interrupt.ino

void loop(){
  // continuously updating individual telemetry fields
  
  if (sentence_needed == true){
    sprintf(buffer, "%s, %d, %s .. ", callsign, sentence_id, latitude ..); // store all telemetry in buffer
    *ptr = buffer; // set data at location being pointed to, to the buffer
  }
}


ISR(Timer1_vect){
	
  switch (tx_status){
    
    case 0: // when the next byte needs to be gotten
      if (ptr){ // if the pointer is valid
        if (currentbyte){ // if there exists a byte at the pointer
          tx_status = 1;
          sentence_needed = False;
        }
        else {
          sentence_needed = true;
          break;
        }
      }
      else {
        sentence_needed = true;
        break;
      }
    
    case 1: // first bit about to be sent
      rtty_txbit(0); // send start bit
      tx_status = 2;
      currentbitcount = 1; // set bit count to 0 ready for incrementing to 7 for last bit of a ASCII-7 byte
      break;
			
    case 2: // normal status, transmitting bits of byte (including first and last)
      rtty_txbit(currentbyte & 1); // transmit the bit, 1 or 0 appropriately
			
      if (currentbitcount == 7){ // if we've just transmitted the final bit of the byte
        tx_status = 3;
      }
      
      currentbyte = currentbyte >> 1; // shift all bits in byte 1 to right so next bit is LSB
      currentbitcount++;
      break;
			
    case 3: // if all bits have been transmitted and we need to send the first of two stop bits
      rtty_txbit(1); // send first stop bit
      tx_status = 4;
      break;
			
    case 4: // ready to send the last of two stop bits
      rtty_txbit(1); // send the final stop bit
      ptr++; // increment the pointer for reading next byte in buffer
      tx_status = 0;
      break;
      
    }

}