#define ledPin 13
int num=0;
void setup() {
Serial.begin(9600);
pinMode(ledPin, OUTPUT);
noInterrupts(); //    initialize timer1 and disable all interrupts
 TCCR1A = 0;  TCCR1B = 0;
 TCNT1 = 34286; // preload timer 65536-16MHz/256/2Hz
 TCCR1B |= (1 << CS12); // 256 prescaler
 TIMSK1 |= (1 << TOIE1); // enable timer overflow interrupt
 interrupts(); }  // enable all interrupts

ISR(TIMER1_OVF_vect) // interrupt service routine
{  TCNT1 = 34286; // preload timer again
 digitalWrite(ledPin, digitalRead(ledPin) ^ 1);
 Serial.println(++num);
  }


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

}