ISR.c

// note, rtty_txbit() function will be replaced by directly operating pins, just using to save space for now

current_byte = "E";

ISR(TIMER1_COMPA_vect)
{

  if (current_bit_count == 0){ // if we're about to send a new byte...
    rtty_txbit(0); // tx a start bit
  }
  
  else if (current_bit_count == 8){ // if we've sent all the bits in the byte
    
    current_bit_count = 0; // reset bit count for next byte transmission
    
    // send two stop bits
    rtty_txbit(1);
    rtty_txbit(1);
    
  }
  
  else {
    if (current_byte & 1){ // if the first bit of current_byte is a 1...
      rtty_txbit(1);
    } 
    else {
      rtty_txbit(0);
    }
    
    current_bit_count++;
    current_byte = current_byte << 1; // shift all bits in byte by 1 so first bit of byte is actually the next bit

}