bool isrEnable = false;
unsigned long int t;
unsigned long int prevTime;
char oldSREG;

void setup ()
{
 Serial.begin(9600);
 oldSREG = SREG;                        // save state register
 cli();                                 // disable interrupts


 TCCR1A = 0;
 TCCR1B = 0;
 TCNT1  = 0;                              // Reset counter value

 OCR1A = 15625;                           // toggle after counting to 16,000,000/1024 = 15625
 TIMSK1 |= (1 << OCIE1A);                 // Output Compare A Match Interrupt Enable
 TCCR1B |= (1 << WGM12);                  // CTC mode
 TCCR1B |= (1 << CS12) | (1 << CS10);     // pre-scaler: 1024


  sei();                                  // enable interrupts
  SREG = oldSREG;                         // restore state register
}

ISR(TIMER1_COMPA_vect)          // interrupt service routine that wraps a user defined function supplied by attachInterrupt
{
  isrEnable = true;
}

void loop () {
  t = millis();
if(isrEnable) {
   printTime();
  isrEnable = false;
  }
}

void printTime(void) {    // diagnostics
  Serial.print(t);        // current time
  Serial.print("\t");     // print tab
  Serial.println((float)(t - prevTime) / 1000.0, 4);
  prevTime = t;}           // previous time