Weird Arduino timer behaviour

Question

I am trying to output something every one second with the following code.

volatile uint64_t timerCounts = 0;
double timenow,pretime;
void setup()
 {
 Serial.begin(19600);
 noInterrupts();
 TCCR2A = TCCR2B = 0;
 TCNT2 = 0;
 OCR2A = 124; //Zero Relative 125
 TCCR2B = B00001101; //prescaler = 1024
 TIMSK2 |= (1 << OCIE1A);
 interrupts();
 Serial.print("Begin... \n");
 }
 //**********************************************************************
 // Timer2 Interrupt Service is invoked by hardware Timer 2 every 0.008s
 // 16Mhz / 125 / 1024 = 125 Hz 0.008s
 ISR(TIMER2_COMPA_vect)
 {
 timerCounts++;
 if (timerCounts == 125){
 Serial.print("1s");
 timerCounts = 0;
 }
 } // end of TIMER2_COMPA_vect
 void loop()
 {
 }

In the above code, I have set the OCR2A(output compare register) to 124``, which is well below 256(2^8). A user defined counter is also used.

However, when I execute the programme, the interval between consecutive outputs is less than 1 second.

When I change it to timer 1, the interval becomes roughly 1 second. When I change it to timer 0, the interval becomes longer than 1 second.

Anyone knows why?

Thanks.

Answer 1

OCR2A doesn't do anything because the timer is in 'normal' mode. In normal mode the timer will just count to 256, not 125. Which would mean the you get a serial.print every 2.05s.

You need to use CTC mode. Do this by setting the WGM21 bit to 1.

TCCR2A = _BV(WGM21);