/*The following code can be added to you sketch to allow the use of
the Timer interrupt to read a GPS character ~1 millisecond*/

boolean usingInterrupt = false;   
/* This variable that is changed when the interrupt is turned on and off.
It can be used to check if there is an interrupt is on or off*/

ISR(TIMER0_COMPA_vect) {
/******* Interrupt Service Routine ************************************************
 *  This is the interrupt service routine. It reads a character from the GPS.
 **********************************************************************************/
  GPS.read();      // Read a character from the GPS
}

void useInterrupt(boolean v) {
/******* useInterrupt *************************************************************
  This function is enables or disables the Timer interrupt.
  So 
 **********************************************************************************/
  /* millis() uses Timer0, so we'll interrupt at the half millisecond point (ie 0.5, 1.5 ... )
  that prevents interference with millis which happens at the top of the millisecond (ie 1.0, 2.0 ..) */
  if (v) {
    OCR0A = 0xAF;    //This line controls when the trigger happens from 0 (very start) to 255 (end)
    TIMSK0 |= _BV(OCIE0A); //This enables the interrupt by changing a bit in a Register to 1
    usingInterrupt = true;
  }

  // Do not call the interrupt function COMPA anymore
  else {
    TIMSK0 &= ~_BV(OCIE0A);  //This enables the interrupt by changing a bit in a Register to 0
    usingInterrupt = false;
  }
}