Add documentation.

const.h

@@ -1,8 +1,21 @@
 #pragma once
 
-#define VERTICAL_OFFSET 30
-#define HORIZONTAL_OFFSET 192
-#define LINE_BUFFER_SIZE 160
+#define FRAME_SYSCLOCK_WIDTH 1024 // [sysclock]
+#define FRAME_LINE_HEIGHT 256     // [line]
 
-#define BLACK 0x00
-#define WHITE 0xff
+#define BACK_PORCH_SYSCLOCK_OFFSET 80  // [sysclock]
+#define FRONT_PORCH_SYSCLOCK_OFFSET 40 // [sysclock]
+
+#define SPRITE_PIXEL_WIDTH 80 // [sysclock]
+#define SPRITE_LINE_HEIGHT 64 // [line]
+
+#define SYSCLOCK_PER_PIXEL 4
+#define SPRITE_SYSCLOCK_WIDTH (SPRITE_PIXEL_WIDTH * SYSCLOCK_PER_PIXEL)
+
+// Time spent from the horizontal sync pulse,
+// to the first displayed dot.
+// This is given as the sum of:
+// + HSYNC_INT_TUNE
+// + HSYNC_TIMER_TUNE
+// + sysclock duration of "C" interrupt part
+#define HSYNC_SYSCLOCK_TUNE 70 // [sysclock] (approx)

main.S

@@ -36,80 +36,82 @@ main_asm:
 1:
     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
+.global int_horizontal_sync_s
+int_horizontal_sync_s:
+    push r31            ; +2
+    push r30            ; +2
 
-    ; 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
-    breq enter          ; +23
-    jmp int_horizontal_sync_end
+    ; load timer to trigger isr at the beginning of the visible scanline
+    ; reset counter to 0
+    ldi r31, 0          ; +1
+    sts TCNT1H, r31     ; +2
+    sts TCNT1L, r31     ; +2
 
-enter:
-    ; here, +23 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
+    ; set counter TOP (MSB first!)
+    lds r31, hpos + 1   ; +2
+    lds r30, hpos       ; +2
+    sts OCR1AH, r31     ; +2
+    sts OCR1AL, r30     ; +2
 
-    ldi r31, 0x7        ; clear any pending interrupt
-    sts TIFR0, r31
+    ; clear any pending interrupt
+    ldi r31, 0x7        ; +1
+    sts TIFR1, r31      ; +2
 
-    lds r31, TIMSK0
-    ori r31, 0x02       ; mask enable interrupt timer A
-    sts TIMSK0, r31
+    ; enable interrupt timer A
+    lds r31, TIMSK1     ; +2
+    ori r31, 0x02       ; +1
+    sts TIMSK1, r31     ; +2
+
+                        ; total: 25
+                        ; HSYNC_INT_TUNE = 25
+                        ; this adds up for HSYNC_SYSCLOCK_TUNE
 
-int_horizontal_sync_end:
     pop r30
     pop r31
-    out IO(SREG), r31
-    pop r31
-    reti
+    ret
 
-.global int_timer_0
-int_timer_0:
+.global int_timer_1
+int_timer_1:
     ; here we are at the beginning of the visible line
-    push r31
-    in r31, IO(SREG)
-    push r31
-    push r30
-    push r29
+    ; jmp (from C)      ; +3
+    push r31            ; +2
+    in r31, IO(SREG)    ; +1
+    push r31            ; +2
+    push r30            ; +2
+    push r29            ; +2
+    push r25            ; +2
 
     ; turn off interrupt
-    lds r31, TIMSK0
-    andi r31, 0xfd      ; mask disable interrupt timer A
-    sts TIMSK0, r31
+    lds r31, TIMSK1     ; +2
+    andi r31, 0xfd      ; mask disable interrupt timer A, +1
+    sts TIMSK1, r31     ; +2
 
-    lds r31, show_image
-    cpi r31, 0x1
-    brne jump_table_return_address
+    lds r31, show_image ; +2
+    cpi r31, 0x1        ; +1
+    brne jump_table_return_address  ; +1 (not taken)
 
-    lds zl, current_jump_table
-    lds zh, current_jump_table + 1
+    lds zl, current_jump_table      ; +2
+    lds zh, current_jump_table + 1  ; +2
 
-;    ldi zl, pm_lo8(line_jump_table_0)
-;    ldi zh, pm_hi8(line_jump_table_0)
+    clr r0              ; +1
+    lds r29, line       ; +2
+    lds r25, vpos       ; +2
+    sub r29, r25        ; +1
 
-    clr r0
-    lds r29, line
-    add zl, r29
-    adc zh, r0
-    add zl, r29
-    adc zh, r0
+    add zl, r29         ; +1
+    adc zh, r0          ; +1
+    add zl, r29         ; +1
+    adc zh, r0          ; +1
 
-    ijmp
+    ijmp                ; +2
+
+                        ; total: 39
+                        ; HSYNC_TIMER_TUNE = 39
+                        ; this adds up for HSYNC_SYSCLOCK_TUNE
 
 .global jump_table_return_address
 jump_table_return_address:
+    pop r25
     pop r29
     pop r30
     pop r31
@@ -117,8 +119,8 @@ jump_table_return_address:
     pop r31
     reti
 
-.global int_vertical_sync
-int_vertical_sync:
+.global int_vertical_sync_s
+int_vertical_sync_s:
     push r31
     in r31, IO(SREG)
     push zl
@@ -126,12 +128,6 @@ int_vertical_sync:
     push yl
     push yh
 
-    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

main.c

@@ -18,17 +18,55 @@ volatile uint16_t frame = 0;
 volatile uint16_t line = 0xfe;
 volatile uint8_t image = 0xff;
 
-ISR(INT0_vect, ISR_NAKED) {
-  // vertical sync interrupt
-  asm("jmp int_vertical_sync");
+// horizontal and vertical position
+// (and current increments)
+// of bouncing sprite
+volatile uint8_t vpos = 0;
+volatile uint16_t hpos = 0;
+volatile int8_t vinc = +1;
+volatile int8_t hinc = +1;
+
+// vertical sync interrupt
+void int_vertical_sync_s(void);
+ISR(INT0_vect) {
+  // count frames
+  frame += 1;
+
+  // move sprite
+  vpos += vinc;
+  hpos += hinc;
+
+  // invert bouncing direction
+  if (vpos == 0)
+    vinc = +1;
+  if (vpos == FRAME_LINE_HEIGHT - SPRITE_LINE_HEIGHT)
+    vinc = -1;
+  if (hpos == BACK_PORCH_SYSCLOCK_OFFSET)
+    hinc = +1;
+  if (hpos == FRAME_SYSCLOCK_WIDTH - SPRITE_SYSCLOCK_WIDTH -
+                  FRONT_PORCH_SYSCLOCK_OFFSET - HSYNC_SYSCLOCK_TUNE)
+    hinc = -1;
+
+  int_vertical_sync_s();
 }
-ISR(INT1_vect, ISR_NAKED) {
-  // horizontal sync interrupt
-  asm("jmp int_horizontal_sync");
+
+// horizontal sync interrupt
+void int_horizontal_sync_s(void);
+ISR(INT1_vect) {
+  // count lines
+  line += 1;
+
+  // we are past the last line of the sprite
+  if (line >= vpos + SPRITE_LINE_HEIGHT)
+    return;
+
+  if (line >= vpos)
+    int_horizontal_sync_s();
 }
-ISR(TIMER0_COMPA_vect, ISR_NAKED) {
-  // back porch timer interrupt
-  asm("jmp int_timer_0");
+
+ISR(TIMER1_COMPA_vect, ISR_NAKED) {
+  // end-of back porch timer interrupt
+  asm("jmp int_timer_1");
 }
 
 // external assembly main loop
@@ -46,9 +84,8 @@ int main() {
   EICRA = 0x0a; // external interrupt 0 and 1, falling edge
   EIMSK = 0x03; // external interrupt 0 and 1, mask enable
 
-  TCCR0A = 0x02; // don't connect output pins to timer, CTC[1:0] mode
-
-  TCCR0B = 0x01; // CTC[2] mode, no prescaler
+  TCCR1A = 0x00; // don't connect output pins to timer, CTC[1:0] mode
+  TCCR1B = 0x09; // CTC[3:2] mode, no prescaler
 
   main_asm();
 }

samples/bouncing-logo.pbm

@@ -0,0 +1,77 @@
+P1
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