videochargen/main.S

#include <avr/io.h>
#include "macro.h"
#include "const.h"

.text

.global main
main:
    ldi r16, 0x30       ; port B, pin 4 and 5 as output
    sts DDRB, r16

    ; set interrupt vectors at address 0x0, not bootloader
    ; timing is important, see atmel datasheet
    ldi r16, (1 << IVCE)
    ldi r17, 0
    out IO(MCUCR), r16
    out IO(MCUCR), r17

    ldi r16, 0xa        ; external interrupt 0 and 1, falling edge
    sts EICRA, r16

    ldi r16, 0x3        ; external interrupt 0 and 1, mask enable
    sts EIMSK, r16

    ldi r16, 0x02       ; don't connect output pins to timer, CTC[1:0] mode
    sts TCCR0A, r16
    ldi r16, 0x01       ; CTC[2] mode, no prescaler
    sts TCCR0B, r16

    ; init variables
    ldi r16, 0
    sts frame, r16
    sts frame + 1, r16
    sts line + 1, r16
    ldi r16, 1
    sts line, r16

    call setup_c

    sei                 ; global interrupt enable

1:
    call loop_c
    rjmp 1b

.global int_horizontal_sync
int_horizontal_sync:    ; +3
    push r31            ; +5
    in r31, IO(SREG)    ; +6, status register
    push r31            ; +8
    push r30            ; +10

    ; if (line >= VERTICAL_OFFSET), then enter
    lds r30, line       ; +12
    lds r31, line + 1   ; +14
    adiw z, 1           ; +16
    sts line, r30       ; +18
    sts line + 1, r31   ; +20
    cpi r31, 0          ; +21
    brne enter                      ; +22, +23
    cpi r30, VERTICAL_OFFSET        ; +23
    brlo int_horizontal_sync_end    ; +24

enter:
    ; here, +23 or +24 cycles have passed since horizontal sync
    ; so, there are still ~168 cycles before first useful data
    ldi r31, 0
    sts TCNT0, r31
    ldi r31, HORIZONTAL_OFFSET  ; set counter TOP
    sts OCR0A, r31

    ldi r31, 0x7        ; clear any pending interrupt
    sts TIFR0, r31

    lds r31, TIMSK0
    ori r31, 0x02       ; mask enable interrupt timer A
    sts TIMSK0, r31

int_horizontal_sync_end:
    pop r30
    pop r31
    out IO(SREG), r31
    pop r31
    reti

.global int_timer_0
int_timer_0:
    ; here we are at the beginning of the visible line
    push r31
    in r31, IO(SREG)
    push r31
    push r30
    push r29

    ; turn off interrupt
    lds r31, TIMSK0
    andi r31, 0xfd      ; mask disable interrupt timer A
    sts TIMSK0, r31

    ; draw things
    ldi r30, lo8(line_buffer)
    ldi r31, hi8(line_buffer)

.rept LINE_BUFFER_SIZE
    ld r29, z+
    out IO(PORTB), r29
    nop
.endr

    pop r29
    pop r30
    pop r31
    out IO(SREG), r31
    pop r31
    reti

.global int_vertical_sync
int_vertical_sync:
    push r31
    in r31, IO(SREG)
    push r31
    push r30

    lds r31, frame + 1
    lds r30, frame
    adiw z, 1
    sts frame + 1, r31
    sts frame, r30

    ldi r30, 1
    ldi r31, 0
    sts line, r30
    sts line + 1, r31

int_vertical_sync_end:
    pop r30
    pop r31
    out IO(SREG), r31
    pop r31
    reti