AVR Assembler

M. Neil - Microprocessor Course

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BRANCHES SUBROUTINES AND MEMORY USAGE

AVR Assembler


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Outline

Memory on the AVR processorAddressing modes for accessing memoryProgram Flow: Subroutines and the stackMemory Test ProgramProgram DesignThe LED display program

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Memory in the AVR

There are Several different types of memory on the AVR which you can use/access

Flash Program Memory

Data SRAM (internal and external)

Registers

EEPROM


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Program Memory

Organized as 64K 16bit words Addressed $0000-$FFFF 128 Kilo-Bytes of memory

This is where the program which is executing is stored The Program Counter (PC)

holds the address in program memory of the current instruction

Data can be stored in program memory The data stored in program

memory needs to be copied into data memory before it can be used – there is a special instruction (LPM) for doing this


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Data Memory

There is some internal memory (SRAM) on the Atmega128 which can be used for storing data (and also the Stack) Internal SRAM memory is 4K X

8 bits (4 Kilo-Bytes $0100-$10FF)

The registers and I/O ports occupy the lower part of this memory (Addresses $0000-$00FF)

There is also the possibility of external memory being installed Not on our boards Can add if your project needs


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Registers

The registers actually occupy the lowest 32 bytes of internal SRAM.

Most registers can be used generally for 8 bit operations

R16-R31 can be used with the ldi instruction

R26-r31 can also be used as the X/Y/Z address registers


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Accessing memory – Addressing modes

neg r20R20 $00-r20

add r20,r24R20 r20+r24


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Addressing Input/Output memory

in r20,PINDR20 PINDout PORTB,r16PORTB r16

There are 6 I/O portsPORTA-PORTGEach has

PINX Input Port

DDRX Data Direction

Register PORTX

Output Port


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Working with SRAM Data

We can define addresses where we want to store variables, and move data in and out of the SRAM with the lds,sts instructions

.equ myVariable = $0208lds r16,

myVariablesbi r16, $22sts

myVariable, r16


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Using the X, Y, Z registers to address SRAM

The registers r26-r31 are special and can be used to store an address (16 bits $0000-$FFFF) which points to the SRAM r26:r27 is XL:XH r28:r29 is YL:YH r30:r31 is ZL:ZH

With these registers, any location in SRAM can be read from or written to ld r16,Y

There are also pre/post increment/decrement modes to make it quicker to address arrays of data ld r3,Z+ ld r7,–X

.equ myVariable = $0208ldi XL, low(myVariable) ;load

XL(r26):XH(r27) withldi XH, high(myVariable) ;address of

myVariableld r16,X

;copy data from SRAMsbi r16, $22

;subtract 34 from the bytest X,r16

;store result in SRAM


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Using the Z register to point at program memory

Program Memory

Z Register LPM will load

$200 $B1

$201 $B2

$202 $B3

$203 $B4

$204 $B5

$205 $B6

Address Bits 8-15 Bits 0-7

$100 $B2 $B1

$101 $B4 $B3

$102 $B6 $B5

$103 $B8 $B7

Program Memory and LPM - a picture

We multiply the address by 2 to select which 16 bit wordThe Least Significant Bit (LSB) is then used to select which byte to read with the LPM instruction


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Copying data from PM to SRAM

.equ myArray=$0800 ; Address in SRAMldi ZH,high(myTable*2) ; Load the Z register with theldi ZL,low(myTable*2) ; address of data in PMldi XH,high(myArray) ; Load X register with the ldi XL,low(myArray) ; address in SRAMldi r16,22 ; there are 22 bytes to copy

loop:lpm r17,Z+ ; move one byte from PM to r17st X+,r17 ; store in SRAM (and increment)dec r16 ; count down bytes copiedbrne loop ; keep going until finishedret

myTable:.db “This is just some data”

Can use only the Z register to copy data from PM to a register. Auto increment modes make more compact code – Use the special lpm instruction


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More details about Program memory

The Z register is only 16 bits wide Can address 64K Bytes There are 128K Bytes of flash

program memory The LPM instruction

addresses the lower 64K Bytes

The RAMPZ register allows access to the upper 64K Bytes with the ELPM instruction

; **** RAMPZ Setup Code **** ldi r16,$00 ; 1 = ELPM acts on

upper 64Kout RAMPZ,r16 ; 0 = ELPM acts on

lower 64K


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Storing data in program memory

You can create initial values for data in your program code These will be stored in the program memory

Accessed using the labels you define See the examples below for the assembler directives needed to create data

in the You can copy them into SRAM (as we did on the previous page)

or use them directly from the program memory Access will be quicker from SRAM and values can be changed

MyByteTable:.DB $00,$01,$02,$03,$04,$05,$06,$07;Table loading with 8 bytesMyWordTable:.DW $0908,$0B0A,$0D0C,$0F0E,$1110 ; Load 5 16-bit words in the table.DW $1514,$1617,$1918,$1B1A,$1D1C ; Next 5 16-bit words in the tableMyStringTable:.DB “abcdefghijklmnopqrstuvwxyz” ; Line of ascii characters

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Using 16 Bit Counters

Sometimes we need to work with numbers larger than 8 bits

There are Word operations which can be used to perform 16 bit operations on the upper 4 register pairs (25:24,27:26,29:28, 31:30)

adiw - add immediate to word

sbiw – subtract immediate to word

We can also use the carry bits for longer numbers – this requires some coding

adc – add with carry

sbc – subtract with carry

longloop:ldi XH,high($DEAD) ;put large

numberldi XL,low($DEAD) ;in X register

loop:sbiw XH:XL,1

;subtract 1 from Xbrne loop

;continue till 0ret


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Program Flow, subroutines, local variables

You will want to break your program up into small pieces or subroutines

You will need to set up loops to repeat small pieces of program

A special register called the stack pointer is used to call subroutines, and temporarily store registers when using subroutines

The simulator can help you understand what is happening in your program as well

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SRAM usageThe on-board RAM must hold the Stack Pointer

The Stack Pointer keeps track of where the program execution has been

Allows temporary storage of registers

SRAM also holds data variables

Typical to have Stack grow downwards and data grow upwards

Must avoid stack/data collisions!

; ***** Stack Pointer Setup Code ldi r16, $10 ; Stack Pointer

Setup out SPH,r16 ; Stack

Pointer High Byte ldi r16, $FF ; Stack Pointer

Setup out SPL,r16 ; Stack

Pointer Low Byte

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Subroutines:call,rcall,icallcall to a small unit of code which does a given task

Good Practice to push/pop any registers used in the subroutine

push r24 – stores r24 register byte at the location in the Stack Pointer (SP) then decrements the SP

pop r24 – loads r24 with the byte pointed to by SP then increments the SP

Must call ret at the end of the subroutine

main:rcall wait ; call our

subroutinerjmp main

wait: push r24 ; preserve r24

ldi r24,$FF ; loop: dec r24 ; count down bytes

brne loop ; keep going until finished

pop r24 ; restore r24

ret

• rcall mysub• PC+1 stored on

stack• PC is changed

to mysub:• ret

• PC is popped from the stack

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Functions:

Call to a small unit if code which does a given task

Can depend on input parameters – passed to the function in registers or on the stack

Good Practice to push/pop any registers used in the subroutine

Must call ret at the end of the subroutine

Result stored in a register

main:ldi r24,3 ; input

parameter in r24rcall mult10 ; call our

subroutineout PORTB,r24 ; put the answer on

the LEDrjmp main

mult10: push r25 ; preserve r25push r26 ; preserve

r26ldi r25,$0A ; we will

multiply by 10ldi r26,$00 ; sum

initialized to 0loop: add r26,r24 ; add r24 to the sum

dec r25brne loop ;

keep going until finishedmov r24,r26 ; put sum

in r24pop r26 ; restore

r26pop r25 ; restore

r25ret ;

return with answer in r24


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Stack Pointer – Examples, Call

SPH SPL

$10 $FF

Location

Data

$10FF 00

$10FE 00

$10FD 00

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe

26: 97 01 movw r18, r14…..;; our subroutine; fe: ef 92 push r14; do some things – and use r14; for whatever we want100:7c 01 movw r14, r24;13e: ef 90 pop

r14140: 08 95 ret

r14

$99

PC

$22


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SPH SPL

$10 $FD

Location

Data

$10FF 00

$10FE 26

$10FD 00

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$99

PC

$FE


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SPH SPL

$10 $FC

Location

Data

$10FF 00

$10FE 26

$10FD 99

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$99

PC

$100


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SPH SPL

$10 $FC

Location

Data

$10FF 00

$10FE 26

$10FD 99

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$xx

PC

$102


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SPH SPL

$10 $FC

Location

Data

$10FF 00

$10FE 26

$10FD 99

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$xx

PC

$13E


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SPH SPL

$10 $FD

Location

Data

$10FF 00

$10FE 26

$10FD 99

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$99

PC

$140


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SPH SPL

$10 $FF

Location

Data

$10FF 00

$10FE 26

$10FD 99

$10FC 00

$10FB 00

$10FA 00

$10F9 00

$10F8 00

$10F7 00

22: 0e 94 7f 00 call0xfe


r14140: 08 95 ret

r14

$99

PC

$26


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Exercises:

Exercise 1 : Last time you made a counter. Now use a subroutine to delay the speed of the counter.

Exercise 2 : You can delay further by cascading delay subroutines.

Exercise 3 : Memory Test Program: Write a program that writes (st) incrementing numbers to the SRAM. Read (ld) them back, compare with what you expect to see and if it is not correct send a pattern of lights to PORTB. What happens if you start writing before address $0100


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LED sequence display

Write a program that puts the contents of a table into program memory and then reads the contents back sequentially, byte by byte, and outputs them to the LEDs on PORTB. Chose the bytes so that they will make a pattern

when they are displayed on the LEDsUse delays to slow down the program so

you can see the different patterns flashing as they light the LEDS on PORTB. Allow the user to change the speed of flashing

using the switches on PORTD Allow the user to start/stop the pattern using

switches on PORTD


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Structured Programming

• Design your programme into a main control segment, which calls sub-routines • marks will be given on the modularity of your code• Before starting to write code make a simple design

diagram outlining the tasks which need to be done• Take 5-10 minutes, and show this to your

demonstrator• Use a Top Down Modular Programming approach to

design

• Subroutines should perform specific tasks or repeated functions

• Put lots of comments in your code


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Top Down Modular Programming

Reset

complexity

time

Calculator

Initialise Get command Calculate Output

Load values

Promptmessage Input Interpret

command Select/run

routine

Format output

Display

Outputcharacter

Inputcharacter

Output character

Outputcharacter

AVR Assembler

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