12
3 3 19 13
4 4 20 14
5 5 21 15
6 6 22 16
7 7 23 17
8 8 24 18
9 9 25 19
10 A 26 1A
11 B 27 1B
12 C 28 1C
13 D 29 1D
14 E 30 1E
15 F 31 1F

1.9 Converting Hexadecimal Numbers into Binary

To convert a hexadecimal number into binary, write the 4-bit binary equivalent of each hexadecimal digit.

Example 1.9

Convert hexadecimal number A916 into binary.

Solution 1.9

Writing the binary equivalent of each hexadecimal digit:

A = 10102 9 = 10012

The binary number is 101010012.

Example 1.10

Convert hexadecimal number FE3C16 into binary.

Solution 1.10

Writing the binary equivalent of each hexadecimal digit:

F = 11112 E = 11102 3 = 00112 C = 11002

The binary number is 11111110001111002.

1.10 Converting Hexadecimal Numbers into Decimal

To convert a hexadecimal number into decimal, calculate the sum of the powers of 16 of the number.

Example 1.11

Convert hexadecimal number 2AC16 into decimal.

Solution 1.11

Calculating the sum of the powers of 16 of the number:

2AC16 = 2 × 162 + 10 × 161 + 12 × 160

      = 512 + 160 + 12

      = 684

The required decimal number is 68410.

Example 1.12

Convert hexadecimal number EE16 into decimal.

Solution 1.12

Calculating the sum of the powers of 16 of the number:

EE16 = 14 × 161 + 14 × 160

     = 224 + 14

     = 238

The decimal number is 23810.

1.11 Converting Decimal Numbers into Hexadecimal

To convert a decimal number into hexadecimal, divide the number repeatedly by 16 and take the remainders. The first remainder is the LSD, and the last remainder is the MSD.

Example 1.13

Convert decimal number 23810 into hexadecimal.

Solution 1.13

Dividing the number repeatedly by 16:

238/16 → 14 Remainder 14 (E) (LSD)

14/16  → 0  Remainder 14 (E) (MSD)

The hexadecimal number is EE16.

Example 1.14

Convert decimal number 68410 into hexadecimal.

Solution 1.14

Dividing the number repeatedly by 16:

684/16 → 42 Remainder 12 (C) (LSD)

42/16  → 2  Remainder 10 (A)

2/16   → 0  Remainder 2      (MSD)

The hexadecimal number is 2AC16.

1.12 Converting Octal Numbers into Decimal

To convert an octal number into decimal, calculate the sum of the powers of 8 of the number.

Example 1.15

Convert octal number 158 into decimal.

Solution 1.15

Calculating the sum of the powers of 8 of the number:

158 = 1 × 81 + 5 × 80

     = 8 + 5

     = 13

The decimal number is 1310.

Example 1.16

Convert octal number 2378 into decimal.

Solution 1.16

Calculating the sum of the powers of 8 of the number:

2378 = 2 × 82 + 3 × 81 + 7 × 80

     = 128 + 24 + 7

     = 159

The decimal number is 15910.

1.13 Converting Decimal Numbers into Octal

To convert a decimal number into octal, divide the number repeatedly by 8 and take the remainders. The first remainder is the LSD, and the last remainder is the MSD.

Example 1.17

Convert decimal number 15910 into octal.

Solution 1.17

Dividing the number repeatedly by 8:

159/8 → 19 Remainder 7 (LSD)

19/8  → 2  Remainder 3

2/8   → 0  Remainder 2 (MSD)

The octal number is 2378.

Example 1.18

Convert decimal number 46010 into octal.

Solution 1.18

Dividing the number repeatedly by 8:

460/8 → 57 Remainder 4 (LSD)

57/8  → 7  Remainder 1

7/8   → 0  Remainder 7 (MSD)

The octal number is 7148.

Table 1.3 shows the octal equivalent of decimal numbers 0 to 31.

Table 1.3: Octal equivalent of decimal numbers

Decimal Octal Decimal Octal
0 0 16 20
1 1 17 21
2 2 18 22
3 3 19 23
4 4 20 24
5 5 21 25
6 6 22 26
7 7 23 27
8 10 24 30
9 11 25 31
10 12 26 32
11 13 27 33
12 14 28 34
13 15 29 35
14 16 30 36
15 17 31 37

1.14 Converting Octal Numbers into Binary

To convert an octal number into binary, write the 3-bit binary equivalent of each octal digit.

Example 1.19

Convert octal number 1778 into binary.

Solution 1.19

Write the binary equivalent of each octal digit:

1 = 0012 7 = 1112 7 = 1112

The binary number is 0011111112.

Example 1.20

Convert octal number 758 into binary.

Solution 1.20

Write the binary equivalent of each octal digit:

7 = 1112 5 = 1012

The binary number is 1111012.

1.15 Converting Binary Numbers into Octal

To convert a binary number into octal, arrange the number in groups of three and write the octal equivalent of each digit.

Example 1.21

Convert binary number 1101110012 into octal.

Solution 1.21

Arranging in groups of three:

110111001 = 110 111 001

             6   7   1

The octal number is 6718.

1.16 Negative Numbers

The most significant bit of a binary number is usually used as the sign bit. By convention, for positive numbers this bit is 0, and for negative numbers this bit is 1. Figure 1.5 shows the 4-bit positive and negative numbers. The largest positive and negative numbers are +7 and –8 respectively.

Binary number Decimal equivalent
0111 +7
0110 +6
0101 +5
0100 +4
0011 +3
0010 +2
0001 +1
0000 0
1111 −1
1110 −2
1101 −3
1100 −4
1011 −5
1010 −6
1001 −7
1000 −8

Figure 1.5: 4-bit positive and negative numbers

To convert a positive number to negative, take the complement of the number and add 1. This process is also called the 2’s complement of the number.

Example 1.22

Write decimal number −6 as a 4-bit number.

Solution 1.22

First, write the number as a positive number, then find the complement and add 1:

0110 +6

1001 complement

   1 add 1

−−−−

1010 which is −6

Example 1.23

Write decimal number −25 as a 8-bit number.

Solution 1.23

First, write the number as a positive number, then find the complement and add 1:

00011001 +25

11100110 complement

       1 add 1

–––––––-

11100111 which is −25

1.17 Adding Binary Numbers

The addition of binary numbers is similar to the addition of decimal numbers. Numbers in each column are added together with a possible carry from a previous column. The primitive addition operations are:

0 + 0 = 0

0 + 1 = 1

1 + 0 = 1

1 + 1 = 10     generate a carry bit

1 + 1 + 1 = 11 generate a carry bit

Some examples follow.

Example 1.24

Find the sum of binary numbers 011 and 110.

Solution 1.24

We can add these numbers as in the addition of decimal numbers:

  011 First column: 1 + 0 + 1

+ 110 Second column: 1 + 1 = 0 and a carry bit

 -––– Third column: 1 + 1 = 10

 1001

Example 1.25

Find the sum of binary numbers 01000011 and 00100010.

Solution 1.25

We can add these numbers as in the addition of decimal numbers:

  01000011 First column: 1 + 0 + 1

+ 00100010 Second column: 1 + 1 = 10

  –––––––– Third column: 0 + carry = 1

  01100101 Fourth column: 0 + 0 = 0

Fifth column: 0 + 0 = 0

Sixth column: 0 + 1 = 1

Seventh column: 1 + 0 = 1

Eighth column: 0 + 0 = 0

1.18 Subtracting Binary Numbers

To subtract one binary number from another, convert the number to be subtracted into negative and then add the two numbers.

Example 1.26

Subtract binary number 0010 from 0110.

Solution 1.26

First, convert the number to be subtracted into negative:

0010 number to be subtracted

1101 complement

   1 add 1

––––

1110

Now add the two numbers:

  0110

+ 1110

  ––––

  0100

Since we are using only 4 bits, we cannot show the carry bit.

1.19 Multiplication of Binary Numbers

Multiplication of two binary numbers is similar to the multiplication of two decimal numbers. The four possibilities are:

0 × 0 = 0

0 × 1 = 0

1 × 0 = 0

1 × 1 = 1

Some examples follow.

Example 1.27

Multiply the two binary numbers 0110 and 0010.

Solution 1.27

Multiplying the numbers:

   0110

   0010

   ----

   0000

  0110

 0000

0000

-------

 001100 or 1100

In this example 4 bits are needed to show the final result.

Example 1.28

Multiply binary numbers 1001 and 1010.

Solution 1.28

Multiplying the numbers:

   1001

   1010

   ----

   0000

  1001

 0000

1001

-------

1011010

In this example 7 bits are required to show the final result.

1.20 Division of Binary Numbers

Division with binary numbers is similar to division with decimal numbers. An example follows.

Example 1.29

Divide binary number 1110 into binary number 10.

Solution 1.29

Dividing the numbers:

   111

10|―――

   1110

   10

   ----

    11

    10

   ----

     10

     10

   ----

     00

gives the result 1112.

1.21 Floating Point Numbers

Floating point numbers are used to represent noninteger fractional numbers, for example, 3.256, 2.1, 0.0036, and so forth. Floating point numbers are used in most engineering and technical calculations. The most common floating point standard is the IEEE standard, according to which floating point numbers are represented with 32 bits (single precision) or 64 bits (double precision).

In this section we are looking at the format of 32-bit floating point numbers only and seeing how mathematical operations can be performed with such numbers.

According to the IEEE standard, 32-bit floating point numbers are represented as:

31   30    23 22                    0

X    XXXXXXXX XXXXXXXXXXXXXXXXXXXXXXX

↑        ↑              ↑

sign exponent       mantissa

The most significant bit indicates the sign of the number, where 0 indicates the number is positive, and 1 indicates it is negative.

The 8-bit exponent shows the power of the number. To make the calculations easy, the sign of the exponent is not shown; instead, the excess-128 numbering system is used. Thus, to find the real exponent we have to subtract 127 from the given exponent. For example, if the mantissa is “10000000,” the real value of the mantissa is 128 – 127 = 1.

The mantissa is 23 bits wide and represents the increasing negative powers of 2. For example, if we assume that the mantissa is “1110000000000000000000,” the value of this mantissa is calculated as 2–1 + 2-2 + 2-3 = 7/8.

The decimal equivalent of a floating point number can be calculated using the formula:

Number = (–1)s 2e-127 1.f

where

s = 0 for positive numbers, 1 for negative numbers

e = exponent (between 0 and 255)

f = mantissa

As shown in this formula, there is a hidden 1 in front of the mantissa (i.e, the mantissa is shown as 1.f ).

The largest number in 32-bit floating point format is:

0 11111110 11111111111111111111111

This number is (2–2–23)2127 or decimal 3.403×1038. The numbers keep their precision up to 6 digits after the decimal point.

The smallest number in 32-bit floating point format is:

0 00000001 00000000000000000000000

This number is 2–126 or decimal 1.175×10–38.

1.22 Converting a Floating Point Number into Decimal

To convert a given floating point number into decimal, we have to find the mantissa and the exponent of the number and then convert into decimal as just shown. Some examples are given here.

Example 1.30

Find the decimal equivalent of the floating point number: 0 10000001 10000000000000000000000

Solution 1.30

Here

sign = positive

exponent = 129 – 127 = 2

mantissa = 2-1 = 0.5

The decimal equivalent of this number is +1.5 × 22 = +6.0.

Example 1.31

Find the decimal equivalent of the floating point number: 0 10000010 11000000000000000000

Solution 1.31

In this example,

sign = positive

exponent = 130 – 127 = 3

mantissa = 2-1 + 2-2 = 0.75

The decimal equivalent of the number is +1.75 × 23 = 14.0.

1.22.1 Normalizing Floating Point Numbers

Floating point numbers are usually shown in normalized form. A normalized number has only one digit before the decimal point (a hidden number 1 is assumed before the decimal point).

To normalize a given floating point number, we have to move the decimal point repeatedly one digit to the left and increase the exponent after each move.

Some examples follow.

Example 1.32

Normalize the floating point number 123.56

Solution 1.32

If we write the number with a single digit before the decimal point we get:

1.2356 × 10²

Example 1.33

Normalize the binary number 1011.12

Solution 1.33

If we write the number with a single digit before the decimal point we get:

1.0111 × 2³

1.22.2 Converting a Decimal Number into Floating Point

To convert a given decimal number into floating point, carry out the following steps:

• Write the number in binary.

• Normalize the number.

• Find the mantissa and the exponent.

• Write the number as a floating point number.

Some examples follow:

Example 1.34

Convert decimal number 2.2510 into floating point.

Solution 1.34

Write the number in binary:

2.2510 = 10.012

Normalize the number:

10.012 = 1.0012 × 21

Here, s = 0, e – 127 = 1 or e = 128, and f = 00100000000000000000000.

(Remember that a number 1 is assumed on the left side, even though it is not shown in the calculation). The required floating point number can be written as:

s     e                 f

0 10000000 (1)001 0000 0000 0000 0000 0000

or, the required 32-bit floating point number is:

01000000000100000000000000000000

Example 1.35

Convert the decimal number 134.062510 into floating point.

Solution 1.35

Write the number in binary:

134.062510 = 10000110.0001

Normalize the number:

10000110.0001 = 1.00001100001 × 27

Here, s = 0, e – 127 = 7 or e = 134, and f = 00001100001000000000000.

The required floating point number can be written as:

s     e                f

0 10000110 (1)00001100001000000000000

or, the required 32-bit floating point number is:

01000011000001100001000000000000

1.22.3 Multiplication and Division of Floating Point Numbers

Multiplication and division of floating point numbers are rather easy. Here are the steps:

• Add (or subtract) the exponents of the numbers.

• Multiply (or divide) the mantissa of the numbers.

• Correct the exponent.

• Normalize the number.

• The sign of the result is the EXOR of the signs of the two numbers.

Since the exponent is processed twice in the calculations, we have to subtract 127 from the exponent.

An example showing the multiplication of two floating point numbers follows.

Example 1.36

Show the decimal numbers 0.510 and 0.7510 in floating point and then calculate their multiplication.

Solution 1.36

Convert the numbers into floating point as:

0.510 = 1.0000 × 2-1

here, s = 0, e – 127 = -1 or e = 126 and f = 0000

or,

0.510 = 0 01110110 (1)000 0000 0000 0000 0000 0000

Similarly,

0.7510 = 1.1000 × 2-1

here, s = 0, e = 126 and f = 1000

or,

0.7510 = 0 01110110 (1)100 0000 0000 0000 0000 0000

Multiplying the mantissas results in “(1)100 0000 0000 0000 0000 0000.” The sum of the exponents is 126+126=252. Subtracting 127 from the mantissa, we obtain 252–127=125. The EXOR of the signs of the numbers is 0. Thus, the result can be shown in floating point as:

0 01111101 (1)100 0000 0000 0000 0000 0000

This number is equivalent to decimal 0.375 (0.5×0.75=0.375), which is the correct result.

1.22.4 Addition and Subtraction of Floating Point Numbers

The exponents of floating point numbers must be the same before they can be added or subtracted. The steps to add or subtract floating point numbers are:

• Shift the smaller number to the right until the exponents of both numbers are the same. Increment the exponent of the smaller number after each shift.

• Add (or subtract) the mantissa of each number as an integer calculation, without considering the decimal points.

• Normalize the result.

An example follows.

Example 1.37

Show decimal numbers 0.510 and 0.7510 in floating point and then calculate the sum of these numbers.

Solution 1.37

As shown in Example 1.36, we can convert the numbers into floating point as:

0.510 = 0 01110110 (1)000 0000 0000 0000 0000 0000

Similarly,

0.7510 = 0 01110110 (1)100 0000 0000 0000 0000 0000

Since the exponents of both numbers are the same, there is no need to shift the smaller number. If we add the mantissa of the numbers without considering the decimal points, we get:

  (1)000 0000 0000 0000 0000 0000

+ (1)100 0000 0000 0000 0000 0000

 --------------------------------

 (10)100 0000 0000 0000 0000 0000

To normalize the number, shift it right by one digit and then increment its exponent. The resulting number is:

0 01111111 (1)010 0000 0000 0000 0000 0000

This floating point number is equal to decimal number 1.25, which is the sum of decimal numbers 0.5 and 0.75.

A program for converting floating point numbers into decimal, and decimal numbers into floating point, is available for free on the following web site:

http://babbage.cs.qc.edu/courses/cs341/IEEE-754.html

1.23 BCD Numbers

BCD (binary coded decimal) numbers are usually used in display systems such as LCDs and 7-segment displays to show numeric values. In BCD, each digit is a 4-bit number from 0 to 9. As an example, Table 1.4 shows the BCD numbers between 0 and 20.

Table 1.4: BCD numbers between 0 and 20

Decimal BCD Binary
0 0000 0000
1 0001 0001
2 0010 0010
3 0011 0011
4 0100 0100
5 0101 0101
6 0110 0110
7 0111 0111
8 1000 1000
9 1001 1001
10 0001 0000 1010
11 0001 0001 1011
12 0001 0010 1100
13 0001 0011 1101
14 0001 0100 1110
15 0001 0101 1111
16 0001 0110 1 0000
17 0001 0111 1 0001
18 0001 1000 1 0010
19 0001 1001 1 0011
20 0010 0000 1 0100

Example 1.38

Write the decimal number 295 as a BCD number.

Solution 1.38

Write the 4-bit binary equivalent of each digit:

2 = 00102 9 = 10012 5 = 01012

The BCD number is 0010 1001 01012.

Example 1.39

Write the decimal equivalent of BCD number 1001 1001 0110 00012.

Solution 1.39

Writing the decimal equivalent of each group of 4-bit yields the decimal number: 9961

1.24 Summary

Chapter 1 has provided an introduction to the microprocessor and microcontroller systems. The basic building blocks of microcontrollers were described briefly. The chapter also provided an introduction to various number systems, and described how to convert a given number from one base into another. The important topics of floating point numbers and floating point arithmetic were also described with examples.

1.25 Exercises

1. What is a microcontroller? What is a microprocessor? Explain the main difference between a microprocessor and a microcontroller.

2. Identify some applications of microcontrollers around you.

3. Where would you use an EPROM memory?

4. Where would you use a RAM memory?

5. Explain the types of memory usually used in microcontrollers.

6. What is an input-output port?

7. What is an analog-to-digital converter? Give an example of how this converter is used.

8. Explain why a watchdog timer could be useful in a real-time system.

9. What is serial input-output? Where would you use serial communication?

10. Why is the current sink/source capability important in the specification of an output port pin?

11. What is an interrupt? Explain what happens when an interrupt is recognized by a microcontroller?

12. Why is brown-out detection important in real-time systems?

13. Explain the difference between an RISC-based microcontroller and a CISC-based microcontroller. What type of microcontroller is PIC?

14. Convert the following decimal numbers into binary:

 a) 23 b) 128 c) 255 d) 1023

 e) 120 f) 32000 g) 160 h) 250

15. Convert the following binary numbers into decimal:

 a) 1111 b) 0110 c) 11110000

 d) 00001111 e) 10101010 f) 10000000

16. Convert the following octal numbers into decimal:

 a) 177 b) 762 c) 777 d) 123

 e) 1777 f) 655 g) 177777 h) 207

17. Convert the following decimal numbers into octal:

 a) 255 b) 1024 c) 129 d) 2450

 e) 4096 f) 256 g) 180 h) 4096

18. Convert the following hexadecimal numbers into decimal:

 a) AA b) EF c) 1FF d) FFFF

 e) 1AA f) FEF g) F0 h) CC

19. Convert the following binary numbers into hexadecimal:

 a) 0101 b) 11111111 c) 1111 d) 1010

 e) 1110 f) 10011111 g) 1001 h) 1100

20. Convert the following binary numbers into octal:

 a) 111000 b) 000111 c) 1111111 d) 010111

 e) 110001 f) 11111111 g) 1000001 h) 110000

21. Convert the following octal numbers into binary:

 a) 177 b) 7777 c) 555 d) 111

 e) 1777777 f) 55571 g) 171 h) 1777

22. Convert the following hexadecimal numbers into octal:

 a) AA b) FF c) FFFF d) 1AC

 e) CC f) EE g) EEFF h) AB

23. Convert the following octal numbers into hexadecimal:

 a) 177 b) 777 c) 123 d) 23

 e) 1111 f) 17777777 g) 349 h) 17

24. Convert the following decimal numbers into floating point:

 a) 23.45 b) 1.25 c) 45.86 d) 0.56

25. Convert the following decimal numbers into floating point and then calculate their sum:

 0.255 and 1.75

26. Convert the following decimal numbers into floating point and then calculate their product:

 2.125 and 3.75

27. Convert the following decimal numbers into BCD:

 a) 128 b) 970 c) 900 d) 125

CHAPTER 2

PIC18F Microcontroller Series

PIC16-series microcontrollers have been around for many years. Although these are excellent general purpose microcontrollers, they have certain limitations. For example, the program and data memory capacities are limited, the stack is small, and the interrupt structure is primitive, all interrupt sources sharing the same interrupt vector. PIC16-series microcontrollers also do not provide direct support for advanced peripheral interfaces such as USB, CAN bus, etc., and interfacing with such devices is not easy. The instruction set for these microcontrollers is also limited. For example, there are no multiplication or division instructions, and branching is rather simple, being a combination of skip and goto instructions.

Microchip Inc. has developed the PIC18 series of microcontrollers for use in high-pincount, high-density, and complex applications. The PIC18F microcontrollers offer cost-efficient solutions for general purpose applications written in C that use a real-time operating system (RTOS) and require a complex communication protocol stack such as TCP/IP, CAN, USB, or ZigBee. PIC18F devices provide flash program memory in sizes from 8 to 128Kbytes and data memory from 256 to 4Kbytes, operating at a range of 2.0 to 5.0 volts, at speeds from DC to 40MHz.

The basic features of PIC18F-series microcontrollers are:

• 77 instructions

• PIC16 source code compatible

• Program memory addressing up to 2 Mbytes

• Data memory addressing up to 4 Kbytes

• DC to 40MHz operation

• 8×8 hardware multiplier

• Interrupt priority levels

• 16-bit-wide instructions, 8-bit-wide data path

• Up to two 8-bit timers/counters

• Up to three 16-bit timers/counters

• Up to four external interrupts

• High current (25mA) sink/source capability

• Up to five capture/compare/PWM modules

• Master synchronous serial port module (SPI and I²C modes)

• Up to two USART modules

• Parallel slave port (PSP)

• Fast 10-bit analog-to-digital converter

• Programmable low-voltage detection (LVD) module

• Power-on reset (POR), power-up timer (PWRT), and oscillator start-up timer (OST)

• Watchdog timer (WDT) with on-chip RC oscillator

• In-circuit programming

In addition, some microcontrollers in the PIC18F family offer the following special features:

• Direct CAN 2.0 bus interface

• Direct USB 2.0 bus interface

• Direct LCD control interface

• TCP/IP interface

• ZigBee interface

• Direct motor control interface

Most devices in the PIC18F family are source compatible with each other. Table 2.1 gives the characteristics of some of the popular devices in this family. This chapter offers a detailed study of the PIC18FXX2 microcontrollers. The architectures of most of the other microcontrollers in the PIC18F family are similar.

Table 2.1: The 18FXX2 microcontroller family

Feature PIC18F242 PIC18F252 PIC18F442 PIC18F452
Program memory (Bytes) 16K 32K 16K 32K
Data memory (Bytes) 768 1536 768 1536
EEPROM (Bytes) 256 256 256 256
I/O Ports A,B,C A,B,C A,B,C,D,E A,B,C,D,E
Timers 4 4 4 4
Interrupt sources 17 17 18 18
Capture/compare/PWM 2 2 2 2
Serial communication MSSP USART MSSP USART MSSP USART MSSP USART
A/D converter (10-bit) 5 channels 5 channels 8 channels 8 channels
Low-voltage detect yes yes yes yes
Brown-out reset yes yes yes yes
Packages 28-pin DIP 28-pin SOIC 28-pin DIP 28-pin SOIC 40-pin DIP 44-pin PLCC 44-pin TQFP 40-pin DIP 44-pin PLCC 44-pin TQFP

The reader may be familiar with the programming and applications of the PIC16F series. Before going into the details of the PIC18F series, it is worthwhile to compare the features of the PIC18F series with those of the PIC16F series.

The following are similarities between PIC16F and PIC18F:

• Similar packages and pinouts

• Similar special function register (SFR) names and functions

• Similar peripheral devices

• Subset of PIC18F instruction set

• Similar development tools

The following are new with the PIC18F series:

• Number of instructions doubled

• 16-bit instruction word

• Hardware 8×8 multiplier

• More external interrupts

• Priority-based interrupts

• Enhanced status register

• Increased program and data memory size

• Bigger stack

• Phase-locked loop (PLL) clock generator

• Enhanced input-output port architecture

• Set of configuration registers

• Higher speed of operation

• Lower power operation

2.1 PIC18FXX2 Architecture

As shown in Table 2.1, the PIC18FXX2 series consists of four devices. PIC18F2X2 microcontrollers are 28-pin devices, while PIC18F4X2 microcontrollers are 40-pin devices. The architectures of the two groups are almost identical except that the larger devices have more input-output ports and more A/D converter channels. In this section we shall be looking at the architecture of the PIC18F452 microcontroller in detail. The architectures of other standard PIC18F-series microcontrollers are similar, and the knowledge gained in this section should be enough to understand the operation of other PIC18F-series microcontrollers.

The pin configuration of the PIC18F452 microcontroller (DIP package) is shown in Figure 2.1. This is a 40-pin microcontroller housed in a DIL package, with a pin configuration similar to the popular PIC16F877.

Figure 2.1: PIC18F452 microcontroller DIP pin configuration

Figure 2.2 shows the internal block diagram of the PIC18F452 microcontroller. The CPU is at the center of the diagram and consists of an 8-bit ALU, an 8-bit working accumulator register (WREG), and an 8×8 hardware multiplier. The higher byte and the lower byte of a multiplication are stored in two 8-bit registers called PRODH and PRODL respectively.

Figure 2.2: Block diagram of the PIC18F452 microcontroller

The program counter and program memory are shown in the upper left portion of the diagram. Program memory addresses consist of 21 bits, capable of accessing 2Mbytes of program memory locations. The PIC18F452 has only 32Kbytes of program memory, which requires only 15 bits. The remaining 6 address bits are redundant and not used. A table pointer provides access to tables and to the data stored in program memory. The program memory contains a 31-level stack which is normally used to store the interrupt and subroutine return addresses.

The data memory can be seen at the top center of the diagram. The data memory bus is 12 bits wide, capable of accessing 4Kbytes of data memory locations. As we shall see later, the data memory consists of special function registers (SFR) and general purpose registers, all organized in banks.

The bottom portion of the diagram shows the timers/counters, capture/compare/PWM registers, USART, A/D converter, and EEPROM data memory. The PIC18F452 consists of:

• 4 timers/counters

• 2 capture/compare/PWM modules

• 2 serial communication modules

• 8 10-bit A/D converter channels

• 256 bytes EEPROM

The oscillator circuit, located at the left side of the diagram, consists of:

• Power-up timer

• Oscillator start-up timer

• Power-on reset

• Watchdog timer

• Brown-out reset

• Low-voltage programming

• In-circuit debugger

• PLL circuit

• Timing generation circuit

The PLL circuit is new to the PIC18F series and provides the option of multiplying up the oscillator frequency to speed up the overall operation. The watchdog timer can be used to force a restart of the microcontroller in the event of a program crash. The in-circuit debugger is useful during program development and can be used to return diagnostic data, including the register values, as the microcontroller is executing a program.

The input-output ports are located at the right side of the diagram. The PIC18F452 has five parallel ports named PORTA, PORTB, PORTC, PORTD, and PORTE. Most port pins have multiple functions. For example, PORTA pins can be used as parallel inputs-outputs or analog inputs. PORTB pins can be used as parallel inputs-outputs or as interrupt inputs.

2.1.1 Program Memory Organization

The program memory map is shown in Figure 2.3. All PIC18F devices have a 21-bit program counter and hence are capable of addressing 2Mbytes of memory space. User memory space on the PIC18F452 microcontroller is 00000H to 7FFFH. Accessing a nonexistent memory location (8000H to 1FFFFFH) will cause a read of all 0s. The reset vector, where the program starts after a reset, is at address 0000. Addresses 0008H and 0018H are reserved for the vectors of high-priority and low-priority interrupts respectively, and interrupt service routines must be written to start at one of these locations.

Figure 2.3: Program memory map of PIC18F452

The PIC18F microcontroller has a 31-entry stack that is used to hold the return addresses for subroutine calls and interrupt processing. The stack is not part of the program or the data memory space. The stack is controlled by a 5-bit stack pointer which is initialized to 00000 after a reset. During a subroutine call (or interrupt) the stack pointer is first incremented, and the memory location it points to is written with the contents of the program counter. During the return from a subroutine call (or interrupt), the memory location the stack pointer has pointed to is decremented. The projects in this book are based on using the C language. Since subroutine and interrupt call/return operations are handled automatically by the C language compiler, their operation is not described here in more detail.

Program memory is addressed in bytes, and instructions are stored as two bytes or four bytes in program memory. The least significant byte of an instruction word is always stored in an even address of the program memory.

An instruction cycle consists of four cycles: A fetch cycle begins with the program counter incrementing in Q1. In the execution cycle, the fetched instruction is latched into the instruction register in cycle Q1. This instruction is decoded and executed during cycles Q2, Q3, and Q4. A data memory location is read during the Q2 cycle and written during the Q4 cycle.

2.1.2 Data Memory Organization

The data memory map of the PIC18F452 microcontroller is shown in Figure 2.4. The data memory address bus is 12 bits with the capability to address up to 4Mbytes. The memory in general consists of sixteen banks, each of 256 bytes, where only 6 banks are used. The PIC18F452 has 1536 bytes of data memory (6 banks × 256 bytes each) occupying the lower end of the data memory. Bank switching happens automatically when a high-level language compiler is used, and thus the user need not worry about selecting memory banks during programming.

Figure 2.4: The PIC18F452 data memory map

The special function register (SFR) occupies the upper half of the top memory bank. SFR contains registers which control operations such as peripheral devices, timers/counters, A/D converter, interrupts, and USART. Figure 2.5 shows the SFR registers of the PIC18F452 microcontroller.

Figure 2.5: The PIC18F452 SFR registers

2.1.3 The Configuration Registers

PIC18F452 microcontrollers have a set of configuration registers (PIC16-series microcontrollers had only one configuration register). Configuration registers are programmed during the programming of the flash program memory by the programming device. These registers are shown in Table 2.2. these registers are given in Table 2.3. Some of the more important configuration registers are described in this section in detail.

Table 2.2: PIC18F452 configuration registers

File Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Default/Unprogrammed Value
300001h CONFIG1H OSCSEN# FOSC2 FOSC1 FOSC0 --1--111
300002h CONFIG2L BORV1 BORV0 BOREN PWRTEN# ---- 1111
300003h CONFIG2H WDTPS2 WDTPS1 WDTPS0 WDTEN ---- 1111
300005h CONFIG3H CCP2MX ---- ---1
300006h CONFIG4L DEBUG LVP STVREN1 --- -1-1
300008h CONFIG5L CP3 CP2 CP1 CP0 ---- 1111
300009h CONFIG5H CPD CPB 11-- ----
30000Ah CONFIG6L WRT3 WRT2 WRT1 WRT0 ---- 1111
30000Bh CONFIG6H WRTD WRTB WRTC 111- ----
30000Ch CONFIG7L EBTR3 EBTR2 EBTR1 EBTR0 ---- 1111
30000Dh CONFIG7H EBTRB -1-----
3FFFFEh DEVID1 DEV2 DEV1 DEV0 REV4 REV3 REV2 REV1 REV0 (1)
3FFFFFh DEVID2 DEV10 DEV9 DEV8 DEV7 DEV6 DEV5 DEV4 DEV3 0000 0100

Legend: x = unknown, u = unchanged, – = unimplemented, q = value depends on condition. Shaded cells are unimplemented, read as ‘0’.

Table 2.3: PIC18F452 configuration register descriptions

Configuration bits Description
OSCSEN Clock source switching enable
FOSC2:FOSC0 Oscillator modes
BORV1:BORV0 Brown-out reset voltage
BOREN Brown-out reset enable
PWRTEN Power-up timer enable
WDTPS2:WDTPS0 Watchdog timer postscale bits
WDTEN Watchdog timer enable
CCP2MX CCP2 multiplex
DEBUG Debug enable
LVP Low-voltage program enable
STVREN Stack full/underflow reset enable
CP3:CP0 Code protection
CPD EEPROM code protection
CPB Boot block code protection
WRT3:WRT0 Program memory write protection
WRTD EPROM write protection
WRTB Boot block write protection
WRTC Configuration register write protection
EBTR3:EBTR0 Table read protection
EBTRB Boot block table read protection
DEV2:DEV0 Device ID bits (001 = 18F452)
REV4:REV0 Revision ID bits
DEV10:DEV3 Device ID bits

CONFIG1H

The CONFIG1H configuration register is at address 300001H and is used to select the microcontroller clock sources. The bit patterns are shown in Figure 2.6.

Figure 2.6: CONFIG1H register bits 

CONFIG2L

The CONFIG2L configuration register is at address 300002H and is used to select the brown-out voltage bits. The bit patterns are shown in Figure 2.7.

Figure 2.7: CONFIG2L register bits

CONFIG2H

The CONFIG2H configuration register is at address 300003H and is used to select the watchdog operations. The bit patterns are shown in Figure 2.8.

Figure 2.8: CONFIG2H register bits

2.1.4 The Power Supply

The power supply requirements of the PIC18F452 microcontroller are shown in Figure 2.9. As shown in Figure 2.10, PIC18F452 can operate with a supply voltage of 4.2V to 5.5V at the full speed of 40MHz. The lower power version, PIC18LF452, can operate from 2.0 to 5.5 volts. At lower voltages the maximum clock frequency is 4MHz, which rises to 40MHz at 4.2V. The RAM data retention voltage is specified as 1.5V and will be lost if the power supply voltage is lowered below this value. In practice, most microcontroller-based systems are operated with a single +5V supply derived from a suitable voltage regulator.

Figure 2.9: The PIC8F452 power supply parameters

Figure 2.10: Operation of PIC18LF452 at different voltages

2.1.5 The Reset

The reset action puts the microcontroller into a known state. Resetting a PIC18F microcontroller starts execution of the program from address 0000H of the program memory. The microcontroller can be reset during one of the following operations:

• Power-on reset (POR)

• MCLR reset

• Watchdog timer (WDT) reset

• Brown-out reset (BOR)

• Reset instruction

• Stack full reset

• Stack underflow reset

Two types of resets are commonly used: power-on reset and external reset using the MCLR pin.

Power-on Reset

The power-on reset is generated automatically when power supply voltage is applied to the chip. The MCLR pin should be tied to the supply voltage directly or, preferably, through a 10K resistor. Figure 2.11 shows a typical reset circuit.

Figure 2.11: Typical reset circuit

For applications where the rise time of the voltage is slow, it is recommended to use a diode, a capacitor, and a series resistor as shown in Figure 2.12.

Figure 2.12: Reset circuit for slow-rising voltages

In some applications the microcontroller may have to be reset externally by pressing a button. Figure 2.13 shows the circuit that can be used to reset the microcontroller externally. Normally the MCLR input is at logic 1. When the RESET button is pressed, this pin goes to logic 0 and resets the microcontroller.

Figure 2.13: External reset circuit

2.1.6 The Clock Sources

The PIC18F452 microcontroller can be operated from an external crystal or ceramic resonator connected to the microcontroller’s OSC1 and OSC2 pins. In addition, an external resistor and capacitor, an external clock source, and in some models internal oscillators can be used to provide clock pulses to the microcontroller. There are eight clock sources on the PIC18F452 microcontroller, selected by the configuration register CONFIG1H. These are:

• Low-power crystal (LP)

• Crystal or ceramic resonator (XT)

• High-speed crystal or ceramic resonator (HS)

• High-speed crystal or ceramic resonator with PLL (HSPLL)

• External clock with FOSC/4 on OSC2 (EC)

• External clock with I/O on OSC2 (port RA6) (ECIO)

• External resistor/capacitor with FOSC/4 output on OSC2 (RC)

• External resistor/capacitor with I/O on OSC2 (port RA6) (RCIO)

Crystal or Ceramic Resonator Operation

The first several clock sources listed use an external crystal or ceramic resonator that is connected to the OSC1 and OSC2 pins. For applications where accuracy of timing is important, a crystal should be used. And if a crystal is used, a parallel resonant crystal must be chosen, since series resonant crystals do not oscillate when the system is first powered.

Figure 2.14 shows how a crystal is connected to the microcontroller. The capacitor values depend on the mode of the crystal and the selected frequency. Table 2.4 gives the recommended values. For example, for a 4MHz crystal frequency, use 15pF capacitors. Higher capacitance increases the oscillator stability but also increases the start-up time.

Figure 2.14: Using a crystal as the clock input

Table 2.4: Capacitor values

Mode Frequency C1,C2 (pF)
LP 32 KHz 33
200 KHz 15
XT 200 KHz 22–68
1.0 MHz 15
4.0 MHz 15
HS 4.0 MHz 15
8.0 MHz 15–33
20.0 MHz 15–33
25.0 MHz 15–33

Resonators should be used in low-cost applications where high accuracy in timing is not required. Figure 2.15 shows how a resonator is connected to the microcontroller.

Figure 2.15: Using a resonator as the clock input

The LP (low-power) oscillator mode is advised in applications to up to 200KHz clock. The XT mode is advised to up to 4MHz, and the HS (high-speed) mode is advised in applications where the clock frequency is between 4MHz to 25MHz.

An external clock source may also be connected to the OSC1 pin in the LP, XT, or HS modes as shown in Figure 2.16.

Figure 2.16: Connecting an external clock in LP, XT, or HS modes

External Clock Operation

An external clock source can be connected to the OSC1 input of the microcontroller in EC and ECIO modes. No oscillator start-up time is required after a power-on reset. Figure 2.17 shows the operation with the external clock in EC mode. Timing pulses at the frequency FOSC/4 are available on the OSC2 pin. These pulses can be used for test purposes or to provide pulses to external devices.

Figure 2.17: External clock in EC mode

The ECIO mode is similar to the EC mode, except that the OSC2 pin can be used as a general purpose digital I/O pin. As shown in Figure 2.18, this pin becomes bit 6 of PORTA (i.e., pin RA6).

Figure 2.18: External clock in ECIO mode

Resistor/Capacitor Operation

In the many applications where accurate timing is not required we can use an external resistor and a capacitor to provide clock pulses. The clock frequency is a function of the resistor, the capacitor, the power supply voltage, and the temperature. The clock frequency is not accurate and can vary from unit to unit due to manufacturing and component tolerances. Table 2.5 gives the approximate clock frequency with various resistor and capacitor combinations. A close approximation of the clock frequency is 1/(4.2RC), where R should be between 3K and 100K and C should be greater than 20pF.

Table 2.5: Clock frequency with RC

C (pF) R (K) Frequency (MHz)
22 3.3 3.3
4.7 2.3
10 1.08
30 3.3 2.4
4.7 1.7
10 0.793

In RC mode, the oscillator frequency divided by 4 (FOSC/4) is available on pin OSC2 of the microcontroller. Figure 2.19 shows the operation at a clock frequency of approximately 2MHz, where R=3.9K and C=30pF. In this application the clock frequency at the output of OSC2 is 2MHz/4=500KHz.

Figure 2.19: 2MHz clock in RC mode

RCIO mode is similar to RC mode, except that the OSC2 pin can be used as a general purpose digital I/O pin. As shown in Figure 2.20, this pin becomes bit 6 of PORTA (i.e., pin RA6).

Figure 2.20: 2MHz clock in RCIO mode

Crystal or Resonator with PLL

One of the problems with using high-frequency crystals or resonators is electromagnetic interference. A Phase Locked Loop (PLL) circuit is provided that can be enabled to multiply the clock frequency by 4. Thus, for a crystal clock frequency of 10MHz, the internal operation frequency will be multiplied to 40MHz. The PLL mode is enabled when the oscillator configuration bits are programmed for HS mode.

Internal Clock

Some devices in the PIC18F family have internal clock modes (although the PIC18F452 does not). In this mode, OSC1 and OSC2 pins are available for general purpose I/O (RA6 and RA7) or as FOSC/4 and RA7. An internal clock can be from 31KHz to 8MHz and is selected by registers OSCCON and OSCTUNE. Figure 2.21 shows the bits of internal clock control registers.

Figure 2.21: Internal clock control registers

Clock Switching

It is possible to switch the clock from the main oscillator to a low-frequency clock source. For example, the clock can be allowed to run fast in periods of intense activity and slower when there is less activity. In the PIC18F452 microcontroller this is controlled by bit SCS of the OSCCON register. In microcontrollers of the PIC18F family that do support an internal clock, clock switching is controlled by bits SCS0 and SCS1 of OSCCON. It is important to ensure that during clock switching unwanted glitches do not occur in the clock signal. PIC18F microcontrollers contain circuitry to ensure error-free switching from one frequency to another.

2.1.7 Watchdog Timer

In PIC18F-series microcontrollers family members the watchdog timer (WDT) is a free-running on-chip RC-based oscillator and does not require any external components. When the WDT times out, a device RESET is generated. If the device is in SLEEP mode, the WDT time-out will wake it up and continue with normal operation.

The watchdog is enabled/disabled by bit SWDTEN of register WDTCON. Setting SWDTEN = 1 enables the WDT, and clearing this bit turns off the WDT. On the PIC18F452 microcontroller an 8-bit postscaler is used to multiply the basic time-out period from 1 to 128 in powers of 2. This postscaler is controlled from configuration register CONFIG2H. The typical basic WDT time-out period is 18ms for a postscaler value of 1.

2.1.8 Parallel I/O Ports

The parallel ports in PIC18F microcontrollers are very similar to those of the PIC16 series. The number of I/O ports and port pins varies depending on which PIC18F microcontroller is used, but all of them have at least PORTA and PORTB. The pins of a port are labeled as RPn, where P is the port letter and n is the port bit number. For example, PORTA pins are labeled RA0 to RA7, PORTB pins are labeled RB0 to RB7, and so on.

When working with a port we may want to:

• Set port direction

• Set an output value

• Read an input value

• Set an output value and then read back the output value

The first three operations are the same in the PIC16 and the PIC18F series. In some applications we may want to send a value to the port and then read back the value just sent. The PIC16 series has a weakness in the port design such that the value read from a port may be different from the value just written to it. This is because the reading is the actual port bit pin value, and this value can be changed by external devices connected to the port pin. In the PIC18F series, a latch register (e.g., LATA for PORTA) is introduced to the I/O ports to hold the actual value sent to a port pin. Reading from the port reads the latched value, which is not affected by any external device.

In this section we shall be looking at the general structure of I/O ports.

PORTA

In the PIC18F452 microcontroller PORTA is 7 bits wide and port pins are shared with other functions. Table 2.6 shows the PORTA pin functions.

Table 2.6: PIC18F452 PORTA pin functions

Pin Description
RA0/AN0  
RA0 Digital I/O
AN0 Analog input 0
RA1/AN1  
RA1 Digital I/O
AN1 Analog input 1
RA2/AN2/VREF–  
RA2 Digital I/O
AN2 Analog input 2
VREF– A/D reference voltage (low) input
RA3/AN3/VREF+  
RA3 Digital I/O
AN3 Analog input 3
VREF+ A/D reference voltage (high) input
RA4/T0CKI  
RA4 Digital I/O
T0CKI Timer 0 external clock input
RA5/AN4/SS/LVDIN  
RA5 Digital I/O
AN4 Analog input 4
SS SPI Slave Select input
RA6 Digital I/O

The architecture of PORTA is shown in Figure 2.22. There are three registers associated with PORTA:

• Port data register — PORTA

• Port direction register — TRISA

• Port latch register — LATA

Figure 2.22: PIC18F452 PORTA RA0–RA3 and RA5 pins

PORTA is the name of the port data register. The TRISA register defines the direction of PORTA pins, where a logic 1 in a bit position defines the pin as an input pin, and a 0 in a bit position defines it as an output pin. LATA is the output latch register which shares the same data latch as PORTA. Writing to one is equivalent to writing to the other. But reading from LATA activates the buffer at the top of the diagram, and the value held in the PORTA/LATA data latch is transferred to the data bus independent of the state of the actual output pin of the microcontroller.

Bits 0 through 3 and 5 of PORTA are also used as analog inputs. After a device reset, these pins are programmed as analog inputs and RA4 and RA6 are configured as digital inputs. To program the analog inputs as digital I/O, the ADCON1 register (A/D register) must be programmed accordingly. Writing 7 to ADCON1 configures all PORTA pins as digital I/O.

The RA4 pin is multiplexed with the Timer 0 clock input (T0CKI). This is a Schmitt trigger input and an open drain output.

RA6 can be used as a general purpose I/O pin, as the OSC2 clock input, or as a clock output providing FOSC/4 clock pulses.

PORTB

In PIC18F452 microcontroller PORTB is an 8-bit bidirectional port shared with interrupt pins and serial device programming pins. Table 2.7 gives the PORTB bit functions.

Table 2.7: PIC18F452 PORTB pin functions

Pin Description
RB0/INT0  
RB0 Digital I/O
INT0 External interrupt 0
RB1/INT1  
RB1 Digital I/O
INT1 External interrupt 1
RB2/INT2  
RB2 Digital I/O
INT2 External interrupt 2
RB3/CCP2  
RB3 Digital I/O
CCP2 Capture 2 input, compare 2, and PWM2 output
RB4 Digital I/O, interrupt on change pin
RB5/PGM  
RB5 Digital I/O, interrupt on change pin
PGM Low-voltage ICSP programming pin
RB6/PGC  
RB6 Digital I/O, interrupt on change pin
PGC In-circuit debugger and ICSP programming pin
RB7/PGD  
RB7 Digital I/O, interrupt on change pin
PGD In-circuit debugger and ICSP programming pin

PORTB is controlled by three registers:

• Port data register — PORTB

• Port direction register — TRISB

• Port latch register — LATB

The general operation of PORTB is similar to that of PORTA. Figure 2.23 shows the architecture of PORTB. Each port pin has a weak internal pull-up which can be enabled by clearing bit RBPU of register INTCON2. These pull-ups are disabled on a power-on reset and when the port pin is configured as an output. On a power-on reset, PORTB pins are configured as digital inputs. Internal pull-ups allow input devices such as switches to be connected to PORTB pins without the use of external pull-up resistors. This saves costs because the component count and wiring requirements are reduced.

Figure 2.23: PIC18F452 PORTB RB4–RB7 pins

Port pins RB4–RB7 can be used as interrupt-on-change inputs, whereby a change on any of pins 4 through 7 causes an interrupt flag to be set. The interrupt enable and flag bits RBIE and RBIF are in register INTCON.

PORTC, PORTD, PORTE, and Beyond

In addition to PORTA and PORTB, the PIC18F452 has 8-bit bidirectional ports PORTC and PORTD, and 3-bit PORTE. Each port has its own data register (e.g., PORTC), data direction register (e.g., TRISC), and data latch register (e.g., LATC). The general operation of these ports is similar to that of PORTA.2.1.

In the PIC18F452 microcontroller PORTC is multiplexed with several peripheral functions as shown in Table 2.8. On a power-on reset, PORTC pins are configured as digital inputs.

Table 2.8: PIC18F452 PORTC pin functions

Pin Description
RC0/T1OSO/T1CKI  
RC0 Digital I/O
T1OSO Timer 1 oscillator output
T1CKI Timer 1/Timer 3 external clock input
RC1/T1OSI/CCP2  
RC1 Digital I/O
T1OSI Timer 1 oscillator input
CCP2 Capture 2 input, Compare 2 and PWM2 output
RC2/CCP1  
RC2 Digital I/O
CCP1 Capture 1 input, Compare 1 and PWM1 output
RC3/SCK/SCL  
RC3 Digital I/O
SCK Synchronous serial clock input/output for SPI
SCL Synchronous serial clock input/output for I²C
RC4/SDI/SDA  
RC4 Digital I/O
SDI SPI data in
SDA I²C data I/O
RC5/SDO  
RC5 Digital I/O
SDO SPI data output
RC6/TX/CK  
RC6 Digital I/O
TX USART transmit pin
CK USART synchronous clock pin
RC7/RX/DT  
RC7 Digital I/O
RX USART receive pin
DT USART synchronous data pin

In the PIC18F452 microcontroller, PORTD has Schmitt trigger input buffers. On a power-on reset, PORTD is configured as digital input. PORTD can be configured as an 8-bit parallel slave port (i.e., a microprocessor port) by setting bit 4 of the TRISE register. Table 2.9 shows functions of PORTD pins.

Table 2.9: PIC18F452 PORTD pin functions

Pin Description
RD0/PSP0  
RD0 Digital I/O
PSP0 Parallel slave port bit 0
RD1/PSP1  
RD1 Digital I/O
PSP1 Parallel slave port bit 1
RD2/PSP2  
RD2 Digital I/O
PSP2 Parallel slave port bit 2
RD3/PSP3  
RD3 Digital I/O
PSP3 Parallel slave port bit 3
RD4/PSP4  
RD4 Digital I/O
PSP4 Parallel slave port bit 4
RD5/PSP5  
RD5 Digital I/O
PSP5 Parallel slave port bit 5
RD6/PSP6  
RD6 Digital I/O
PSP6 Parallel slave port bit 6
RD7/PSP7  
RD7 Digital I/O
PSP7 Parallel slave port bit 7

In the PIC18F452 microcontroller, PORTE is only 3 bits wide. As shown in Table 2.10, port pins are shared with analog inputs and with parallel slave port read/write control bits. On a power-on reset, PORTE pins are configured as analog inputs and register ADCON1 must be programmed to change these pins to digital I/O.

Table 2.10: PIC18F452 PORTE pin functions

Pin Description
RE0/RD/AN5  
RE0 Digital I/O
RD Parallel slave port read control pin
AN5 Analog input 5
RE1/WR/AN6  
RE1 Digital I/O
WR Parallel slave port write control pin
AN6 Analog input 6
RE2/CS/AN7  
RE2 Digital I/O
CS Parallel slave port CS
AN7 Analog input 7

2.1.9 Timers

The PIC18F452 microcontroller has four programmable timers which can be used in many tasks, such as generating timing signals, causing interrupts to be generated at specific time intervals, measuring frequency and time intervals, and so on.

This section introduces the timers available in the PIC18F452 microcontroller.

Timer 0

Timer 0 is similar to the PIC16 series Timer 0, except that it can operate either in 8-bit or in 16-bit mode. Timer 0 has the following basic features:

• 8-bit or 16-bit operation

• 8-bit programmable prescaler

• External or internal clock source

• Interrupt generation on overflow

Timer 0 control register is T0CON, shown in Figure 2.24. The lower 6 bits of this register have similar functions to the PIC16-series OPTION register. The top two bits are used to select the 8-bit or 16-bit mode of operation and to enable/disable the timer.

Figure 2.24: Timer 0 control register, T0CON

Timer 0 can be operated either as a timer or as a counter. Timer mode is selected by clearing the T0CS bit, and in this mode the clock to the timer is derived from FOSC/4. Counter mode is selected by setting the T0CS bit, and in this mode Timer 0 is incremented on the rising or falling edge of input RA4/T0CKI. Bit T0SE of T0CON selects the edge triggering mode.

An 8-bit prescaler can be used to change the timer clock rate by a factor of up to 256. The prescaler is selected by bits PSA and T0PS2:T0PS0 of register T0CON.

8-Bit Mode Figure 2.25 shows Timer 0 in 8-bit mode. The following operations are normally carried out in a timer application:

• Clear T0CS to select clock FOSC/4

• Use bits T0PS2:T0PS0 to select a suitable prescaler value

• Clear PSA to select the prescaler

• Load timer register TMR0L

• Optionally enable Timer 0 interrupts

• The timer counts up and an interrupt is generated when the timer value overflows from FFH to 00H in 8-bit mode (or from FFFFH to 0000H in 16-bit mode)

Figure 2.25: Timer 0 in 8-bit mode

By loading a value into the TMR0 register we can control the count until an overflow occurs. The formula that follows can be used to calculate the time it will take for the timer to overflow (or to generate an interrupt) given the oscillator period, the value loaded into the timer, and the prescaler value:

Overflow time = 4 × TOSC × Prescaler × (256–TMR0)    (2.1)

where

 Overflow time is in ms

 TOSC is the oscillator period in μs

 Prescaler is the prescaler value

 TMR0 is the value loaded into TMR0 register

For example, assume that we are using a 4MHz crystal, and the prescaler is chosen as 1:8 by setting bits PS2:PS0 to 010. Also assume that the value loaded into the timer register TMR0 is decimal 100. The overflow time is then given by:

4MHZ clock has a period; T = 1/f = 0.25 μs

using the above formula

Overflow time = 4 × 0.25 × 8 × (256 – 100) = 1248 μs

Thus, the timer will overflow after 1.248 msec, and a timer interrupt will be generated if the timer interrupt and global interrupts are enabled.

What we normally want is to know what value to load into the TMR0 register for a required overflow time. This can be calculated by modifying Equation (2.1) as follows:

TMR0 = 256 – (Overflow time)/(4 × TOSC × Prescaler)    (2.2)

For example, suppose we want an interrupt to be generated after 500ms and the clock and the prescaler values are as before. The value to be loaded into the TMR0 register can be calculated using Equation (2.2) as follows:

TMR0 = 256 – 500/(4 × 0.25 × 8) = 193.5

The closest number we can load into TMR0 register is 193.

16-Bit Mode The Timer 0 in 16-bit mode is shown in Figure 2.26. Here, two timer registers named TMR0L and TMR0 are used to store the 16-bit timer value. The low byte TMR0L is directly loadable from the data bus. The high byte TMR0 can be loaded through a buffer called TMR0H. During a read of TMR0L, the high byte of the timer (TMR0) is also loaded into TMR0H, and thus all 16 bits of the timer value can be read. To read the 16-bit timer value, first we have to read TMR0L, and then read TMR0H in a later instruction. Similarly, during a write to TMR0L, the high byte of the timer is also updated with the contents of TMR0H, allowing all 16 bits to be written to the timer. Thus, to write to the timer the program should first write the required high byte to TMR0H. When the low byte is written to TMR0L, then the value stored in TMR0H is automatically transferred to TMR0, thus causing all 16 bits to be written to the timer.

Figure 2.26: Timer 0 in 16-bit mode

Timer 1

PIC18F452 Timer 1 is a 16-bit timer controlled by register T1CON, as shown in Figure 2.27. Figure 2.28 shows the internal structure of Timer 1.

Figure 2.27: Timer 1 control register, T1CON

Figure 2.28: Internal structure of Timer 1

Timer 1 can be operated as either a timer or a counter. When bit TMR1CS of register T1CON is low, clock FOSC/4 is selected for the timer. When TMR1CS is high, the module operates as a counter clocked from input T1OSI. A crystal oscillator circuit, enabled from bit T1OSCEN of T1CON, is built between pins T1OSI and T1OSO where a crystal up to 200KHz can be connected between these pins. This oscillator is primarily intended for a 32KHz crystal operation in real-time clock applications. A prescaler is used in Timer 1 that can change the timing rate as a factor of 1, 2, 4, or 8.

Timer 1 can be configured so that read/write can be performed either in 16-bit mode or in two 8-bit modes. Bit RD16 of register T1CON controls the mode. When RD16 is low, timer read and write operations are performed as two 8-bit operations. When RD16 is high, the timer read and write operations are as in Timer 0 16-bit mode (i.e., a buffer is used between the timer register and the data bus) (see Figure 2.29).

Figure 2.29: Timer 1 in 16-bit mode

If the Timer 1 interrupts are enabled, an interrupt will be generated when the timer value rolls over from FFFFH to 0000H.

Timer 2

Timer 2 is an 8-bit timer with the following features:

• 8-bit timer (TMR2)

• 8-bit period register (PR2)

• Programmable prescaler

• Programmable postscaler

• Interrupt when TM2 matches PR2

Timer 2 is controlled from register T2CON, as shown in Figure 2.30. Bits T2CKPS1:T2CKPS0 set the prescaler for a scaling of 1, 4, and 16. Bits TOUTPS3:TOUTPS0 set the postscaler for a scaling of 1:1 to 1:16. The timer can be turned on or off by setting or clearing bit TMR2ON.

Figure 2.30: Timer 2 control register, T2CON

The block diagram of Timer 2 is shown in Figure 2.31. Timer 2 can be used for the PWM mode of the CCP module. The output of Timer 2 can be software selected by the SSP module as a baud clock. Timer 2 increments from 00H until it matches PR2 and sets the interrupt flag. It then resets to 00H on the next cycle.

Figure 2.31: Timer 2 block diagram

Timer 3

The structure and operation of Timer 3 is the same as for Timer 1, having registers TMR3H and TMR3L. This timer is controlled from register T3CON as shown in Figure 2.32.

Figure 2.32: Timer 3 control register, T3CON

The block diagram of Timer 3 is shown in Figure 2.33.

Figure 2.33: Block diagram of Timer 3

2.1.10 Capture/Compare/PWM Modules (CCP)

The PIC18F452 microcontroller has two capture/compare/PWM (CCP) modules, and they work with Timers 1, 2, and 3 to provide capture, compare, and pulse width modulation (PWM) operations. Each module has two 8-bit registers. Module 1 registers are CCPR1L and CCPR1H, and module 2 registers are CCPR2L and CCPR2H. Together, each register pair forms a 16-bit register and can be used to capture, compare, or generate waveforms with a specified duty cycle. Module 1 is controlled by register CCP1CON, and module 2 is controlled by CCP2CON. Figure 2.34 shows the bit allocations of the CCP control registers.

Figure 2.34: CCPxCON register bit allocations

Capture Mode

In capture mode, the registers operate like a stopwatch. When an event occurs, the time of the event is recorded, although the clock continues running (a stopwatch, on the other hand, stops when the event time is recorded).

Figure 2.35 shows the capture mode of operation. Here, CCP1 will be considered, but the operation of CCP2 is identical with the register and port names changed accordingly. In this mode CCPR1H:CCPR1L captures the 16-bit value of the TMR1 or TMR3 registers when an event occurs on pin RC2/CCP1 (pin RC2/CCP1 must be configured as an input pin using TRISC). An external signal can be prescaled by 4 or 16. The event is selected by control bits CCP1M3:CCP1M0, and any of the following events can be selected:

• Every falling edge

• Every rising edge

• Every fourth rising edge

• Every sixteenth rising edge

Figure 2.35: Capture mode of operation

If the capture interrupt is enabled, the occurrence of an event causes an interrupt to be generated in software. If another capture occurs before the value in register CCPR1 is read, the old captured value is overwritten by the new captured value.

Either Timer 1 or Timer 3 can be used in capture mode. They must be running in timer mode, or in synchronized counter mode, selected by register T3CON.

Compare Mode

In compare mode, a digital comparator is used to compare the value of Timer 1 or Timer 3 to the value in a 16-bit register pair. When a match occurs, the output state of a pin is changed. Figure 2.36 shows the block diagram of compare mode in operation.

Figure 2.36: Compare mode of operation

Here only module CCP1 is considered, but the operation of module CCP2 is identical.

The value of the 16-bit register pair CCPR1H:CCPR1L is continuously compared against the Timer 1 or Timer 3 value. When a match occurs, the state of the RC2/CCP1 pin is changed depending on the programming of bits CCP1M2:CCP1M0 of register CCP1CON. The following changes can be programmed:

• Force RC2/CCP1 high

• Force RC2/CCP1 low

• Toggle RC2/CCP1 pin (low to high or high to low)

• Generate interrupt when a match occurs

• No change

Timer 1 or Timer 3 must be running in timer mode or in synchronized counter mode, selected by register T3CON.

PWM Module

The pulse width modulation (PWM) mode produces a PWM output at 10-bit resolution. A PWM output is basically a square waveform with a specified period and duty cycle. Figure 2.37 shows a typical PWM waveform.

Figure 2.37: Typical PWM waveform

Figure 2.38 shows the PWM module block diagram. The module is controlled by Timer 2. The PWM period is given by:

PWM period = (PR2 + 1) * TMR2PS * 4 * TOSC    (2.3)

or

     (2.4)

where

 PR2 is the value loaded into Timer 2 register

 TMR2PS is the Timer 2 prescaler value

 TOSC is the clock oscillator period (seconds)

 The PWM frequency is defined as 1/(PWM period).

Figure 2.38: PWM module block diagram

The resolution of the PWM duty cycle is 10 bits. The PWM duty cycle is selected by writing the eight most significant bits into the CCPR1L register and the two least significant bits into bits 4 and 5 of CCP1CON register. The duty cycle (in seconds) is given by:

PWM duty cycle = (CCPR1L:CCP1CON<5:4>) * TMR2PS * TOSC    (2.5)

or

     (2.6)

The steps to configure the PWM are as follows:

• Specify the required period and duty cycle.

• Choose a value for the Timer 2 prescaler (TMR2PS).

• Calculate the value to be written into the PR2 register using Equation (2.2).

• Calculate the value to be loaded into the CCPR1L and CCP1CON registers using Equation (2.6).

• Clear bit 2 of TRISC to make CCP1 pin an output pin.

• Configure the CCP1 module for PWM operation using register CCP1CON.

The following example shows how the PWM can be set up.

Example 2.1

PWM pulses must be generated from pin CCP1 of a PIC18F452 microcontroller. The required pulse period is 44ms and the required duty cycle is 50%. Assuming that the microcontroller operates with a 4MHz crystal, calculate the values to be loaded into the various registers.

Solution 2.1

Using a 4MHz crystal; TOSC = 1/4 = 0.25 × 10–6

The required PWM duty cycle is 44/2 = 22μs.

From Equation (2.4), assuming a timer prescaler factor of 4, we have:

 

or

     i.e., 0AH

and from Equation (2.6)

 

or

 

But the equivalent of number 22 in 10-bit binary is:

“00 00010110”

Therefore, the value to be loaded into bits 4 and 5 of CCP1CON is “00.” Bits 2 and 3 of CCP1CON must be set to high for PWM operation. Therefore, CCP1CON must be set to bit pattern (“X” is “don’t care”):

XX001100

Taking the don’t-care entries as 0, we can set CCP1CON to hexadecimal 0CH.

The value to be loaded into CCPR1L is “00010110” (i.e., hexadecimal number 16H). The required steps are summarized as follows:

• Load Timer 2 with prescaler of 4 (i.e., load T2CON) with 00000101 (i.e., 05H).

• Load 0AH into PR2.

• Load 16H into CCPR1L.

• Load 0 into TRISC (make CCP1 pin output).

• Load 0CH into CCP1CON.

One period of the generated PWM waveform is shown in Figure 2.39.

Figure 2.39: Generated PWM waveform

2.1.11 Analog-to-Digital Converter (A/D) Module

An analog-to-digital converter (A/D) is another important peripheral component of a microcontroller. The A/D converts an analog input voltage into a digital number so it can be processed by a microcontroller or any other digital system. There are many analog-to-digital converter chips available on the market, and an embedded systems designer should understand the characteristics of such chips so they can be used efficiently.

As far as the input and output voltage are concerned A/D converters can be classified as either unipolar and bipolar. Unipolar A/D converters accept unipolar input voltages in the range 0 to +0V, and bipolar A/D converters accept bipolar input voltages in the range ±V. Bipolar converters are frequently used in signal processing applications, where the signals by nature are bipolar. Unipolar converters are usually cheaper, and they are used in many control and instrumentation applications.

Figure 2.40 shows the typical steps involved in reading and converting an analog signal into digital form, a process also known as signal conditioning. Signals received from sensors usually need to be processed before being fed to an A/D converter. This processing usually begins with scaling the signal to the correct value. Unwanted signal components are then removed by filtering the signal using classical filters (e.g., a low-pass filter). Finally, before feeding the signal to an A/D converter, the signal is passed through a sample-and-hold device. This is particularly important with fast real-time signals whose value may be changing between the sampling instants. A sample-and-hold device ensures that the signal stays at a constant value during the actual conversion process. Many applications required more than one A/D, which normally involves using an analog multiplexer at the input of the A/D. The multiplexer selects only one signal at any time and presents this signal to the A/D converter. An A/D converter usually has a single analog input and a digital parallel output. The conversion process is as follows:

• Apply the processed signal to the A/D input

• Start the conversion

• Wait until conversion is complete

• Read the converted digital data

Figure 2.40: Signal conditioning and A/D conversion process

The A/D conversion starts by triggering the converter. Depending on the speed of the converter, the conversion process itself can take several microseconds. At the end of the conversion, the converter either raises a flag or generates an interrupt to indicate that the conversion is complete. The converted parallel output data can then be read by the digital device connected to the A/D converter.

Most members of the PIC18F family contain a 10-bit A/D converter. If the chosen voltage reference is +5V, the voltage step value is:

  

Therefore, for example, if the input voltage is 1.0V, the converter will generate a digital output of 1.0/0.00489 = 205 decimal. Similarly, if the input voltage is 3.0V, the converter will generate 3.0/0.00489 = 613.

The A/D converter used by the PIC18F452 microcontroller has eight channels, named AN0–AN7, which are shared by the PORTA and PORTE pins. Figure 2.41 shows the block diagram of the A/D converter.

Figure 2.41: Block diagram of the PIC18F452 A/D converter

The A/D converter has four registers. Registers ADRESH and ADRESL store the higher and lower results of the conversion respectively. Register ADCON0, shown in Figure 2.42, controls the operation of the A/D module, such as selecting the conversion clock together with register ADCON1, selecting an input channel, starting a conversion, and powering up and shutting down the A/D converter.

Figure 2.42: ADCON0 register

Register ADCON1 (see Figure 2.43) is used for selecting the conversion format, configuring the A/D channels for analog input, selecting the reference voltage, and selecting the conversion clock together with register ADCON0.

Figure 2.43: ADCON1 register

A/D conversion starts by setting the GO/DONE bit of ADCON0. When the conversion is complete, the 2 bits of the converted data is written into register ADRESH, and the remaining 8 bits are written into register ADRESL. At the same time the GO/DONE bit is cleared to indicate the end of conversion. If required, interrupts can be enabled so that a software interrupt is generated when the conversion is complete.

The steps in carrying out an A/D conversion are as follows:

• Use ADCON1 to configure required channels as analog and configure the reference voltage.

• Set the TRISA or TRISE bits so the required channel is an input port.

• Use ADCON0 to select the required analog input channel.

• Use ADCON0 and ADCON1 to select the conversion clock.

• Use ADCON0 to turn on the A/D module.

• Configure the A/D interrupt (if desired).

• Set the GO/DONE bit to start conversion.

• Wait until the GO/DONE bit is cleared, or until a conversion complete interrupt is generated.

• Read the converted data from ADRESH and ADRESL.

• Repeat these steps as required.

For correct A/D conversion, the A/D conversion clock must be selected to ensure a minimum bit conversion time of 1.6μs. Table 2.11 gives the recommended A/D clock sources for various microcontroller operating frequencies. For example, if the microcontroller is operated from a 10MHz clock, the A/D clock source should be FOSC/16 or higher (e.g., FOSC/32).

Table 2.11: A/D conversion clock selection

A/D clock source  
Operation ADCS2:ADCS0 Maximum microcontroller frequency
2 TOSC 000 1.25 MHz
4 TOSC 100 2.50 MHz
8 TOSC 001 5.0 MHz
16 TOSC 101 10.0 MHz
32 TOSC 010 20.0 MHz
64 TOSC 110 40.0 MHz
RC 011

Bit ADFM of register ADCON1 controls the format of a conversion. When ADFM is cleared, the 10-bit result is left justified (see Figure 2.44) and lower 6 bits of ADRESL are cleared to 0. When ADFM is set to 1 the result is right justified and the upper 6 bits of ADRESH are cleared to 0. This is the mode most commonly used, in which ADRESL contains the lower 8 bits, and bits 0 and 1 of ADRESH contain the upper 2 bits of the 10-bit result.

Figure 2.44: Formatting the A/D conversion result

Analog Input Model and Acquisition Time

An understanding of the A/D analog input model is necessary to interface the A/D to external devices. Figure 2.45 shows the analog input model of the A/D. The analog input voltage VAIN and the source resistance RS are shown on the left side of the diagram. It is recommended that the source resistance be no greater than 2.5K. The analog signal is applied to the pin labeled ANx. There is a small capacitance (5pF) and a leakage current to the ground of approximately 500nA. RIC is the interconnect resistance, which has a value of less than 1K. The sampling process is shown with switch SS having a resistance RSS whose value depends on the voltage as shown in the small graph at the bottom of Figure 2.45. The value of RSS is approximately 7K at 5V supply voltage.

Figure 2.45: Analog input model of the A/D converter

The A/D converter is based on a switched capacitor principle, and capacitor CHOLD shown in Figure 2.45 must be charged fully before the start of a conversion. This is a 120pF capacitor which is disconnected from the input pin once the conversion is started.

The acquisition time can be calculated by using Equation (2.7), provided by Microchip Inc:

TACQ = Amplifier settling time + Holding capacitor charging time + temperature coefficient    (2.7)

The amplifier settling time is specified as a fixed 2μs. The temperature coefficient, which is only applicable if the temperature is above 25°C, is specified as:

Temperature coefficient = (Temperature – 25°C)(0.05μs/°C)    (2.8)

Equation (2.8) shows that the effect of the temperature is very small, creating about 0.5μs delay for every 10°C above 25°C. Thus, assuming a working environment between 25°C and 35°C, the maximum delay due to temperature will be 0.5μs, which can be ignored for most practical applications.

The holding capacitor charging time as specified by Microchip Inc is:

Holding capacitor charging time = –(120pF)(1K + RSS + RS)Ln(1/2048)    (2.9)

Assuming that RSS = 7K, RS = 2.5K, Equation (2.9) gives the holding capacitor charging time as 9.6μs.

The acquisition time is then calculated as:

TACQ = 2 + 9.6 + 0.5 = 12.1μs

A full 10-bit conversion takes 12 A/D cycles, and each A/D cycle is specified at a minimum of 1.6μs. Thus, the fastest conversion time is 19.2μs. Adding this to the best possible acquisition time gives a total time to complete a conversion of 19.2+12.1=31.3μs.

When a conversion is complete, it is specified that the converter should wait for two conversion periods before starting a new conversion. This corresponds to 2×1.6=3.2μs. Adding this to the best possible conversion time of 31.3μs gives a complete conversion time of 34.5μs. Assuming the A/D converter is used successively, and ignoring the software overheads, this implies a maximum sampling frequency of about 29KHz.

2.1.12 Interrupts

An interrupt is an event that requires the CPU to stop normal program execution and then execute a program code related to the event causing the interrupt. Interrupts can be generated internally (by some event inside the chip) or externally (by some external event). An example of an internal interrupt is a timer overflowing or the A/D completing a conversion. An example of an external interrupt is an I/O pin changing state.

Interrupts can be useful in many applications such as:

• Time critical applications. Applications which require the immediate attention of the CPU can use interrupts. For example, in an emergency such as a power failure or fire in a plant the CPU may have to shut down the system immediately in an orderly manner. In such applications an external interrupt can force the CPU to stop whatever it is doing and take immediate action.

• Performing routine tasks. Many applications require the CPU to perform routine work at precise times, such as checking the state of a peripheral device exactly every millisecond. A timer interrupt scheduled with the required timing can divert the CPU from normal program execution to accomplish the task at the precise time required.

• Task switching in multi-tasking applications. In multi-tasking applications, each task may have a finite time to execute its code. Interrupt mechanisms can be used to stop a task should it consume more than its allocated time.

• To service peripheral devices quickly. Some applications may need to know when a task, such as an A/D conversion, is completed. This can be accomplished by continuously checking the completion flag of the A/D converter. A more elegant solution would be to enable the A/D completion interrupt so the CPU is forced to read the converted data as soon as it becomes available.

The PIC18F452 microcontroller has both core and peripheral interrupt sources. The core interrupt sources are:

• External edge-triggered interrupt on INT0, INT1, and INT2 pins.

• PORTB pins change interrupts (any one of the RB4–RB7 pins changing state)

• Timer 0 overflow interrupt

The peripheral interrupt sources are:

• Parallel slave port read/write interrupt

• A/D conversion complete interrupt

• USART receive interrupt

• USART transmit interrupt

• Synchronous serial port interrupt

• CCP1 interrupt

• TMR1 overflow interrupt

• TMR2 overflow interrupt

• Comparator interrupt

• EEPROM/FLASH write interrupt

• Bus collision interrupt

• Low-voltage detect interrupt

• Timer 3 overflow interrupt

• CCP2 interrupt

Interrupts in the PIC18F family can be divided into two groups: high priority and low priority. Applications that require more attention can be placed in the higher priority group. A high-priority interrupt can stop a low-priority interrupt that is in progress and gain access to the CPU. However, high-priority interrupts cannot be stopped by low-priority interrupts. If the application does not need to set priorities for interrupts, the user can choose to disable the priority scheme so all interrupts are at the same priority level. High-priority interrupts are vectored to address 00008H and low-priority ones to address 000018H of the program memory. Normally, a user program code (interrupt service routine, ISR) should be at the interrupt vector address to service the interrupting device.

In the PIC18F452 microcontroller there are ten registers that control interrupt operations. These are:

• RCON

• INTCON

• INTCON2

• INTCON3

• PIR1, PIR2

• PIE1, PIE2

• IPR1, IPR2

Every interrupt source (except INT0) has three bits to control its operation. These bits are:

• A flag bit to indicate whether an interrupt has occurred. This bit has a name ending in …IF

• An interrupt enable bit to enable or disable the interrupt source. This bit has the name ending in …IE

• A priority bit to select high or low priority. This bit has a name ending in …IP

RCON Register

The top bit of the RCON register, called IPEN, is used to enable the interrupt priority scheme. When IPEN = 0, interrupt priority levels are disabled and the microcontroller interrupt structure is similar to that of the PIC16 series. When IPEN = 1, interrupt priority levels are enabled. Figure 2.46 shows the bits of register RCON.

Figure 2.46: RCON register bits

Enabling/Disabling Interrupts — No Priority Structure

When the IPEN bit is cleared, the priority feature is disabled. All interrupts branch to address 00008H of the program memory. In this mode, bit PEIE of register INTCON enables/disables all peripheral interrupt sources. Similarly, bit GIE of INTCON enables/disables all interrupt sources. Figure 2.47 shows the bits of register INTCON.

Figure 2.47: INTCON register bits

For an interrupt to be accepted by the CPU the following conditions must be satisfied:

• The interrupt enable bit of the interrupt source must be enabled. For example, if the interrupt source is external interrupt pin INT0, then bit INT0IE of register INTCON must be set to 1.

• The interrupt flag of the interrupt source must be cleared. For example, if the interrupt source is external interrupt pin INT0, then bit INT0IF of register INTCON must be cleared to 0.

• The peripheral interrupt enable/disable bit PEIE of INTCON must be set to 1 if the interrupt source is a peripheral.

• The global interrupt enable/disable bit GIE of INTCON must be set to 1.

With an external interrupt source we normally have to define whether the interrupt should occur on the low-to-high or high-to-low transition of the interrupt source. With INT0 interrupts, for example, this is done by setting/clearing bit INTEDG0 of register INTCON2.

When an interrupt occurs, the CPU stops its normal flow of execution, pushes the return address onto the stack, and jumps to address 00008H in the program memory where the user interrupt service routine program resides. Once the CPU is in the interrupt service routine, the global interrupt enable bit (GIE) is cleared to disable further interrupts. When multiple interrupt sources are enabled, the source of the interrupt can be determined by polling the interrupt flag bits. The interrupt flag bits must be cleared in the software before reenabling interrupts to avoid recursive interrupts. When the CPU has returned from the interrupt service routine, the global interrupt bit GIE is automatically set by the software.

Enabling/Disabling Interrupts — Priority Structure

When the IPEN bit is set to 1, the priority feature is enabled and the interrupts are grouped into two: low priority and high priority. Low-priority interrupts branch to address 00008H and high-priority interrupts branch to address 000018H of the program memory. Setting the priority bit makes the interrupt source a high-priority interrupt, and clearing this bit makes the interrupt source a low-priority interrupt.

Setting the GIEH bit of INTCON enables all high-priority interrupts that have the priority bit set. Similarly, setting the GIEL bit of INTCON enables all low-priority interrupts (the priority is bit cleared).

For a high-priority interrupt to be accepted by the CPU, the following conditions must be satisfied:

• The interrupt enable bit of the interrupt source must be enabled. For example, if the interrupt source is external interrupt pin INT1, then bit INT1IE of register INTCON3 must be set to 1.

• The interrupt flag of the interrupt source must be cleared. For example, if the interrupt source is external interrupt pin INT1, then bit INT1IF of register INTCON3 must be cleared to 0.

• The priority bit must be set to 1. For example, if the interrupt source is external interrupt INT1, then bit INT1P of register INTCON3 must be set to 1.

• The global interrupt enable/disable bit GIEH of INTCON must be set to 1.

For a low-priority interrupt to be accepted by the CPU, the following conditions must be satisfied:

• The interrupt enable bit of the interrupt source must be enabled. For example, if the interrupt source is external interrupt pin INT1, then bit INT1IE of register INTCON3 must be set to 1.

• The interrupt flag of the interrupt source must be cleared. For example, if the interrupt source is external interrupt pin INT1, then bit INT1IF of register INTCON3 must be cleared to 0.

• The priority bit must be cleared to 0. For example, if the interrupt source is external interrupt INT1, then bit INT1P of register INTCON3 must be cleared to 0.

• Low-priority interrupts must be enabled by setting bit GIEL of INTCON to 1.

• The global interrupt enable/disable bit GIEH of INTCON must be set to 1.

Table 2.12 gives a listing of the PIC18F452 microcontroller interrupt bit names and register names for every interrupt source.

Table 2.12: PIC18F452 interrupt bits and registers

Interrupt source Flag bit Enable bit Priority bit
INT0 external INT0IF INT0IE
INT1 external INT1IF INT1IE INT1IP
INT2 external INT2IF INT2IE INT2IP
RB port change RBIF RBIE RBIP
TMR0 overflow TMR0IF TMR0IE TMR0IP
TMR1 overflow TMR1IF TMR1IE TMR1IP
TMR2 match PR2 TMR2IF TMR2IE TMR2IP
TMR3 overflow TMR3IF TMR3IE TMR3IP
A/D complete ADIF ADIE ADIP
CCP1 CCP1IF CCP1IE CCP1IP
CCP2 CCP2IF CCP2IE CCP2IP
USART RCV RCIF RCIE RCIP
USART TX TXIF TXIE TXIP
Parallel slave port PSPIF PSPIE PSPIP
Sync serial port SSPIF SSPIE SSPIP
Low-voltage detect LVDIF LVDIE LVDIP
Bus collision BCLIF BCLIE BCLIP
EEPROM/FLASH write EEIF EEIE EEIP

Figures 2.48 to 2.55 show the bit definitions of interrupt registers INTCON2, INTCON3, PIR1, PIR2, PIE1, PIE2, IPR1, and IPR2.

Figure 2.48: INTCON2 bit definitions

Figure 2.49: INTCON3 bit definitions

Figure 2.50: PIR1 bit definitions

Figure 2.51: PIR2 bit definitions

Figure 2.52: PIE1 bit definitions

Figure 2.53: PIE2 bit definitions

Figure 2.54: IPR1 bit definitions

Figure 2.55: IPR2 bit definitions

Examples are given in this section to illustrate how the CPU can be programmed for an interrupt.

Example 2.2

Set up INT1 as a falling-edge triggered interrupt input having low priority.

Solution 2.2

The following bits should be set up before the INT1 falling-edge triggered interrupts can be accepted by the CPU in low-priority mode:

• Enable the priority structure. Set IPEN = 1

• Make INT1 an input pin. Set TRISB = 1

• Set INT1 interrupts for falling edge. SET INTEDG1 = 0

• Enable INT1 interrupts. Set INT1IE = 1

• Enable low priority. Set INT1IP = 0

• Clear INT1 flag. Set INT1IF = 0

• Enable low-priority interrupts. Set GIEL = 1

• Enable all interrupts. Set GIEH = 1

When an interrupt occurs, the CPU jumps to address 00008H in the program memory to execute the user program at the interrupt service routine.

Example 2.3

Set up INT1 as a rising-edge triggered interrupt input having high priority.

Solution 2.3

The following bits should be set up before the INT1 rising-edge triggered interrupts can be accepted by the CPU in high-priority mode:

• Enable the priority structure. Set IPEN = 1

• Make INT1 an input pin. Set TRISB = 1

• Set INT1 interrupts for rising edge. SET INTEDG1 = 1

• Enable INT1 interrupts. Set INT1IE = 1

• Enable high priority. Set INT1IP = 1

• Clear INT1 flag. Set INT1IF = 0

• Enable all interrupts. Set GIEH = 1

When an interrupt occurs, the CPU jumps to address 000018H of the program memory to execute the user program at the interrupt service routine.

2.2 Summary

This chapter has described the architecture of the PIC18F family of microcontrollers. The PIC18F452 was used as a typical sample microcontroller in this family. Other members of the same family, such as the PIC18F242, have smaller pin counts and less functionality. And some, such as the PIC18F6680, have larger pin counts and more functionality.

Important parts and peripheral circuits of the PIC18F series have been described, including data memory, program memory, clock circuits, reset circuits, watchdog timer, general purpose timers, capture and compare module, PWM module, A/D converter, and the interrupt structure.

2.3 Exercises

1. Describe the data memory structure of the PIC18F452 microcontroller. What is a bank? How many banks are there?

2. Explain the differences between a general purpose register (GPR) and a special function register (SFR).

3. Explain the various ways the PIC18F microcontroller can be reset. Draw a circuit diagram to show how an external push-button switch can be used to reset the microcontroller.

4. Describe the various clock sources that can be used to provide a clock to a PIC18F452 microcontroller. Draw a circuit diagram to show how a 10MHz crystal can be connected to the microcontroller.

5. Draw a circuit diagram to show how a resonator can be connected to a PIC18F microcontroller.

6. In a non-time-critical application a clock must be provided for a PIC18F452 microcontroller using an external resistor and a capacitor. Draw a circuit diagram to show how this can be done and find the component values for a required clock frequency of 5MHz.

7. Explain how an external clock can provide clock pulses to a PIC18F microcontroller.

8. What are the registers of PORTA? Explain the operation of the port by drawing the port block diagram.

9. The watchdog timer must be set to provide an automatic reset every 0.5 seconds. Describe how to do this, including the appropriate register bits.

10. PWM pulses must be generated from pin CCP1 of a PIC18F452 microcontroller. The required pulse period is 100ms, and the required duty cycle is 50%. Assuming the microcontroller is operating with a 4MHz crystal, calculate the values to be loaded into the various registers.

11. Again, with regard to PWM pulses generated from pin CCP1 of a PIC18F452 microcontroller: If the required pulse frequency is 40KHz, and the required duty cycle is 50%, and assuming the microcontroller is operating with a 4MHz crystal, calculate the values to be loaded into the various registers.

12. An LM35DZ-type analog temperature sensor is connected to analog port AN0 of a PIC18F452 microcontroller. The sensor provides an analog output voltage proportional to the temperature (i.e., V0 = 10 mV/°C). Show the steps required to read the temperature.

13. Explain the difference between a priority interrupt and a nonpriority interrupt.

14. Show the steps required to set up INT2 as a falling-edge triggered interrupt input having low priority. What is the interrupt vector address?

15. Show the steps required to set up both INT1 and INT2 as falling-edge triggered interrupt inputs having low priority.

16. Show the steps required to set up INT1 as falling-edge triggered and INT2 as rising-edge triggered interrupt inputs having high priorities. Explain how to find the source of the interrupt when an interrupt occurs.

17. Show the steps required to set up Timer 0 to generate interrupts every millisecond with a high priority. What is the interrupt vector address?

18. In an application the CPU registers have been configured to accept interrupts from external sources INT0, INT1, and INT2. An interrupt has been detected. Explain how to find the source of the interrupt.

CHAPTER 3

C Programming Language

There are several C compilers on the market for the PIC18 series of microcontrollers. These compilers have many similar features, and they can all be used to develop C-based high-level programs for PIC18 microcontrollers.

Some of the C compilers used most often in commercial, industrial, and educational PIC18 microcontroller applications are:

• mikroC

• PICC18

• C18

• CCS

The popular and powerful mikroC, developed by MikroElektronika (web site: www.microe.com), is easy to learn and comes with rich resources, such as a large number of library functions and an integrated development environment with a built-in simulator and an in-circuit debugger (e.g., mikroICD). A demo version of the compiler with a 2K program limit is available from MikroElektronika.

PICC18, another popular C compiler, was developed by Hi-Tech Software (web site: www.htsoft.com) and is available in two versions: standard and professional. A powerful simulator and an integrated development environment (Hi-Tide) are provided by the company. PICC18 is supported by the PROTEUS simulator (www.labcenter.co.uk) which can be used to simulate PIC microcontroller–based systems. A limited-period demo version of this compiler is available on the developer’s web site.

C18 is a product of Microchip Inc. (web site: www.microchip.com). A limited-period demo version, as well as a limited functionality version of C18 with no time limit, are available from the Microchip web site. C18 includes a simulator and supports hardware and software development tools such as in-circuit emulators (e.g., ICE2000) and in-circuit debuggers (e.g., ICD2).

CCS has been developed by the Custom Computer Systems Inc. (web site: www. ccsinfo.com). The company offers a limited-period demo version of their compiler. CCS provides a large number of built-in functions and supports an in-circuit debugger (e.g., ICD-U40) which are very helpful in the development of PIC18 microcontroller–based systems. In this book we are mainly concentrating on the use of the mikroC compiler, and most of the projects are based on this compiler.

3.1 Structure of a mikroC Program

Figure 3.1 shows the simplest structure of a mikroC program. This program flashes an LED connected to port RB0 (bit 0 of PORTB) of a PIC microcontroller in one-second intervals. Do not worry if you don’t understand the operation of the program at this stage, as all will come clear as this chapter progresses. Some of the programming elements in Figure 3.1 are described in detail here.

/********************************************************************

                        LED FLASHING PROGRAM

                 *********************************

This program flashes an LED connected to port pin RB0 of PORTB with

 one second intervals.

Programmer : D. Ibrahim

File       : LED.C

Date       : May, 2007

Micro      : PIC18F452

**********************************************************************/

void main() {

 for(;;) // Endless loop

 {

  TRISB = 0; // Configure PORTB as output

  PORTB.0 = 0; // RB0 = 0

  Delay_Ms(1000); // Wait 1 second

  PORTB.0 = 1; // RB0 = 1

  Delay_Ms(1000); // Wait 1 second

 } // End of loop

}

Figure 3.1: Structure of a simple C program

3.1.1 Comments

Comments are used to clarify the operation of the program or a programming statement. Comment lines are ignored and not compiled by the compiler. In mikroC programs comments can be of two types: long comments, extending several lines, and short comments, occupying only a single line. Comment lines at the beginning of a program can describe briefly the program’s operation and provide the author’s name, the program filename, the date the program was written, and a list of version numbers, together with the modifications in each version. As shown in Figure 3.1, comments can also be added after statements to describe the operations that the statements perform. Clear and succinct comment lines are important for the maintenance and thus the lifetime of a program, as a program with good comments is easier to modify and/or update.

As shown in Figure 3.1, long comments start with the character “/*” and terminate with the character “*/”. Similarly, short comments start with the character “//” and do not need a terminating character.

3.1.2 Beginning and Ending of a Program

In C language, a program begins with the keywords:

void main()

After this, a curly opening bracket is used to indicate the beginning of the program body. The program is terminated with a closing curly bracket. Thus, as shown in Figure 3.1, the program has the following structure:

void main() {

 program body

}

3.1.3 Terminating Program Statements

In C language, all program statements must be terminated with the semicolon (“;”) character; otherwise a compiler error will be generated:

j = 5; // correct

j = 5 // error

3.1.4 White Spaces

White spaces are spaces, blanks, tabs, and newline characters. The C compiler ignores all white spaces. Thus, the following three sequences are identical:

int i;  char j;

or

int i;

char j;

or

int i;

      char j;

Similarly, the following sequences are identical:

i = j + 2;

or

i = j

     + 2;

3.1.5 Case Sensitivity

In general, C language is case sensitive and variables with lowercase names are different from those with uppercase names. Currently, however, mikroC variables are not case sensitive (although future releases of mikroC may offer case sensitivity) so the following variables are equivalent:

total TOTAL Total ToTal total totaL

The only exception is the identifiers main and interrupt, which must be written in lowercase in mikroC. In this book we are assuming that the variables are case sensitive, for the sake of compatibility with other C compilers, and variables with the same name but different cases are not used.

3.1.6 Variable Names

In C language, variable names can begin with an alphabetical character or with the underscore character. In essence, variable names can include any of the characters a to z and A to Z, the digits 0 to 9, and the underscore character “_”. Each variable name should be unique within the first 31 characters of its name. Variable names can contain uppercase and lowercase characters (see Section 3.1.5), and numeric characters can be used inside a variable name. Examples of valid variable names are:

Sum count sum100 counter i1 UserName

_myName

Some names are reserved for the compiler itself and cannot be used as variable names in a program. Table 3.1 gives a list of these reserved names.

Table 3.1: mikroC reserved names

asm enum signed
auto extern sizeof
break float static
case for struct
char goto switch
const if typedef
continue int union
default long unsigned
do register void
double return volatile
else short while

3.1.7 Variable Types

The mikroC language supports the variable types shown in Table 3.2. Examples of variables are given in this section.

Table 3.2: mikroC variable types

Type Size (bits) Range
unsigned char 8 0 to 255
unsigned short int 8 0 to 255
unsigned int 16 0 to 65535
unsigned long int 32 0 to 4294967295
signed char 8 –128 to 127
signed short int 8 –128 to 127
signed int 16 –32768 to 32767
signed long int 32 –2147483648 to 2147483647
float 32 ±1.17549435082E-38 to ±6.80564774407E38
double 32 ±1.17549435082E-38 to ±6.80564774407E38
long double 32 ±1.17549435082E-38 to ±6.80564774407E38

(unsigned) char or unsigned short (int)

The variables (unsigned) char, or unsigned short (int), are 8-bit unsigned variables with a range of 0 to 255. In the following example two 8-bit variables named total and sum are created, and sum is assigned decimal value 150:

unsigned char total, sum;

sum = 150;

or

char total, sum;

sum = 150;

Variables can be assigned values during their declaration. Thus, the above statements can also be written as:

char total, sum = 150;

signed char or (signed) short (int)

The variables signed char, or (signed) short (int), are 8-bit signed character variables with a range of –128 to +127. In the following example a signed 8-bit variable named counter is created with a value of –50:

signed char counter = -50;

or

short counter = -50;

or

short int counter = -50;

(signed) int

Variables called (signed) int are 16-bit variables with a range –32768 to +32767. In the following example a signed integer named Big is created:

int Big;

unsigned (int)

Variables called (unsigned) int are 16-bit unsigned variables with a range 0 to 65535. In the following example an unsigned 16-bit variable named count is created and is assigned value 12000:

unsigned int count = 12000;

(signed) long (int)

Variables called (signed) long (int) are 32 bits long with a range –2147483648 to +2147483647. An example is:

signed long LargeNumber;

unsigned long (int)

Variables called (unsigned) long (int) are 32-bit unsigned variables having the range 0 to 4294967295. An example is:

unsigned long VeryLargeNumber;

float or double or long double

The variables called float or double or long double, are floating point variables implemented in mikroC using the Microchip AN575 32-bit format, which is IEEE 754 compliant. Floating point numbers range from ±1.17549435082E-38 to ±6.80564774407E38. In the following example, a floating point variable named area is created and assigned the value 12.235:

float area;

area = 12.235;

To avoid confusion during program development, specifying the sign of the variable (signed or unsigned) as well as the type of variable is recommended. For example, use unsigned char instead of char only, and unsigned int instead of unsigned only.

In this book we are using the following mikroC data types, which are easy to remember and also compatible with most other C compilers:

unsigned char 0 to 255
signed char –128 to 127
unsigned int 0 to 65535
signed int –32768 to 32767
unsigned long 0 to 4294967295
signed long –2147483648 to 2147483647
float ±1.17549435082E-38 to ±6.80564774407E38

3.1.8 Constants

Constants represent fixed values (numeric or character) in programs that cannot be changed. Constants are stored in the flash program memory of the PIC microcontroller, thus not wasting valuable and limited RAM memory. In mikroC, constants can be integers, floating points, characters, strings, or enumerated types.

Integer Constants

Integer constants can be decimal, hexadecimal, octal, or binary. The data type of a constant is derived by the compiler from its value. But suffixes can be used to change the type of a constant.

In Table 3.2 we saw that decimal constants can have values from –2147483648 to +4294967295. For example, constant number 210 is stored as an unsigned char (or unsigned short int). Similarly, constant number –200 is stored as a signed int.

Using the suffix u or U forces the constant to be unsigned. Using the suffix L or l forces the constant to be long. Using both U (or u) and L (or l) forces the constant to be unsigned long.

Constants are declared using the keyword const and are stored in the flash program memory of the PIC microcontroller, thus not wasting valuable RAM space. In the following example, constant MAX is declared as 100 and is stored in the flash program memory of the PIC microcontroller:

const MAX = 100;

Hexadecimal constants start with characters 0x or 0X and may contain numeric data 0 to 9 and hexadecimal characters A to F. In the following example, constant TOTAL is given the hexadecimal value FF:

const TOTAL = 0xFF;

Octal constants have a zero at the beginning of the number and may contain numeric data 0 to 7. In the following example, constant CNT is given octal value 17:

const CNT = 017;

Binary constant numbers start with 0b or 0B and may contain only 0 or 1. In the following example a constant named Min is declared as having the binary value 11110000:

const Min = 0b11110000

Floating Point Constants

Floating point constant numbers have integer parts, a dot, a fractional part, and an optional e or E followed by a signed integer exponent. In the following example, a constant named TEMP is declared as having the fractional value 37.50:

const TEMP = 37.50

or

const TEMP = 3.750E1

Character Constants

A character constant is a character enclosed within single quote marks. In the following example, a constant named First_Alpha is declared as having the character value “A”:

const First_Alpha = 'A';

String Constants

String constants are fixed sequences of characters stored in the flash memory of the microcontroller. The string must both begin and terminate with a double quote character (“). The compiler automatically inserts a null character as a terminator. An example string constant is:

"This is an example string constant"

A string constant can be extended across a line boundary by using a backslash character (“\”):

"This is first part of the string \

and this is the continuation of the string"

This string constant declaration is the same as:

"This is first part of the string and this is the continuation of the string"

Enumerated Constants

Enumerated constants are integer type and are used to make a program easier to follow. In the following example, constant colors stores the names of colors. The first element is given the value 0:

enum colors {black, brown, red, orange, yellow, green, blue, gray, white};

3.1.9 Escape Sequences

Escape sequences are used to represent nonprintable ASCII characters. Table 3.3 shows some commonly used escape sequences and their representation in C language. For example, the character combination “\n” represents the newline character.

Table 3.3: Some commonly used escape sequences

Escape sequence Hex value Character
\a 0x07 BEL (bell)
\b 0x08 BS (backspace)
\t 0x09 HT (horizontal tab)
\n 0x0A LF (linefeed)
\v 0x0B VT (vertical feed)
\f 0x0C FF (formfeed)
\r 0x0D CR (carriage return)
\xH   String of hex digits

An ASCII character can also be represented by specifying its hexadecimal code after a backslash. For example, the newline character can also be represented as “\x0A”.

3.1.10 Static Variables

Static variables are local variables used in functions (see Chapter 4) when the last value of a variable between successive calls to the function must be preserved. As the following example shows, static variables are declared using the keyword static:

static unsigned int count;

3.1.11 External Variables

Using the keyword extern before a variable name declares that variable as external. It tells the compiler that the variable is declared elsewhere in a separate source code module. In the following example, variables sum1 and sum2 are declared as external unsigned integers:

extern int sum1, sum2;

3.1.12 Volatile Variables

Volatile variables are especially important in interrupt-based programs and input-output routines. Using the keyword volatile indicates that the value of the variable may change during the lifetime of the program independent of the normal flow of the program. Variables declared as volatile are not optimized by the compiler, since their values can change unexpectedly. In the following example, variable Led is declared as a volatile unsigned char:

volatile unsigned char Led;

3.1.13 Enumerated Variables

Enumerated variables are used to make a program more readable. In an enumerated variable, a list of items is specified and the value of the first item is set to 0, the next item is set to 1, and so on. In the following example, type Week is declared as an enumerated list and MON = 0, TUE = 1, WED = 2, and so on):

enum Week {MON, TUE, WED, THU, FRI, SAT, SUN};

It is possible to imply the values of the elements in an enumerated list. In the following example, black = 2, blue = 3, red = 4, and so on.

enum colors {black = 2, blue, red, white, gray};

Similarly, in the following example, black = 2, blue = 3, red = 8, and gray = 9:

enum colors {black = 2, blue, red = 8, gray};

Variables of type enumeration can be declared by specifying them after the list of items. For example, to declare variable My_Week of enumerated type Week, use the following statement:

enum Week {MON, TUE, WED, THU, FRI, SAT, SUN} My_Week;

Now we can use variable My_Week in a program:

My_Week = WED // assign 2 to My_Week

or

My_Week = 2 // same as above

After defining the enumerated type Week, we can declare variables This_Week and Next_Week of type Week as:

enum Week This_Week, Next_Week;

3.1.14 Arrays

Arrays are used to store related items in the same block of memory and under a specified name. An array is declared by specifying its type, name, and the number of elements it will store. For example:

unsigned int Total[5];

This array of type unsigned int has the name Total and has five elements. The first element of an array is indexed with 0. Thus, in this example, Total[0] refers to the first element of the array and Total[4] refers to the last element. The array Total is stored in memory in five consecutive locations as follows:

Total[0]
Total[1]
Total[2]
Total[3]
Total[4]

Data can be stored in the array by specifying the array name and index. For example, to store 25 in the second element of the array we have to write:

Total[1] = 25;

Similarly, the contents of an array can be read by specifying the array name and its index. For example, to copy the third array element to a variable called Temp we have to write:

Temp = Total[2];

The contents of an array can be initialized during the declaration of the array by assigning a sequence of comma-delimited values to the array. An example follows where array months has twelve elements and months[0] = 31, months[1] = 28, and so on:

unsigned char months[12] = {31,28,31,30,31,30,31,31,30,31,30,31};

The same array can also be declared without specifying its size:

unsigned char months[] = {31,28,31,30,31,30,31,31,30,31,30,31};

Character arrays can be declared similarly. In the following example, a character array named Hex_Letters is declared with 6 elements:

unsigned char Hex_Letters[] = {'A', 'B', 'C', 'D', 'E', 'F'};

Strings are character arrays with a null terminator. Strings can be declared either by enclosing the string in double quotes, or by specifying each character of the array within single quotes and then terminating the string with a null character. The two string declarations in the following example are identical, and both occupy five locations in memory:

unsigned char Mystring[] = "COMP";

and

unsigned char Mystring[] = {'C', 'O', 'M', 'P', '\0'};

In C programming language, we can also declare arrays with multiple dimensions. One-dimensional arrays are usually called vectors, and two-dimensional arrays are called matrices. A two-dimensional array is declared by specifying the data type of the array, the array name, and the size of each dimension. In the following example, a two-dimensional array named P is created having three rows and four columns. Altogether, the array has twelve elements. The first element of the array is P[0][0], and the last element is P[2][3]. The structure of this array is shown below:

P[0][0] P[0][1] P[0][2] P[0][3]
P[1][0] P[1][1] P[1][2] P[1][3]
P[2][0] P[2][1] P[2][2] P[2][3]

Elements of a multidimensional array can be specified during the declaration of the array. In the following example, two-dimensional array Q has two rows and two columns, its diagonal elements are set to 1, and its nondiagonal elements are cleared to 0:

unsigned char Q[2][2] = { {1,0}, {0,1} };

3.1.15 Pointers

Pointers are an important part of the C language, as they hold the memory addresses of variables. Pointers are declared in the same way as other variables, but with the character (“*”) in front of the variable name. In general, pointers can be created to point to (or hold the addresses of) character variables, integer variables, long variables, floating point variables, or functions (although mikroC currently does not support pointers to functions).

In the following example, an unsigned character pointer named pnt is declared:

unsigned char *pnt;

When a new pointer is created, its content is initially unspecified and it does not hold the address of any variable. We can assign the address of a variable to a pointer using the (“&”) character:

pnt = &Count;

Now pnt holds the address of variable Count. Variable Count can be set to a value by using the character (“*”) in front of its pointer. For example, Count can be set to 10 using its pointer:

*pnt = 10; // Count = 10

which is the same as

Count = 10; // Count = 10

Or, the value of Count can be copied to variable Cnt using its pointer:

Cnt = *pnt; // Cnt = Count

Array Pointers

In C language the name of an array is also a pointer to the array. Thus, for the array:

unsigned int Total[10];

The name Total is also a pointer to this array, and it holds the address of the first element of the array. Thus the following two statements are equal:

Total[2] = 0;

and

*(Total + 2) = 0;

Also, the following statement is true:

&Total[j] = Total + j

In C language we can perform pointer arithmetic which may involve:

• Comparing two pointers

• Adding or subtracting a pointer and an integer value

• Subtracting two pointers

• Assigning one pointer to another

• Comparing a pointer to null

For example, let’s assume that pointer P is set to hold the address of array element Z[2]:

P = &Z[2];

We can now clear elements 2 and 3 of array Z, as in the two examples that follow. The two examples are identical except that in the first example pointer P holds the address of Z[3] at the end of the statements, and it holds the address of Z[2] at the end of the second set of statements:

*P = 0;    // Z[2] = 0

P = P + 1; // P now points to element 3 of Z

*P = 0;    // Z[3] = 0

or

*P = 0;       // Z[2] = 0

*(P + 1) = 0; // Z[3] = 0

A pointer can be assigned to another pointer. In the following example, variables Cnt and Tot are both set to 10 using two different pointers:

unsigned int *i, *j;   // declare 2 pointers

unsigned int Cnt, Tot; // declare two variables

i = Cnt;               // i points to Cnt

*i = 10;               // Cnt = 10

j = i;                 // copy pointer i to pointer j

Tot = *j;              // Tot = 10

3.1.16 Structures

A structure can be used to collect related items that are then treated as a single object. Unlike an array, a structure can contain a mixture of data types. For example, a structure can store the personal details (name, surname, age, date of birth, etc.) of a student.

A structure is created by using the keyword struct, followed by a structure name and a list of member declarations. Optionally, variables of the same type as the structure can be declared at the end of the structure.

The following example declares a structure named Person:

struct Person {

 unsigned char name[20];

 unsigned char surname[20];

 unsigned char nationality[20];

 unsigned char age;

}

Declaring a structure does not occupy any space in memory; rather, the compiler creates a template describing the names and types of the data objects or member elements that will eventually be stored within such a structure variable. Only when variables of the same type as the structure are created do these variables occupy space in memory. We can declare variables of the same type as the structure by giving the name of the structure and the name of the variable. For example, two variables Me and You of type Person can be created by the statement:

struct Person Me, You;

Variables of type Person can also be created during the declaration of the structure as follows:

struct Person {

unsigned char name[20];

unsigned char surname[20];

unsigned char nationality[20];

unsigned char age;

} Me, You;

We can assign values to members of a structure by specifying the name of the structure, followed by a dot (“.”) and the name of the member. In the following example, the age of structure variable Me is set to 25, and variable M is assigned to the value of age in structure variable You:

Me.age = 25;

M = You.age;

Structure members can be initialized during the declaration of the structure. In the following example, the radius and height of structure Cylinder are initialized to 1.2 and 2.5 respectively:

struct Cylinder {

 float radius;

 float height;

} MyCylinder = {1.2, 2.5};

Values can also be set to members of a structure using pointers by defining the variable types as pointers. For example, if TheCylinder is defined as a pointer to structure Cylinder, then we can write:

struct Cylinder {

 float radius;

 float height;

} *TheCylinder;

TheCylinder->radius = 1.2;

TheCylinder->height = 2.5;

The size of a structure is the number of bytes contained within the structure. We can use the sizeof operator to get the size of a structure. Considering the above example,

sizeof(MyCylinder)

returns 8, since each float variable occupies 4 bytes in memory.

Bit fields can be defined using structures. With bit fields we can assign identifiers to bits of a variable. For example, to identify bits 0, 1, 2, and 3 of a variable as LowNibble and to identify the remaining 4 bits as HighNibble we can write:

struct {

 LowNibble  : 4;

 HighNibble : 4;

} MyVariable;

We can then access the nibbles of variable MyVariable as:

MyVariable.LowNibble = 12;

MyVariable.HighNibble = 8;

In C language we can use the typedef statements to create new types of variables. For example, a new structure data type named Reg can be created as follows:

typedef struct {

 unsigned char name[20];

 unsigned char surname[20];

 unsigned age;

} Reg;

Variables of type Reg can then be created in the same way other types of variables are created. In the following example, variables MyReg, Reg1, and Reg2 are created from data type Reg:

Reg MyReg, Reg1, Reg2;

The contents of one structure can be copied to another structure, provided that both structures are derived from the same template. In the following example, structure variables of the same type, P1 and P2, are created, and P2 is copied to P1:

struct Person {

 unsigned char name[20];

 unsigned char surname[20];

 unsigned int age;

 unsigned int height;

 unsigned weight;

}

struct Person P1, P2;

........................

........................

P2 = P1;

3.1.17 Unions

Unions are used to overlay variables. A union is similar to a structure and is even defined in a similar manner. Both are based on templates, and the members of both are accessed using the “.” or “->” operators. A union differs from a structure in that all variables in a union occupy the same memory area, that is, they share the same storage. An example of a union declaration is:

union flags {

 unsigned char x;

 unsigned int y;

} P;

In this example, variables x and y occupy the same memory area, and the size of this union is 2 bytes long, which is the size of the biggest member of the union. When variable y is loaded with a 2-byte value, variable x will have the same value as the low byte of y. In the following example, y is loaded with 16-bit hexadecimal value 0xAEFA, and x is loaded with 0xFA:

P.y = 0xAEFA;

The size of a union is the size (number of bytes) of its largest member. Thus, the statement:

sizeof(P)

returns 2.

This union can also be declared as:

union flags {

 unsigned char x;

 unsigned int y;

}

union flags P;

3.1.18 Operators in C

Operators are applied to variables and other objects in expressions to cause certain conditions or computations to occur.

mikroC language supports the following operators:

• Arithmetic operators

• Relational operators

• Logical operators

• Bitwise operators

• Assignment operators

• Conditional operators

• Preprocessor operators

• Arithmetic Operators

Arithmetic operators are used in arithmetic computations. Arithmetic operators associate from left to right, and they return numerical results. The mikroC arithmetic operators are listed in Table 3.4.

Table 3.4: mikroC arithmetic operators

Operator Operation
+ Addition
- Subtraction
* Multiplication
/ Division
% Remainder (integer division)
++ Auto increment
- Auto decrement

The following example illustrates the use of arithmetic operators:

/* Adding two integers */

5 + 12 // equals 17

/* Subtracting two integers */

120 – 5 // equals 115

10 – 15 // equals -5

/* Dividing two integers */

5 / 3  // equals 1

12 / 3 // equals 4

/* Multiplying two integers */

3 * 12 // equals 36

/* Adding two floating point numbers */

3.1 + 2.4 // equals 5.5

/* Multiplying two floating point numbers */

2.5 * 5.0 // equals 12.5

/* Dividing two floating point numbers */

25.0 / 4.0 // equals 6.25

/* Remainder (not for float) */

7 % 3 // equals 1

/* Post-increment operator */

j = 4;

k = j++; // k = 4, j = 5

/* Pre-increment operator */

j = 4;

k = ++j; // k = 5, j = 5

/* Post-decrement operator */

j = 12;

k = j--; // k = 12, j = 11

/* Pre-decrement operator */

j = 12;

k = --j; // k = 11, j = 11

Relational Operators

Relational operators are used in comparisons. If the expression evaluates to TRUE, a 1 is returned; otherwise a 0 is returned.

All relational operators associate from left to right. A list of mikroC relational operators is given in Table 3.5.

Table 3.5: mikroC relational operators

Operator Operation
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

The following example illustrates the use of relational operators:

x = 10

x > 8   // returns 1

x == 10 // returns 1

x < 100 // returns 1

x > 20  // returns 0

x != 10 // returns 0

x >= 10 // returns 1

x <= 10 // returns 1

Logical Operators

Logical operators are used in logical and arithmetic comparisons, and they return TRUE (i.e., logical 1) if the expression evaluates to nonzero, and FALSE (i.e., logical 0) if the expression evaluates to zero. If more than one logical operator is used in a statement, and if the first condition evaluates to FALSE, the second expression is not evaluated. The mikroC logical operators are listed in Table 3.6.

Table 3.6: mikroC logical operators

Operator Operation
&& AND
|| OR
! NOT

The following example illustrates the use of logical operators:

/* Logical AND */

x = 7;

x > 0 && x < 10   // returns 1

x > 0 || x < 10   // returns 1

x >= 0 && x <= 10 // returns 1

x >= 0 && x < 5   // returns 0

a = 10; b = 20; c = 30; d = 40;

a > b && c > d // returns 0

b > a && d > c // returns 1

a > b || d > c // returns 1

Bitwise Operators

Bitwise operators are used to modify the bits of a variable. The mikroC bitwise operators are listed in Table 3.7.

Table 3.7: mikroC bitwise operators

Operator Operation
& Bitwise AND
| Bitwise OR
^ Bitwise EXOR
$ Bitwise complement
<< Shift left
>> Shift right

Bitwise AND returns 1 if both bits are 1, otherwise it returns 0.

Bitwise OR returns 0 if both bits are 0, otherwise it returns 1.

Bitwise XOR returns 1 if both bits are complementary, otherwise it returns 0.

Bitwise complement inverts each bit.

Bitwise shift left and shift right move the bits to the left or right respectively.

The following example illustrates the use of bitwise operators:

   i. 0xFA 0xEE returns 0xEA

      0xFA: 1111 1010

      0xEE: 1110 1110

      - - - - - - - -

      0xEA: 1110 1010

  ii. 0x01 | 0xFE returns 0xFF

      0x01: 0000 0001

      0xFE: 1111 1110

      - - - - - - - -

      0xFE: 1111 1111

 iii. 0xAA ^ 0x1F returns

      0xAA: 1010 1010

      0x1F: 0001 1111

      - - - - - - - -

      0xB5: 1011 0101

  iv. ~0xAA returns 0x55

      0xAA: 1010 1010

      ~ :   0101 0101

      - - - - - - - -

      0x55: 0101 0101

   v. 0x14 >> 1 returns 0x0A (shift 0x14 right by 1 digit)

      0x14: 0001 0100

      >> 1: 0000 1010

      - - - - - - - -

      0x0A: 0000 1010

  vi. 0x14 >> 2 returns 0x05 (shift 0x14 right by 2 digits)

      0x14: 0001 0100

      >> 2: 0000 0101

      - - - - - - - -

      0x05: 0000 0101

 vii. 0x235A << 1 returns 0x46B4 (shift left 0x235A left by 1 digit)

      0x235A: 0010 0011 0101 1010

      << 1 :  0100 0110 1011 0100

      - - - - - - - - - - - - - -

      0x46B4: 0100 0110 1011 0100

viii. 0x1A << 3 returns 0xD0 (shift left 0x1A by 3 digits)

      0x1A: 0001 1010

      << 3: 1101 0000

      - - - - - - - -

      0xD0: 1101 0000

Assignment Operators

In C language there are two types of assignments: simple and compound. In simple assignments an expression is simply assigned to another expression, or an operation is performed using an expression and the result is assigned to another expression:

Expression1 = Expression2

or

Result = Expression1 operation Expression2

Examples of simple assignments are:

Temp = 10;

Cnt = Cnt + Temp;

Compound assignments have the general format:

Result operation = Expression1

Here the specified operation is performed on Expression1 and the result is stored in Result. For example:

j += k;

is same as:

j = j + k;

also

p *= m;

is same as

p = p * m;

The following compound operators can be used in mikroC programs:

+= -= *= /=  %=

&= |= ^= >>= <<=

Conditional Operators

The syntax of a conditional operator is:

Result = Expression1 ? Expression2 : Expression3

Expression1 is evaluated first, and if its value is true, Expression2 is assigned to Result, otherwise Expression3 is assigned to Result. In the following example, the maximum of x and y is found where x is compared with y and if x y then max = x, otherwise max = y:

max = (x > y) ? x : y;

In the following example, lowercase characters are converted to uppercase. If the character is lowercase (between a and z), then by subtracting 32 from the character we obtain the equivalent uppercase character:

c = (c >= a && c <= z) ? (c - 32) : c;

Preprocessor Operators

The preprocessor allows a programmer to:

• Compile a program conditionally, such that parts of the code are not compiled

• Replace symbols with other symbols or values

• Insert text files into a program

The preprocessor operator is the (“#”) character, and any line of code leading with a (“#”) is assumed to be a preprocessor command. The semicolon character (“;”) is not needed to terminate a preprocessor command.

mikroC compiler supports the following preprocessor commands:

#define #undef

#if     #elif  #endif

#ifdef  #ifndef

#error

#line

#define, #undef, #ifdef, #ifndef  The #define preprocessor command provides macro expansion where every occurrence of an identifier in the program is replaced with the value of that identifier. For example, to replace every occurrence of MAX with value 100 we can write:

#define MAX 100

An identifier that has already been defined cannot be defined again unless both definitions have the same value. One way to get around this problem is to remove the macro definition:

#undef MAX

Alternatively, the existence of a macro definition can be checked. In the following example, if MAX has not already been defined, it is given value 100, otherwise the #define line is skipped:

#ifndef MAX

 #define MAX 100

#endif

Note that the #define preprocessor command does not occupy any space in memory. We can pass parameters to a macro definition by specifying the parameters in a parenthesis after the macro name. For example, consider the macro definition:

#define ADD(a, b) (a + b)

When this macro is used in a program, ADD(a, b) will be replaced with (a + b) as shown:

p = ADD(x, y)

will be transformed into

p = (x + y)

Similarly, we can define a macro to calculate the square of two numbers:

#define SQUARE(a) (a * a)

We can now use this macro in a program:

p = SQUARE(x)

will be transformed into

p = (x * x)

#include  The preprocessor directive #include is used to include a source file in our program. Usually header files with extension “.h” are used with #include. There are two formats for using #include:

#include <file>

and

#include "file"

In first option the file is searched in the mikroC installation directory first and then in user search paths. In second option the specified file is searched in the mikroC project folder, then in the mikroC installation folder, and then in user search paths. It is also possible to specify a complete directory path as:

#include "C:\temp\last.h"

The file is then searched only in the specified directory path.

#if, #elif, #else, #endif  The preprocessor commands #if, #elif, #else, and #endif are used for conditional compilations, where parts of the source code can be compiled only if certain conditions are met. In the following example, the code section where variables A and B are cleared to zero is compiled if M has a nonzero value, otherwise the code section where A and B are both set to 1 is compiled. Notice that the #if must be terminated with #endif:

#if M

 A = 0;

 B = 0;

#else

 A = 1;

 B = 1;

#endif

We can also use the #elif condition, which tests for a new condition if the previous condition was false:

#if M

 A = 0;

 B = 0;

#elif N

 A = 1;

 B = 1;

#else

 A = 2;

 B = 2;

#endif

In the above example, if M has a nonzero value code section, A = 0; B = 0; are compiled. Otherwise, if N has a nonzero value, then code section A = 1; B = 1; is compiled. Finally, if both M and N are zero, then code section A = 2; B = 2; is compiled. Notice that only one code section is compiled between #if and #endif and that a code section can contain any number of statements.

3.1.19 Modifying the Flow of Control

Statements are normally executed sequentially from the beginning to the end of a program. We can use control statements to modify this normal sequential flow in a C program. The following control statements are available in mikroC programs:

• Selection statements

• Unconditional modifications of flow

• Iteration statements

Selection Statements

There are two selection statements: if and switch.

if Statement The general format of the if statement is:

if (expression)

 Statement1;

else

 Statement2;

or

if (expression) Statement1; else Statement2;

If the expression evaluates to TRUE, Statement1 is executed, otherwise Statement2 is executed. The else keyword is optional and may be omitted. In the following example, if the value of x is greater than MAX then variable P is incremented by 1, otherwise it is decremented by 1:

if (x > MAX) P++;

else P--;

We can have more than one statement by enclosing the statements within curly brackets. For example:

if (x > MAX) {

 P++;

 Cnt = P;

 Sum = Sum + Cnt;

} else P--;

In this example, if x is greater than MAX then the three statements within the curly brackets are executed, otherwise the statement P-- is executed. Another example using the if statement is:

if (x > 0 && x < 10) {

 Total += Sum;

 Sum++;

} else {

 Total = 0;

 Sum = 0;

}

switch Statement  The switch statement is used when a number of conditions and different operations are performed if a condition is true. The syntax of the switch statement is:

switch (condition) {

case condition1:

 Statements;

break;

case condition2:

 Statements;

 break;

.....................

.....................

case condition:

 Statements;

 break;

default:

 Statements;

}

The switch statement functions as follows: First the condition is evaluated. The condition is then compared to condition1 and if a match is found, statements in that case block are evaluated and control jumps outside the switch statement when the break keyword is encountered. If a match is not found, condition is compared to condition2 and if a match is found, statements in that case block are evaluated and control jumps outside the switch statements, and so on. The default is optional, and statements following default are evaluated if the condition does not match any of the conditions specified after the case keywords.

In the following example, the value of variable Cnt is evaluated. If Cnt = 1, A is set to 1. If Cnt = 10, B is set to 1, and if Cnt = 100, C is set to 1. If Cnt is not equal to 1, 10, or 100 then D is set to 1:

switch (Cnt) {

case 1:

 A = 1;

 break;

case 10:

 B = 1;

 break;

case 100:

 C = 1;

 break;

default:

 D = 1;

}

Because white spaces are ignored in C language we can also write the preceding code as:

switch (Cnt) {

case 1:

 A = 1;

 break;

case 10:

 B = 1;

 break;

case 100:

 C = 1;

 break;

default:

 D = 1;

}

Example 3.1

In an experiment the relationship between X and Y values are found to be:

X Y

1 3.2

2 2.5

3 8.9

4 1.2

5 12.9

Write a switch statement that will return the Y value, given the X value.

Solution 3.1

The required switch statement is:

switch (X) {

case 1:

 Y = 3.2;

 break;

case 2:

 Y = 2.5;

 break;

case 3:

 Y = 8.9;

 break;

case 4:

 Y = 1.2;

 break;

case 5:

 Y = 12.9;

}

Iteration Statements

Iteration statements enable us to perform loops in a program, where part of a code must be repeated a number of times. In mikroC iteration can be performed in four ways. We will look at each one with examples:

• Using for statement

• Using while statement

• Using do statement

• Using goto statement

for Statement  The syntax of a for statement is:

for(initial expression; condition expression; increment expression) {

 Statements;

}

The initial expression sets the starting variable of the loop, and this variable is compared against the condition expression before entry into the loop. Statements inside the loop are executed repeatedly, and after each iteration the value of the increment expression is incremented. The iteration continues until the condition expression becomes false. An endless loop is formed if the condition expression is always true.

The following example shows how a loop can be set up to execute 10 times. In this example, variable i starts from 0 and increments by 1 at the end of each iteration. The loop terminates when i=10, in which case the condition i<10 becomes false. On exit from the loop, the value of i is 10:

for(i = 0; i < 10; i++) {

 statements;

}

This loop could also be started by an initial expression with a nonzero value. Here, i starts with 1 and the loop terminates when i = 11. Thus, on exit from the loop, the value of i is 11:

for(i = 1; i <= 10; i++) {

 Statements;

}

The parameters of a for loop are all optional and can be omitted. If the condition expression is left out, it is assumed to be true. In the following example, an endless loop is formed where the condition expression is always true and the value of i starts with 0 and is incremented after each iteration:

/* Endless loop with incrementing i */

for(i=0; ; i++) {

 Statements;

}

In the following example of an endless loop all the parameters are omitted:

/* Example of endless loop */

for(; ;) {

 Statements;

}

In the following endless loop, i starts with 1 and is not incremented inside the loop:

/* Endless loop with i = 1 */

for(i=1; ;) {

 Statements;

}

If there is only one statement inside the for loop, he curly brackets can be omitted as shown in the following example:

for(k = 0; k < 10; k++)Total  = Total + Sum;

Nested for loops can also be used. In a nested for loop, the inner loop is executed for each iteration of the outer loop. In the following example the inner loop is executed five times and the outer loop is executed ten times. The total iteration count is fifty:

/* Example of nested for loops */

for(i = 0; i < 10; i++) {

 for(j = 0; j < 5; j++) {

  Statements;

 }

}

In the following example, the sum of all the elements of a 3×4 matrix M is calculated and stored in a variable called Sum:

/* Add all elements of a 3x4 matrix */

Sum = 0;

for(i = 0; i < 3; i++) {

 for(j = 0; j < 4; j++) {

  Sum = Sum + M[i][j];

 }

}

Since there is only one statement to be executed, the preceding example could also be written as:

/* Add all elements of a 3x4 matrix */

Sum = 0;

for(i = 0; i < 3; i++) {

 for(j = 0; j < 4; j++) Sum = Sum + M[i][j];

}

while Statement  The syntax of a while statement is:

while (condition) {

 Statements;

}

Here, the statements are executed repeatedly until the condition becomes false, or the statements are executed repeatedly as long as the condition is true. If the condition is false on entry to the loop, then the loop will not be executed and the program will continue from the end of the while loop. It is important that the condition is changed inside the loop, otherwise an endless loop will be formed.

The following code shows how to set up a loop to execute 10 times, using the while statement:

/* A loop that executes 10 times */

k = 0;

while (k < 10) {

 Statements;

 k++;

}

At the beginning of the code, variable k is 0. Since k is less than 10, the while loop starts. Inside the loop the value of k is incremented by 1 after each iteration. The loop repeats as long as k 10 and is terminated when k = 10. At the end of the loop the value of k is 10.

Notice that an endless loop will be formed if k is not incremented inside the loop:

/* An endless loop */

k = 0;

while (k < 10) {

 Statements;

}

An endless loop can also be formed by setting the condition to be always true:

/* An endless loop */

while (k == k) {

 Statements;

}

Here is an example of calculating the sum of numbers from 1 to 10 and storing the result in a variable called sum:

/* Calculate the sum of numbers from 1 to 10 */

unsigned int k, sum;

k = 1;

sum = 0;

while(k <= 10) {

 sum = sum + k;

 k++;

}

It is possible to have a while statement with no body. Such a statement is useful, for example, if we are waiting for an input port to change its value. An example follows where the program will wait as long as bit 0 of PORTB (PORTB.0) is at logic 0. The program will continue when the port pin changes to logic 1:

while(PORTB.0 == 0); // Wait until PORTB.0 becomes 1

or

while(PORTB.0);

It is also possible to have nested while statements.

do Statement  A do statement is similar to a while statement except that the loop executes until the condition becomes false, or, the loop executes as long as the condition is true. The condition is tested at the end of the loop. The syntax of a do statement is:

do {

 Statements;

} while (condition);

The first iteration is always performed whether the condition is true or false. This is the main difference between a while statement and a do statement. The following code shows how to set up a loop to execute 10 times using the do statement:

/* Execute 10 times */

k = 0;

do {

 Statements;

 k++;

} while (k < 10);

The loop starts with k = 0, and the value of k is incremented inside the loop after each iteration. At the end of the loop k is tested, and if k is not less than 10, the loop terminates. In this example because k = 0 is at the beginning of the loop, the value of k is 10 at the end of the loop.

An endless loop will be formed if the condition is not modified inside the loop, as shown in the following example. Here k is always less than 10:

/* An endless loop */

k = 0;

do {

 Statements;

} while (k < 10);

An endless loop can also be created if the condition is set to be true all the time:

/* An endless loop */

do {

 Statements;

} while (k == k);

It is also possible to have nested do statements.

Unconditional Modifications of Flow

goto Statement  A goto statement can be used to alter the normal flow of control in a program. It causes the program to jump to a specified label. A label can be any alphanumeric character set starting with a letter and terminating with the colon (“:”) character.

Although not recommended, a goto statement can be used together with an if statement to create iterations in a program. The following example shows how to set up a loop to execute 10 times using goto and if statements:

/* Execute 10 times */

k = 0;

Loop:

 Statements;

 k++;

if (k < 10) goto Loop;

The loop starts with label Loop and variable k = 0 at the beginning of the loop. Inside the loop the statements are executed and k is incremented by 1. The value of k is then compared with 10 and the program jumps back to label Loop if k 10. Thus, the loop is executed 10 times until the condition at the end becomes false. At the end of the loop the value of k is 10.

continue and break Statements  continue and break statements can be used inside iterations to modify the flow of control. A continue statement is usually used with an if statement and causes the loop to skip an iteration. An example follows that calculates the sum of numbers from 1 to 10 except number 5:

/* Calculate sum of numbers 1,2,3,4,6,7,8,9,10 */

Sum = 0;

i = 1;

for(i = 1; i <= 10; i++) {

 if (i == 5) continue; // Skip number 5

 Sum = Sum + i;

}

Similarly, a break statement can be used to terminate a loop from inside the loop. In the following example, the sum of numbers from 1 to 5 is calculated even though the loop parameters are set to iterate 10 times:

/* Calculate sum of numbers 1,2,3,4,5 */

Sum = 0;

i = 1;

for(i = 1; i <= 10; i++) {

 if (i > 5) break; // Stop loop if i > 5

 Sum = Sum + i;

}

3.1.20 Mixing mikroC with Assembly Language Statements

It sometimes becomes necessary to mix PIC microcontroller assembly language statements with the mikroC language. For example, very accurate program delays can be generated by using assembly language statements. The topic of assembly language is beyond the scope of this book, but techniques for including assembly language instructions in mikroC programs are discussed in this section for readers who are familiar with the PIC microcontroller assembly languages.

Assembly language instructions can be included in a mikroC program by using the keyword asm (or _asm, or __asm). A group of assembly instructions or a single such instruction can be included within a pair of curly brackets. The syntax is:

asm {

 assembly instructions

}

Assembly language style comments (a line starting with a semicolon character) are not allowed, but mikroC does allow both types of C style comments to be used with assembly language programs:

asm {

 /* This assembly code introduces delay to the program*/

 MOVLW 6 // Load W with 6

 ................

 ................

}

User-declared C variables can be used in assembly language routines, but they must be declared and initialized before use. For example, C variable Temp can be initialized and then loaded to the W register as:

unsigned char Temp = 10;

asm {

 MOVLW Temp // W = Temp = 10

..................

..................

}

Global symbols such as predefined port names and register names can be used in assembly language routines without having to initialize them:

asm {

 MOVWF PORTB

 .....................

 .....................

}

3.2 PIC Microcontroller Input-Output Port Programming

Depending on the type of microcontroller used, PIC microcontroller input-output ports are named as PORTA, PORTB, PORTC, and so on. Port pins can be in analog or digital mode. In analog mode, ports are input only and a built-in analog-to-digital converter and multiplexer circuits are used. In digital mode, a port pin can be configured as either input or output. The TRIS registers control the port directions, and there are TRIS registers for each port, named as TRISA, TRISB, TRISC, and so on. Clearing a TRIS register bit to 0 sets the corresponding port bit to output mode. Similarly, setting a TRIS register bit to 1 sets the corresponding port bit to input mode.

Ports can be accessed as a single 8-bit register, or individual bits of a port can be accessed. In the following example, PORTB is configured as an output port and all its bits are set to a 1:

TRISB = 0;    // Set PORTB as output

PORTB = 0xFF; // Set PORTB bits to 1

Similarly, the following example shows how the 4 upper bits of PORTC can be set as input and the 4 lower bits of PORTC can be set as output:

TRISC = 0xF0;

Bits of an input-output port can be accessed by specifying the required bit number. In the following example, variable P2 is loaded with bit 2 of PORTB:

P2 = PORTB.2;

All the bits of a port can be complemented by the statement:

PORTB = ~PORTB;

3.3 Programming Examples

In this section, some simple programming examples are given to familiarize the reader with programming in C.

Example 3.2

Write a program to set all eight port pins of PORTB to logic 1.

Solution 3.2

PORTB is configured as an output port, and then all port pins are set to logic 1 by sending hexadecimal number 0xFF:

void main() {

 TRISB = 0;    // Configure PORTB as output

 PORTB = 0xFF; // Set all port pins to logic a

}

Example 3.3

Write a program to set the odd-numbered PORTB pins (bits 1, 3, 5, and 7) to logic 1.

Solution 3.3

Odd-numbered port pins can be set to logic 1 by sending the bit pattern 10101010 to the port. This bit pattern is the hexadecimal number 0xAA and the required program is:

void main() {

 TRISB = 0;    // Configure PORTB as output

 PORTB = 0xAA; // Turn on odd numbered port pins

}

Example 3.4

Write a program to continuously count up in binary and send this data to PORTB. Thus PORTB requires the binary data:

00000000

00000001

00000010

00000011

........

........

11111110

11111111

00000000

........

Solution 3.4

A for loop can be used to create an endless loop, and inside this loop the value of a variable can be incremented and then sent to PORTB:

void main() {

 unsigned char Cnt = 0;

 for(;;) // Endless loop

 {

  PORTB = Cnt; // Send Cnt to PORTB

  Cnt++;       // Increment Cnt

 }

}

Example 3.5

Write a program to set all bits of PORTB to logic 1 and then to logic 0, and to repeat this process ten times.

Solution 3.5

A for statement can be used to create a loop to repeat the required operation ten times:

void main() {

 unsigned char j;

 for(j = 0; j < 10; j++) // Repeat 10 times

 {

  PORTB = 0xFF; // Set PORTB pins to 1

  PORTB = 0;    // Clear PORTB pins

 }

}

Example 3.6

The radius and height of a cylinder are 2.5cm and 10cm respectively. Write a program to calculate the volume of this cylinder.

Solution 3.6

The required program is:

void main() {

 float Radius = 2.5, Height = 10;

 float Volume;

 Volume = PI * Radius * Radius * Height;

}

Example 3.7

Write a program to find the largest element of an integer array having ten elements.

Solution 3.7

At the beginning, variable m is set to the first element of the array. A loop is then formed and the largest element of the array is found:

void main() {

 unsigned char j;

 int m, A[10];

 m = A[0]; // First element of array

 for(j = 1; j < 10; j++) {

  if(A[j] > m) m = A[j];

 }

}

Example 3.8

Write a program using a while statement to clear all ten elements of an integer array M.

Solution 3.8

As shown in the program that follows, NUM is defined as 10 and variable j is used as the loop counter:

#define NUM 10

void main() {

 int M[NUM];

 unsigned char j = 0;

 while (j < NUM) {

  M[j] = 0;

  j++;

 }

}

Example 3.9

Write a program to convert the temperature from °C to °F starting from 0°C, in steps of 1°C up to and including 100°C, and store the results in an array called F.

Solution 3.9

Given the temperature in °C, the equivalent in °F is calculated using the formula:

F = (C – 32.0)/1.8

A for loop is used to calculate the temperature in °F and store in array F:

void main() {

 float F[100];

 unsigned char C;

 for(C = 0; C <= 100; C++) {

  F[C] = (C – 32.0) / 1.8;

 }

}

3.4 Summary

There are many assembly and high-level languages for the PIC18 series of microcontrollers. This book focuses on the mikroC compiler, since it is easy to learn and a free demo version is available that allows users to develop programs as large as 2K in size.

This chapter presented an introduction to the mikroC language. A C program may contain a number of functions and variables plus a main program. The beginning of the main program is indicated by the statement void main().

A variable stores a value used during the computation. All variables in C must be declared before they are used. A variable can be an 8-bit character, a 16-bit integer, a 32-bit long, or a floating point number. Constants are stored in the flash program memory of PIC microcontrollers, so using them avoids using valuable and limited RAM memory.

Various flow control and iteration statements such as if, switch, while, do, break, and so on have been described in the chapter, with examples.

Pointers are used to store the addresses of variables. As we shall see in the next chapter, pointers can be used to pass information back and forth between a function and its calling point. For example, pointers can be used to pass variables between a main program and a function.

3.5 Exercises

1. Write a C program to set bits 0 and 7 of PORTC to logic 1.

2. Write a C program to count down continuously and send the count to PORTB.

3. Write a C program to multiply each element of a ten element array by 2.

4. Write a C program to add two matrices P and Q. Assume that the dimension of each matrix is 3×3 and store the result in another matrix called W.

5. Repeat Exercise 4 but this time multiply matrices P and Q and store the product in matrix R.

6. What do the terms variable and constant mean?

7. What does program repetition mean? Describe the operation of while, do-while, and for loops in C.

8. What is an array? Write example statements to define the following arrays:

 a) An array of ten integers

 b) An array of thirty floats

 c) A two-dimensional array having six rows and ten columns

9. Trace the operation of the following loops. What will be the value of variable z at the end of each loop?

 a)

unsigned char j = 0, z = 0;

while (j < 10) {

 z++;

 j++;

}

 b)

 unsigned char z = 10;

 for (j = 0; j < 10; j++) z--;

10. Given the following variable definitions, list the outcome of the following conditional tests in terms of “true” or “false”:

unsigned int a = 10, b = 2;

if (a > 10)

if (b >= 2)

if(a == 10)

if (a > 0)

11. Write a program to calculate whether a number is odd or even.

12. Determine the value of the following bitwise operations using AND, OR, and EXOR operations:

Operand 1: 00010001

Operand 2: 11110001

13. How many times does each of the following loops iterate, and what is the final value of the variable j in each case?

 a) for(j = 0; j < 5; j++)

 b) for(j = 1; j < 10; j++)

 c) for(j = 0; j <= 10; j++)

 d) for(j = 0; j <= 10; j += 2)

 e) for(j = 10; j > 0; j -= 2)

14. Write a program to calculate the sum of all positive integer numbers from 1 to 100.

15. Write a program to evaluate factorial n, where 0! and 1! evaluate to 1 and n! = n × (n – 1)!

16. Write a program to calculate the average value of the numbers stored in an array. Assume that the array is called M and has twenty elements.

17. Modify the program in Exercise 16 to find the smallest and largest values of the array. Store the smallest value in a variable called Sml and the largest value in a variable called Lrg.

18. Derive equivalent if-else statements for the following tests:

 a) (a > b) ? 0 : 1

 b) (x < y) ? (a > b) : (c > d)

19. Given that f1 and f2 are both floating point variables, explain why the following test expression controlling the while loop may not be safe:

do {

 ...............

 ...............

} while(f1 != f2);

Why would the problem not occur if both f1 and f2 were integers? How would you correct this while loop?

20. What can you say about the following while loop?

k = 0;

Total = 0;