UNO Timer 1 CTC mode not resetting TCNT1

Question

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
 sei();
 }
ISR(TIMER1_COMPA_vect){
 SET(MOTOR_PORT,LEFT_MOTOR_STEP); //set the pin high
 delayMicroseconds(3); //keep high for a bit
 CLR(MOTOR_PORT,LEFT_MOTOR_STEP); //set it low
}

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

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);

• Nick Gammon: I normally assign to TCCR1A and TCCR1B rather than "or". That way I know it is set to a fixed value.