videochargen/main.c

/**
 * @file main.c
 * @author giomba@glgprograms.it
 * @brief OSDY main.
 *
 * @copyright Copyright RetrOfficina GLG Programs (c) 2022
 *
 * Video image generator superimposition for analog PAL signals,
 * with Atmega328, for your retro OSD titles.
 *
 */
#include "const.h"
#include <avr/interrupt.h>
#include <avr/io.h>
#include <string.h>

volatile uint16_t frame = 0;
volatile uint16_t line = 0xfe;
volatile uint8_t image = 0xff;

// 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_PIXEL_HEIGHT - SPRITE_HEIGHT)
    vinc = -1;
  if (hpos == BACK_PORCH_OFFSET)
    hinc = +1;
  if (hpos ==
      FRAME_PIXEL_WIDTH - SPRITE_WIDTH - FRONT_PORCH_OFFSET - HSYNC_TUNE)
    hinc = -1;

  int_vertical_sync_s();
}

// 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_HEIGHT)
    return;

  if (line >= vpos)
    int_horizontal_sync_s();
}

ISR(TIMER1_COMPA_vect, ISR_NAKED) {
  // end-of back porch timer interrupt
  asm("jmp int_timer_1");
}

// external assembly main loop
void main_asm(void);

int main() {
  // port B, pin 4 and 5 as output, others as input
  DDRB |= 0x30;

  // set interrupt vectors at address 0x0, not bootloader
  // timing is important, see atmel datasheet
  _SFR_IO8(MCUCR) = (1 << IVCE);
  _SFR_IO8(MCUCR) = 0;

  EICRA = 0x0a; // external interrupt 0 and 1, falling edge
  EIMSK = 0x03; // external interrupt 0 and 1, mask enable

  TCCR1A = 0x00; // don't connect output pins to timer, CTC[1:0] mode
  TCCR1B = 0x09; // CTC[3:2] mode, no prescaler

  main_asm();
}