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;
;	Author: MLH
;	Date: 9 May 2007
;	Purpose:
;		This example demonstrates the use of the Timer/Counter subsystem.
;
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.NOLIST
.include "m32def.inc"
.LIST

; data segment, reserve data memory in SRAM
.DSEG
overflow_counter: .byte 1	; this byte stores the number of Counter0 overflows

; code segment for program instructions
.CSEG
.ORG 0
	rjmp start

.ORG 0x14				; Output Comparator vector
	rjmp comparator_matchISR
.ORG 0x16				; Counter0 Overflow vector
	rjmp counter0_overflowISR

.DEF temp=r18			; use arbitrary register for temporary operations
.org 0x2a
start:
; Initialize the Stack Pointer (SP) CPU register for subsequent use
	ldi		temp, LOW(RAMEND)	; get low byte of the address of the highest byte in Data Memory we can use
	out		SPL, temp			; load it into the low byte of SP
	ldi		temp, HIGH(RAMEND)	; high byte of address of highest Data Memory byte
	out		SPH, temp			; load it into the high byte of SP

	ldi 	temp, 0xFF			; setup PortB for digital output
	out 	DDRB, temp
	andi	temp, 0x00
	out		PORTB, temp			; turn LEDs off

	clr		temp
	sts		overflow_counter, temp	; initialize overflow_counter to 0

; initialize Timer/Counter0
	out		TCNT0, temp			; clear Counter0 counter register; this is what Counter0 increments
	in		temp, TCCR0			; get current state of Timer/Counter0 Control Register
	andi	temp, 0b11111000	; clear CS02:CS00 that control frequency
	ori		temp, 0b00000100	; set CS02:CS00 to increment Counter0 every 256 CPU clock cycles (16us @ 16MHz)
	out		TCCR0, temp
; counter is now running, TCNT0 is incrementing automatically every 16us

; next, enable Counter0 interrupts
	in		temp, TIMSK			; get state of Timer/Counter Interrupt Mask
	andi	temp, 0b11111100	; clear TOIE0 and OCIE0; keep other bits 
	ori		temp, 0b00000001	; set bit TOIE0 to enable Counter0 overflow interrupt
;	ori		temp, 0b00000010	; set bit OCIE0 to enable Output Comparator interrupt
	out		TIMSK, temp

	sei							; this enables interrupts globally
	
loop:
	rjmp 	loop				; loop forever, waiting for Timer/Counter interrupts


; interrupt handler counter0_overflowISR
; Purpose: implements the handling of the Timer/Counter0 overflow interrupt
; Every time the Counter0 overflow interrupt occurs, the overflow_counter (a byte in data memory)
; is incremented. The Counter0 is incremented every 256 clock cycles (16us), which
; corresponds to an overflow every 4096us, or 4.096ms. This means 244 overflows takes
; 999.424ms, or 0.999424s. After counting this many overflows, the LEDs are blinked.
counter0_overflowISR:
	push	temp

	lds		temp, overflow_counter	; load last-stored value of overflow_counter from data memory 
	inc		temp				; increment value of overflow_counter in temp register
	cpi		temp, 244			; check if overflow_counter has reached 244
	breq	reset				; if so, reset the overflow_counter to 0

	sts		overflow_counter, temp	; store incremented value of overflow_counter

	clr		temp				; all bits 0
	out		PORTB, temp
	rjmp	end_ISR

reset:							; reset the overflow_counter to 0
	clr		temp				
	com		temp				; all bits 1
	out		PORTB, temp			; toggle LEDs
	clr		temp
	sts		overflow_counter, temp	; here's where we actually clear the overflow_counter

end_ISR:
	pop		temp				; restore temporary registers
	reti
; end interrupt handler counter0_overflowISR

; interrupt handler comparator_matchISR
; Purpose: implements the handling of the Output Comparator match
comparator_matchISR:
// TODO: implement if using Output Comparator
	reti
; end interrupt handler comparator_matchISR