I must be overwrought or something... I just can't get the calculation of the timer frequency right. I have a 16 MHz crystal on the Atmega328. I want 1 kHz timer interrupt rate. So I chose prescaler = 64 and counter top = 249, that is

fTimer = fCPU/prescaler/(top+1) = 16 MHz/64/250 = 1 kHz

But my oscilloscope shows 2 kHz... !? Why?

Code:

const int signalLED = 1; //14;

void setup() 
{
  pinMode(signalLED, OUTPUT);
  digitalWrite(signalLED, LOW);

  TCCR2A = 0; // set TCCR2A register to 0
  TCCR2B = 0; // set TCCR2B register to 0
  TCNT2  = 0; // reset counter
  OCR2A = 249; // top value in CTC mode
  TCCR2A |= (1 << WGM21); // enable timer2 CTC mode
  TCCR2B |= (1 << CS21) | (1 << CS20); // 1:64 prescaling for timer 2
  TIMSK2 |= (1 << OCIE2A); // enable timer2 compare interrupt
  sei(); // allow interrupts
}

ISR(TIMER2_COMPA_vect) 
{
  // generate peak
  digitalWrite(signalLED, HIGH);
  digitalWrite(signalLED, LOW);
}

void loop() 
{
}