AVR Assembly Language An Introduction

1
AVR Assembly LanguageAn Introduction

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
Assemblers
objectfile

• Translate source code to machine language

source
Assembler
hexfile
include files
mapfile
listfile
3
Source Files

• Usually a text file containing assembly language
  statements
• Include directives are used to insert text from
  other files as part of the source

4
Object File

• Contains machine program and debugging
  information used by AVR Studio

5
Hex File

• Contains data for AVR Processor flash memory
• This is used by the programmer utility to
  download to AVR memory

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Map File

• A list of labels and symbols used in the program
• Serves as a reference to locate data in memory

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Listing File

• A complete report of the assembly process
• Includes error messages, address information,
  bytes generated, and statistics

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Syntax

• There are 3 valid formats for AVR assembly
  language statements
• label directive operands comment
• label instruction operands comment
• label comment
• .. indicates optional stuff
• Each line is a directive, an instruction
  (machine), a label, or is blank
• Any line can have a comment

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Comments

• Single line comments
• Begin with a semi-colon or double slash and
  extend to the end of the line
• Block comments
• Begin with /, end with /, and may span several
  lines

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Segment Directives

• The assembler generates bytes for flash and
  EEPROM and reserves addresses in SRAM
• Segment directives set the active memory for
  subsequent commands
• .cseg code segment (flash)
• This is the default segment if none is specified
• .eseg EEPROM
• .dseg data segment (SRAM)

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Origin Directive

• The assembler maintains a location counter for
  each segment
• .org value
• sets the location counter of the current segment
  to the specified value
• The default origin for the code segment and
  EEPROM is 0000
• The location counter for the data segment is the
  first address following the I/O registers (0060)

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LED Flasher Program

• A simple AVR example to illustrate output to a
  port.
• Designed to be executed in a simulator under
  debug control
• This program counts from 0 to 255 (and repeats)
  outputting
• the current counter value to PORTB.
• .cseg select current segment as code
• .org 0 begin assembling at address 0
• .def count r16 Reg 16 will hold counter
  value
• .def temp r17 used as a temporary register
• .equ PORTB 0x18 Port B's output register
• .equ DDRB 0x17 Port B's Data Direction
  Register
• ldi temp,0xFF configure PORTB as output
• out DDRB,temp
• ldi count,0x00 Initialize count at 0
• lp
• out PORTB,count Put counter value on PORT B
• inc count increment counter
• rjmp lp repeat (forever)

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Assembling the Program

• After creating a new project and entering the
  source code, the program is assembled using the
  Build command.
• The option to create a listing file must be
  selected under Assembler Options under the
  Project menu
• If errors occur, correct them and reassemble

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Simulating the Program

• Start debugging (Debug menu)
• Select the I/O View and be sure that Register 16,
  the Processor, and PORT B are visible
• Single step, observing changes in the I/O View
• Use Auto-step to run the simulator while
  observing changes at each step

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Using the STK-500

• Setup the STK-500 as described in the included
  documentation
• Connect the STK-500 to your computer's serial
  port and turn on the STK-500
• Activate the AVR Programmer
• Tools menu Program AVR Connect
• Usually the Auto detect finds the board

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Program Flash from HEX

• On the Program Tab, you can accept defaults for
  everything but the input file under Flash
• Use the browse () button to locate the HEX file
  with the same name as your project
• It will be in the project folder
• Click Program when ready

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Internal Clock Speed

• This program runs too fast under the default
  processor settings (4MHz internal oscillator)
• The LEDs appear to all stay illuminated
• Under the Fuses tab, select the Ext. Clock
• Click Program to send the options to the processor

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External Clock

• Under the Board tab, set the STK500 Osc to 1024Hz
  and click Write
• The processor should run slow enough to observe
  the leading bit cycling once per second
• Note that the dialog indicates the clock will be
  set to the attainable frequency of 1019 Hz
• Not all frequencies can be produced exactly

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Backwards Counter

• You should notice that the counter appears to run
  backwards
• The circuit used to control the LEDs causes the
  LED to emit light when the port output is 0
• 00000000
• 00000001
• etc appears to be counting backwards

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External Oscillator

• Be sure to set the STK500 Osc back to a larger
  value (4 MHz is fine) before downloading other
  programs
• You must click Write to send the setting to the
  processor
• If this value is too slow, the programmer cannot
  download code to flash