6. Der Code

6.1. Code Example 1 (Timer0)

This code toggles PD5 every 0.5s. The delay_timer0 subroutine uses Timer0 in CTC Mode with a 1024 prescaler and a compare value of 156, creating a 10ms hardware delay per call. This subroutine is called 50 times using software loop with R18 as the counter.

;------------------------
; Assembly Code
;------------------------
#define __SFR_OFFSET 0x00
#include "avr/io.h"
;------------------------
.global main
;===============================================================
    main:
    LDI R16, 0b00100000 ; to toggle PD5
    LDI R17, 0b00000000
    ;--------------------------------------------
    SBI DDRD, 5         ; set PD5 for o/p
    OUT PORTD, R17      ; PD5 = 0
    ;--------------------------------------------
    LDI R18, 50         ; set loop counter
l1: RCALL delay_timer0  ; apply delay via timer0
    DEC R18
    BRNE l1             ; & go back & repeat
    ;--------------------------------------------
    EOR R17, R16        ; R17 = R17 XOR R16
    OUT PORTD, R17      ; toggle PD5
    LDI R18, 50         ; re-set loop counter
    RJMP l1             ; go back & repeat toggle
;===============================================================
delay_timer0:           ; ~10ms delay via Timer0
    ;---------------------------------------------------------
    CLR R20
    OUT TCNT0, R20      ; initialize timer0 with count=0
    LDI R20, 156
    OUT OCR0A, R20      ; OCR0 = 9
    LDI R20, 0b00000010
    OUT TCCR0A, R20
    LDI R20, 0b00000101
    OUT TCCR0B, R20     ; timer0: CTC mode, prescaler 1024
    ;---------------------------------------------------------
l2: IN R20, TIFR0       ; get TIFR0 byte & check
    SBRS R20, OCF0A     ; if OCF0=1, skip next instruction
    RJMP l2             ; else, loop back & check OCF0 flag
    ;---------------------------------------------------------
    CLR R20
    OUT TCCR0B, R20     ; stop timer0
    ;---------------------------------------------------------
    LDI R20, (1<<OCF0A)
    OUT TIFR0, R20      ; clear OCF0 flag
    RET

6.2. Code Example 2 (Timer1)

This code toggles PD5 every 0.5s (just like Code Example 1). The delay_timer1 subroutine uses Timer1 in Normal Mode with a 1024 prescaler and a preload value of 57724, creating a 500ms hardware delay per call.

;------------------------
; Assembly Code
;------------------------
#define __SFR_OFFSET 0x00
#include "avr/io.h"
;------------------------
.global main
;===============================================================
main:
    LDI R16, 0b00100000 ; to toggle PD5
    LDI R17, 0b00000000
    ;---------------------------------------------
    SBI DDRD, 5         ; set PD5 for o/p
    OUT PORTD, R17      ; PD5 = 0
    ;---------------------------------------------
l1: RCALL delay_timer1  ; 0.5 sec delay via timer1
    ;---------------------------------------------
    EOR R17, R16        ; R17 = R17 XOR R16
    OUT PORTD, R17      ; toggle PD5
    LDI R18, 61         ; re-set loop counter
    RJMP l1             ; go back & repeat toggle
;===============================================================
delay_timer1:           ; 0.5 sec delay via timer1
;-------------------------------------------------------
    .EQU value, 57724   ; value to give 0.5 sec delay
    LDI R20, hi8(value)
    STS TCNT1H, R20
    LDI R20, lo8(value)
    STS TCNT1L, R20     ; initialize counter TCNT1 = value
    ;-------------------------------------------------------
    LDI R20, 0b00000000
    STS TCCR1A, R20
    LDI R20, 0b00000101
    STS TCCR1B, R20     ; normal mode, prescaler = 1024
    ;-------------------------------------------------------
l2: IN R20, TIFR1       ; get TIFR1 byte & check
    SBRS R20, TOV1      ; if TOV1=1, skip next instruction
    RJMP l2             ; else, loop back & check TOV1 flag
    ;-------------------------------------------------------
    LDI R20, 1<<TOV1
    OUT TIFR1, R20      ; clear TOV1 flag
    ;-------------------------------------------------------
    LDI R20, 0b00000000
    STS TCCR1B, R20     ; stop timer1
    RET