;
; vga_test.asm
;
; Created: 2/20/2016 6:44:02 PM
; Author : jimlkosmo
;
.org 0
rjmp RESET ;reset 
.org OC1Aaddr
rjmp TIM1_COMPA ;isr for tc1 Compare Match A
.org OC1Baddr
rjmp TIM1_COMPB ;isr for tc1 Compare Match B

RESET:
sbi ddrb,4 ; HORIZONTAL SYNC PULSE
sbi ddrb,3 ;red
sbi ddrb,2 ;green
sbi ddrb,0 ;blue
sbi ddrb,1 ; VERTICAL SYNC PULSE
ldi r16,(1<<PORTB4); H_SYNC PIN HIGH
out PORTB,r16
in r16,PORTB; V_SYNC PIN LOW
sbr r16,(0<<PORTB1)
out PORTB,r16

;stack_pointer
ldi r16,ramend
out spl,r16

;setup interrupts
clr r16
ldi r16,(1<<COM1B0); Toggle in OC1B when Comp Match on OCR1B
out TCCR1A,r16
ldi r16,(1<<CS10 | 1<<WGM12) ;CTC on OCR1A Comp Match and set CLKin 
out TCCR1B,r16
ldi r16,(1<<OCIE1A | 1<<OCIE1B) ;TC1 Output Comp Match Interrupt Enabled on A and B channel
out timsk,r16
ldi r16,high(635); Load Max for TC1
out ocr1ah,r16
ldi r16,low(635)
out ocr1al,r16
ldi r16,0
out tcnt1h,r16; clear tc1
out tcnt1l,r16
ldi r16,high(12); Load first toggle value for H_SYNC (13 clock pulses)
out ocr1bh,r16
ldi r16,low(12)
out ocr1bl,r16
ldi r16,0
out tcnt1h,r16; clear tc1
out tcnt1l,r16
ldi r18,10; Load r18 with the first value of vsync toggle
ldi r19,44 ;[1] load r19 with the value of the first visible line
ldi r20,0 ;[1]
ldi r21,low(126); load r21,r22 with the value of the first visible pixel
ldi r22,high(126)
clr r28
clr r29
clr r16
sei; interrupts globally enabled
main:

cp r28,r19 ;[1]
cpc r29,r20 ;[1]
brlo no_vid ;[1/2] Are we in vertical visible area?
in r26,tcnt1l;
in r27,tcnt1h
cp r26,r21 ;[1]
cpc r24,r22 ;[1]
brlo no_vid ;[1/2] Are we in horizontal visible area?
in r16,PORTB; white space
sbr r16,(1<<PORTB0)|(1<<PORTB2)|(1<<PORTB3)
out PORTB,r16

rjmp main

no_vid:
in r16,PORTB; black space
sbr r16,(0<<PORTB0)|(0<<PORTB2)|(0<<PORTB3)
out PORTB,r16
rjmp main


TIM1_COMPA:
in r16,sreg ; SAVE STATUS REGISTER [1]
push r16;[2]
ldi r16,low(12); Load first toggle value for H_SYNC (13 clock pulses) [1]
ldi r17,high(12);
out ocr1bh,r17;[2]
out ocr1bl,r16
adiw r29:r28,1 ;[2] V_SYNC counter++
ldi r17,0 ;[1]
cp r18,r28 ;[1]
cpc r17,r29 ;[1]
breq vsync_toggle ;[1/2] If we got 10 or 12 lines
ldi r16,low(525) ;[1]
ldi r17,high(525) ;[1]
cp r16,r28 ;[1]
cpc r17,r29 ;[1]
breq cls_vsync ;[1/2] If we got 525 lines
cont:
pop r16 ;[2] RESTORE sreg
out sreg,r16 ;1
reti ;[4] return operation to software

TIM1_COMPB:
in r16,sreg ; SAVE STATUS REGISTER [1]
push r16;[2]
ldi r16,low(87); Load second toggle value for H_SYNC (13+76 clock pulses)
out ocr1bl,r16
pop r16 ;[2] RESTORE sreg
out sreg,r16 ;1
reti ;[4] return operation to software

vsync_toggle:

sbi PINB,1;toggle v_sync
ldi r18,12 ;set r18 with 12
rjmp cont

cls_vsync:
ldi r18,10 ;set r18 with 10
ldi r28,0; reset v_sync counter
ldi r29,0
rjmp cont