void setup() {
  pinMode(13, OUTPUT);        //Set the pin to be OUTPUT
  
  noInterrupts(); // disable interrupts while we setup 

  // reset Timer 1 configurations, default to interrupt on overflow
  TCCR1A = 0; 
  TCCR1B = 0;

  // set bits 0-2 to select the scalar value
  // 001 = 1
  // 010 = 8
  // 011 = 64
  // 100 = 256
  // 101 = 1024
  TCCR1B = (TCCR1B & B11111000) | B101;   // Pre-scalar = 1024
  
  // enable OCM1 & 2  interrupts by setting bit 1 and 2 of TIMSK1
  TIMSK1 |= B110; 
  OCR1A = 20000;  // set the first OCM value
  OCR1B = 40000;  // set the second OCM value

  interrupts();  // enable all interrupts
}

void loop() {
  // put your main code here, to run repeatedly:

}

ISR(TIMER1_COMPA_vect) {
  digitalWrite(13,HIGH); 
}

ISR(TIMER1_COMPB_vect) {
  TCNT1 = 0;
  digitalWrite(13,LOW); 
}