#include <sync_gen_12F1571.h>

#define  sync     PIN_A0   //TV Synchronism Output

#use fast_io(A)

int image_block=0;
int line_count=0;
int field=1;
int interlace=1;

#INT_TIMER0
void  TIMER0_isr(void) {
   set_timer0(13);    //Since the timer overflows at 64.0 us, this offset
   output_low(sync);  //creates a 63.5 us overflow (horizontal period: H).
   switch (image_block){         
      case 0:  line_count++;        //Pre-equalizing pulses: 2.3 us, spaced 1/2  
               delay_cycles(11);    //horizontal period. Total: 6 pulses (3H).
               output_high(sync);   
               delay_us(29);
               delay_cycles(2);
               output_low(sync);
               delay_cycles(18);
               output_high(sync);
               if(line_count==3){
                  line_count=0;
                  image_block=1;
               }
               break;
      case 1:  line_count++;     //Vertical synchronism, including 6 serrated
               delay_us(24);     //pulses, spaced 1/2H. Total duration: 3H.
               delay_cycles(3); 
               output_high(sync);
               delay_cycles(47);
               output_low(sync);
               delay_us(25);     
               delay_cycles(5);  
               output_high(sync);
               if(line_count==3){
                  line_count=0;
                  image_block=2;
               }
               break;
      case 2:  line_count++;        //Post-equalizing pulses: 2.3 us, spaced 1/2
               delay_cycles(5);     //horizontal period. Total: 6 pulses (3H).
               output_high(sync);
               delay_us(29);
               delay_cycles(2);
               output_low(sync);
               delay_cycles(18);
               output_high(sync);
               if(line_count==3){   
                  line_count=0;  //Next horizontal synchronism pulse will arrive
                  image_block=3; //at 1/2H if on Field 1. Otherwise (Field 2)
                  if(field==2){  //it will arrive at 1H (interlaced scanning).
                     set_timer0(31);
                     line_count=1;
                  }
               }
               break;
      case 3:  line_count++;     //Horizontal synchronism pulses: 4.7 us, spaced
               delay_cycles(20); //1 H.
               output_high(sync);
               if(line_count==254){
                  line_count=0;  //Last horizontal line of Field 1 (line 263)
                  image_block=0; //is just 1/2H (interlaced scanning).
                  if(interlace==1){
                     if(field==1){
                        set_timer0(171);
                     }   
                     field=3-field;
                  }
                  else field=1;
               }
               break;
      default: break;
   }      
}

void main() {
   setup_timer_0(RTCC_INTERNAL|RTCC_DIV_2|RTCC_8_BIT);      //64.0 us overflow
  
   SET_TRIS_A(0x00);
   output_low(sync);

   enable_interrupts(INT_TIMER0);
   enable_interrupts(GLOBAL);

   while(TRUE);
}