I'm trying to set up timer 1 to run an isr in CTC mode. no matter what i set OCR1A to it always executes at the same frequency and if i print out TCNT1 I get values higher than OCR1A.

here is my Code

    void timer1_init()
    // set up timer with prescaler = 8 and CTC mode
    TCCR1B |= (1 << WGM12)|(0 << CS12)|(1 << CS11)|(0 << CS10);

    // initialize counter
    TCNT1 = 0;

    // initialize compare value
    OCR1A = 100;

    // enable compare interrupt
    TIMSK1 |= (1 << OCIE1A);

      // enable global interrupts

    SET(MOTOR_PORT,LEFT_MOTOR_STEP); //set the pin high
    delayMicroseconds(3); //keep high for a bit

I'm using this to drive a stepper motor so using PWM mode wont work.

1 Answer 1


It sounds as though timer 1 has not in fact been set to wavegen mode 4. You can ensure that it is by also clearing the other bits of WGM1:

TCCR1A &= ~(_BV(WGM10) | _BV(WGM11));
TCCR1B &= ~_BV(WGM13);
  • 1
    I normally assign to TCCR1A and TCCR1B rather than "or". That way I know it is set to a fixed value.
    – Nick Gammon
    Jul 25, 2015 at 21:34

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