do using polling
actually this does not work, because polling requires:
in
andi
brne
which takes 4 clock cycles, that, on average, means a 2 clock offset,
while interrupts occur during a rjmp, which takes 2 clocks, which means,
on average, only 1 clock cycle of offset
I am not interested in higher or lower overhead: I am just interested in
minimizing its variance.
Being late of a few clock cycles, is not a problem: it just means that
the picture will be slightly shifted toward the right border; but,
starting the picture 1 or 2 clock cycles before (on one line), and 1 or
2 clock cycles after, on another line, means that it just wiggles sligthly
left and right
This commit is contained in:
parent
53da6b7171
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d91201dcfd
139
main.S
139
main.S
@ -8,24 +8,8 @@
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main:
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ldi r16, 0x30 ; port B, pin 4 and 5 as output
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sts DDRB, r16
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; set interrupt vectors at address 0x0, not bootloader
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; timing is important, see atmel datasheet
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ldi r16, (1 << IVCE)
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ldi r17, 0
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out IO(MCUCR), r16
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out IO(MCUCR), r17
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ldi r16, 0xa ; external interrupt 0 and 1, falling edge
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sts EICRA, r16
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ldi r16, 0x3 ; external interrupt 0 and 1, mask enable
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sts EIMSK, r16
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ldi r16, 0x02 ; don't connect output pins to timer, CTC[1:0] mode
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sts TCCR0A, r16
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ldi r16, 0x01 ; CTC[2] mode, no prescaler
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sts TCCR0B, r16
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cbi IO(DDRD), 2 ; port D, pin 2 and 3 as input
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cbi IO(DDRD), 3
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; init variables
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ldi r16, 0
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@ -35,85 +19,68 @@ main:
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ldi r16, 1
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sts line, r16
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cli ; global interrupt disable
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call setup_c
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sei ; global interrupt enable
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1:
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call loop_c
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rjmp 1b
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.global int_horizontal_sync
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int_horizontal_sync: ; +3
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push r31 ; +5
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in r31, IO(SREG) ; +6, status register
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push r31 ; +8
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push r30 ; +10
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wait_next_line:
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in r16, IO(PIND)
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andi r16, 0x08
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brne wait_next_line
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/*
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; if (line >= VERTICAL_OFFSET), then enter
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lds r30, line ; +12
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lds r31, line + 1 ; +14
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adiw z, 1 ; +16
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sts line, r30 ; +18
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sts line + 1, r31 ; +20
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cpi r31, 0 ; +21
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brne enter ; +22, +23
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cpi r30, VERTICAL_OFFSET ; +23
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brlo int_horizontal_sync_end ; +24
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lds r30, line ; +2, 2
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lds r31, line + 1 ; +2, 4
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adiw z, 1 ; +2, 6
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sts line, r30 ; +2, 8
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sts line + 1, r31 ; +2, 10
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cpi r31, 0 ; +1, 11
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brne enter ; +2, 13
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cpi r30, VERTICAL_OFFSET ; TODO timing bad at 256 lines
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brsh enter
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jmp horizontal_line_end
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*/
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enter:
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; here, +23 or +24 cycles have passed since horizontal sync
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; so, there are still ~168 cycles before first useful data
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ldi r31, 0
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sts TCNT0, r31
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ldi r31, HORIZONTAL_OFFSET ; set counter TOP
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sts OCR0A, r31
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; here, +13 cycles have passed since horizontal sync
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; so, there are still (192 - 13) = 179 cycles before first useful data
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ldi r31, 64 ; approx
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1:
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dec r31 ; 1
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brne 1b ; 2
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ldi r31, 0x7 ; clear any pending interrupt
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sts TIFR0, r31
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lds r31, TIMSK0
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ori r31, 0x02 ; mask enable interrupt timer A
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sts TIMSK0, r31
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int_horizontal_sync_end:
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pop r30
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pop r31
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out IO(SREG), r31
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pop r31
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reti
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.global int_timer_0
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int_timer_0:
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; here we are at the beginning of the visible line
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push r31
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in r31, IO(SREG)
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push r31
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push r30
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push r29
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ldi r31, 16
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1:
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dec r31
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brne 1b
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; turn off interrupt
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lds r31, TIMSK0
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andi r31, 0xfd ; mask disable interrupt timer A
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sts TIMSK0, r31
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sbi IO(PORTB), 5
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; draw things
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ldi r30, lo8(line_buffer)
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ldi r31, hi8(line_buffer)
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.rept LINE_BUFFER_SIZE
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ld r29, z+
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out IO(PORTB), r29
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sbi IO(PORTB), 4
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nop
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.endr
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nop
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nop
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nop
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cbi IO(PORTB), 4
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pop r29
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pop r30
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pop r31
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out IO(SREG), r31
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pop r31
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reti
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# ; draw things
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# ldi r30, lo8(line_buffer)
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# ldi r31, hi8(line_buffer)
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#.rept LINE_BUFFER_SIZE
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# ld r29, z+
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# out IO(PORTB), r29
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# nop
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#.endr
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cbi IO(PORTB), 5
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horizontal_line_end:
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jmp wait_next_line
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/*
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.global int_vertical_sync
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int_vertical_sync:
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push r31
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@ -139,4 +106,4 @@ int_vertical_sync_end:
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pop r31
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reti
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*/
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21
main.c
21
main.c
@ -7,23 +7,13 @@ volatile uint16_t frame;
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volatile uint16_t line;
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volatile char line_buffer[LINE_BUFFER_SIZE];
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ISR(INT0_vect, ISR_NAKED) {
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asm("jmp int_vertical_sync");
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}
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ISR(INT1_vect, ISR_NAKED) {
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asm("jmp int_horizontal_sync");
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}
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ISR(TIMER0_COMPA_vect, ISR_NAKED) {
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asm("jmp int_timer_0");
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void setup_c() {
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for (int i = 0; i < LINE_BUFFER_SIZE; ++i) {
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line_buffer[i] = (i > 50 && i < 100) ? '1' : 'A';
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}
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}
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void setup_c() {
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/*
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for (int i = 0; i < LINE_BUFFER_SIZE; ++i) {
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line_buffer[i] = (i % 2) ? 0x0 : 0xff;
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}
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*/
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}
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/*
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void loop_c() {
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for (;;) {
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@ -53,3 +43,4 @@ void loop_c() {
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}
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}
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}
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*/
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