void setup()
{
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  //----------------------
  noInterrupts();
  TCNT1 = 0; //reset timer1 counter register
  TCCR1A = 0x00; //reset timer 1 A register
  TCCR1B = 0b00000111; //set up timer1 to use external clk on pin D5
  TIMSK1 &= ~(1 << OCIE1A); // turn off the timer1 interrupt
  interrupts();
}
void loop()
{
    if(TCNT1 == 10){
      digitalWrite(LED_BUILTIN,!digitalRead(LED_BUILTIN));
      TCNT1 = 0;
    }
}