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;

ISR(INT0_vect, ISR_NAKED) {
  // vertical sync interrupt
  asm("jmp int_vertical_sync");
}
ISR(INT1_vect, ISR_NAKED) {
  // horizontal sync interrupt
  asm("jmp int_horizontal_sync");
}
ISR(TIMER0_COMPA_vect, ISR_NAKED) {
  // back porch timer interrupt
  asm("jmp int_timer_0");
}

// 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

  TCCR0A = 0x02; // don't connect output pins to timer, CTC[1:0] mode

  TCCR0B = 0x01; // CTC[2] mode, no prescaler

  main_asm();
}