;
; Lab1-b assembler program
;
.include "m8515def.inc"  ; include the ATMEGA8515(L) definitions file
.org $000
rjmp RESET
.org $007  ; set start address to Timer 0 overflow 
rjmp ISRTIME  ; jump if timer0 overflows
.def outval = r16  ; labelling regisers
.def temp = r17  ;
.def counter = r18  ;
RESET:
; Initialise the stack pointer (standard method)
ldi temp,low(RAMEND)  ; lower half
out SPL,temp 
ldi temp,high(RAMEND)  ; upper half
out SPH,temp
; set up portb
ser temp
out DDRB, temp  ; set all pins of PORTB as outputs
ldi outval, 0xAA  ; set to zero
out PORTB, outval  ; output to port B
; initialize timer 0
ldi temp, 0x05 
out TCCR0, temp  ; set timer 0 prescaling to lowest /1024
clr temp  ; clear all bits
out TCNT0, temp  ; clear the timer
; setup timer 0's overflow interrupt
in temp, TIMSK  ; load Timer Interrupt Mask Register
sbr temp, 1<<TOIE0  ; set the timer 0 overflow interrupt enable bit
out TIMSK, temp  ; save Timer Interrupt Mask Register
ldi counter, 0x04
sei  ; enable interrupts
LOOP:
; do nothing, just loop forever(waiting for timer0 overflow)
rjmp LOOP
ISRTIME:
; interrupt service routine
; is executed every 2^16 pre-scaled cycles
dec counter
cpi counter, 0x00
brne BACK
com outval ; Toggle ODD and EVEN Pins
out PORTB, outval  ; output to port B
ldi  counter, 0x04
BACK:
reti  ; return from interrupt (different to normal return)