Some assembly required: Assembly language programming with the AVR microcontroller

A family of internationally popular microcontrollers, the Atmel AVR

microcontroller series is a low-cost hardware development platform suitable

for an educational environment. Until now, no text focused on the assembly

language programming of these microcontrollers. Through detailed

coverage of assembly language programming principles and techniques,

Some Assembly Required: Assembly Language Programming with the

AVR Microcontroller teaches the basic system capabilities of 8-bit AVR

microcontrollers.

The text illustrates fundamental computer architecture and programming

structures using AVR assembly language. It employs the core AVR 8-bit

RISC microcontroller architecture and a limited collection of external

devices, such as push buttons, LEDs, and serial communications, to

describe control structures, memory use and allocation, stacks, and I/O.

Each chapter contains numerous examples and exercises, including

programming problems.

Features

• Presents a comprehensive treatment of assembly language

programming and basic computer organization

• Focuses on the popular Atmel AVR microcontroller series

• Covers function call mechanisms and parameter passing techniques

• Describes data representation for integers, floating point, and

character data

• Explains how to use the XPLAIN Evaluation Kit

By studying assembly languages, computer scientists gain an understanding

of the functionality of basic processors and how their capabilities support

high level languages and applications. Exploring this connection between

hardware and software, this book provides a foundation for understanding

compilers, linkers, loaders, and operating systems in addition to the

processors themselves.

K11091

SOME ASSEMBLY REQUIRED

MARGUSH

with the AVR Microcontroller

Assembly Language Programming

Computer Science

K11091_Cover.indd 1 6/27/11 10:40 AM

SOME ASSEMBLY

REQUIRED

Assembly Language Programming

with the AVR Microcontroller

SOME ASSEMBLY

REQUIRED

TIMOTHY S. MARGUSH

Assembly Language Programming

with the AVR Microcontroller

CRC Press

Taylor & Francis Group

6000 Broken Sound Parkway NW, Suite 300

Boca Raton, FL 33487-2742

© 2012 by Taylor & Francis Group, LLC

CRC Press is an imprint of Taylor & Francis Group, an Informa business

No claim to original U.S. Government works

Version Date: 20110818

International Standard Book Number-13: 978-1-4398-9700-3 (eBook - PDF)

This book contains information obtained from authentic and highly regarded sources. Reasonable

efforts have been made to publish reliable data and information, but the author and publisher cannot

assume responsibility for the validity of all materials or the consequences of their use. The authors and

publishers have attempted to trace the copyright holders of all material reproduced in this publication

and apologize to copyright holders if permission to publish in this form has not been obtained. If any

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Visit the Taylor & Francis Web site at

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My husband, Tim, died before completing the dedication page

so I am writing this for him. To Michael Decker who put in

hours of time proofreading text and code during a very busy

time in his life. He is a credit to his profession and a blessing to

our family. I would also like to dedicate this book to Tim's

brother, Philip Margush, whom Tim loved very much.

—Gail Margush

vii

Contents

Acknowledgments, xxi

Author, xxiii

INTRODUCTION, XXV

Chapter 1  ◾ Computer Systems 1

BASIC PROCESSOR ARCHITECTURE 1

COMPONENTS OF A COMPUTER SYSTEM 2

Central Processing Unit 2

Storage 3

Input and Output 4

CLASSIFICATION OF PROCESSORS 5

CISC versus RISC 5

Functionality 5

Architecture 7

NUMERATION SYSTEMS 7

The Nature of Data 8

Binary, Octal, Hexadecimal, Decimal . . . 10

Conversions 12

Repeated Division Algorithm 14

Polynomial Evaluation Algorithm 14

Horner’s Algorithm 14

BOOLEAN DATA 16

Boolean Operations 17

viii   ◾  Contents

Applications of Boolean Operations 18

Mask Out, Clear, Zero 20

Set and Union 20

Toggle 21

Shifting and Rotating Bits 22

EXERCISES 23

Chapter 2  ◾  The Atmel AVR Microcontroller Family 27

THE AVR CORE 27

Instructions 28

Registers 28

Clock 29

MACHINE LANGUAGE FOR THE AVR

MICROCONTROLLER 30

One Plus One Equals? 30

Load Immediate 31

Add 34

Expanding Opcodes 36

Execution Trace 37

Jump 39

AVR STUDIO 42

New Project 42

Editor 43

Assembling 43

Debugger 44

Mnemonics 48

AVR DEVELOPMENT PLATFORMS 50

STK-500 Development Kit 50

The ATAVRXPLAIN Demonstration Kit 52

INTRODUCTION TO AVR ASSEMBLY LANGUAGE 53

Assembly Language Source Files 53

Line Format 54

Additional Features 54

Contents   ◾  ix

SAMPLE AVR ASSEMBLY LANGUAGE PROGRAM 55

A Counting Program 56

Explanation of the Statements 57

Assembling the Program 58

Obtaining a Listing File 58

The Map File 59

Simulating Execution 59

Downloading to the AVR Processor 60

Adjusting the Clock Speed 64

EXERCISES 66

PROGRAMMING EXERCISES 68

ALTERNATE PROGRAMS FOR THE XPLAIN

DEMONSTRATION KIT 69

Program 2.2a: Counter 69

Chapter 3  ◾ Assembly Language 73

DIRECTIVES 75

The Assembler’s Location Counters 75

EXPRESSIONS 79

Symbols and Literals 80

DATA DEFINITION DIRECTIVES 82

Operators Used in Expressions 83

Functions Used in Expressions 86

INSTRUCTIONS 87

Zero-Operand Instructions 89

One-Operand Instructions 90

Two-Operand Instructions 90

THE TOGGLER PROGRAM: SAMPLE I/O 90

AVR Digital I/O Ports and STK-500/XPLAIN LEDs 92

AVR Digital I/O Ports and STK-500/XPLAIN Pushbuttons 94

The Toggler Program 95

EXERCISES 99

x   ◾  Contents

PROGRAMMING EXERCISES 102

ALTERNATE PROGRAMS FOR THE XPLAIN

DEMONSTRATION KIT 103

Program 3.1a: Toggler 103

Program Notes 106

Chapter 4  ◾  Integer Data Representation 109

UNSIGNED INTEGER DATA 109

CHARACTER DATA 115

SIGNED INTEGER DATA 116

Two’s Complement 116

One’s Complement 121

Sign and Magnitude 121

Excess-N 122

BINARY CODED DECIMAL 123

DATA VALUES IN PROGRAMS 124

Define Byte, Word, and Others 124

Data Encoding 125

ASSIGNMENT STATEMENTS 128

Simple Variables 128

Assignment 130

Register-Based Variables 132

Evaluating Expressions 134

BRANCH INSTRUCTIONS 135

EXERCISES 137

PROGRAMMING EXERCISES 142

Chapter 5  ◾ AVR Core: Startup, Reset, and Memories 145

AVR RESET SIGNAL 147

Interrupt Jump Table 148

BASIC SYSTEM ORGANIZATION 150

Program Memory 150

Contents   ◾  xi

General Purpose Registers 150

I/O Registers and Data Space 150

Stack 151

EEPROM Storage 154

PROGRAM MEMORY 154

Program Indirect Addressing 154

DATA SPACE 158

Data Direct Addressing 158

I/O Direct Addressing 160

Data Indirect Addressing 160

Data Indirect Addressing with Postincrement

or Predecrement 161

Data Indirect Addressing with Displacement 162

EEPROM 163

The Standard Include File 166

EEPROM Read 167

Defining EEPROM Data 167

Using EEPROM Data in the AVR Simulator 168

Downloading EEPROM Data to the Microcontroller 171

Writing Data to EEPROM 172

EXERCISES 173

PROGRAMMING EXERCISES 176

ALTERNATE PROGRAMS FOR THE XPLAIN

DEMONSTRATION KIT 177

Program 5.1a: LED Patterns 177

The Standard Include File 180

EEPROM Read 181

Defining EEPROM Data 182

Using EEPROM Data in the AVR Simulator 2 183

Downloading EEPROM Data 184

Writing Data to EEPROM 185

xii   ◾  Contents

Chapter 6  ◾  The Stack and Function Calls 189

STACK 189

Stack Pointer 190

Push and Pop 190

FUNCTIONS 191

Defining a Function 192

Function Call Instructions 193

Return Address and RET Instructions 195

ARGUMENTS, PARAMETERS, AND

RETURN VALUES 196

Return Values 196

Arguments and Parameters: Call by Value/Call

by Reference 199

Memory Allocation for Parameters 200

Register Parameters 201

Stack Parameters 204

Inline Parameters 207

LOCAL STORAGE 211

Parameter and Local Storage Access

in Stack Frames 215

Fibonacci Numbers Example 215

Stack Frame Naming Conventions 218

Function Fibonacci Continued 219

Stack Cleanup: Automatic Parameter Removal 221

Preprocessor Macros 222

AUXILIARY DATA STACK (SOFTWARE STACK) 224

Macros for the Data Stack 226

Fibonacci with Data Stack 228

CALL BY REFERENCE EXAMPLE 230

EXERCISES 233

PROGRAMMING EXERCISES 238

Contents   ◾  xiii

Chapter 7  ◾  Serial Communications Using the

Universal Synchronous and Asynchronous

Serial Receiver and Transmitter 241

RS-232 241

SERIAL COMMUNICATION BASICS 243

Mark and Space 244

Serial Data Frame 244

AVR USART CONFIGURATION 246

Baud Rate 246

Data, Parity, and Stop Bits 248

Physical Connections 251

Terminal Programs 253

XPLAIN Programming Notes 254

AVR USART TRANSMIT AND RECEIVE 255

Enabling the Transmitter and Receiver 255

Transmitting Data 256

Receiving Data 259

Disabling the Transmitter or Receiver 259

SERIAL TRANSMISSION ERRORS 260

Detecting Errors 260

POLLED I/O 262

SERIAL COMMUNICATIONS EXAMPLE 262

Transmit Time 266

EXERCISES 266

PROGRAMMING EXERCISES 268

ALTERNATE PROGRAMS FOR THE XPLAIN

DEMONSTRATION KIT 270

Program 7.1a: Serial Echo 270

Chapter 8  ◾  Logical Operations 275

BITWISE LOGICAL OPERATIONS 275

Status Flags 276

xiv   ◾  Contents

USES OF LOGICAL INSTRUCTIONS 278

Clearing, Setting, and Toggling 280

Masks 281

SHIFT AND ROTATE 282

Arithmetic Shifting 284

Other Applications of Shift and Rotate 285

SPECIAL BITWISE OPERATIONS 287

AVR TIMER/COUNTER 288

Timer/Counter 1 Configuration: ATMEGA16A 289

Timer/Counter 1 Compare Match (ATMEGA16A) 290

Timer/Counter Configuration: ATxmega128A1 292

Timer/Counter Period: ATxmega128A1 293

TIMER/COUNTER EXAMPLE: LED BLINKER 295

EXERCISES 297

PROGRAMMING EXERCISES 299

ALTERNATE PROGRAMS FOR THE XPLAIN

DEMONSTRATION KIT 301

Program 8.1a: LED Blinker 301

Chapter 9  ◾ Control Structures 305

ALTERING THE FLOW OF CONTROL 306

Jump 306

Call and Return 307

Conditional Branch 307

JUMP AND CONDITIONAL BRANCH INSTRUCTIONS 308

Comparisons 310

Conditional Skip Instructions 311

SELECTION 311

If Else 312

Compound Conditions 312

Multiway Selection Structure 318

Computed Goto 319