Hey I'm trying to do blinking led using interrupts on 16 bit timer. My atmega model is Atmega168A. Chip clock rate is 12Mhz.

Here is my code:

#include <avr/io.h>
#include <avr/interrupt.h>

    PORTC ^= (1 << PC5);

int main()

    DDRC |= (1 << PC5);

    PORTC |= (1 << PC5); //led on


    TIMSK1 |= (1 <<  OCIE1A); // cmp interruupt enable
    TCCR1B |= (1 << CS12) | (1 << CS10); //prescalser 64
    OCR1A = 11718; // value to compare
    TCCR1A |= (1 << WGM12); // set ctc mode



    return 0;

When the program startup led turn on. After compare match it should turn off. And it works, but with one strange problem.

If I change OCR1A, only the first peroid of time before led on and off changes.

When I set OCR1A e.g 0x100 the led turns off very quickly.

When I set OCR1A e.g 0xfffa the led turns off slowly.

But only the first state change.

Another led blinks do not take over about changes of OCR1A.

Whats wrong?

Do I need reset some flags or something like this?

  • \$\begingroup\$ You're assuming the timer resets to zero when it reaches the compare value. It doesn't. \$\endgroup\$
    – brhans
    Commented Jul 2, 2018 at 22:48
  • \$\begingroup\$ @brahans Hmmm I added TCNT1 = 0x00 in ISR and it works. But why? Does the TCNT1 shouldn't be cleared automaticly/ As the doc says: "The counter value (TCNT1) increases until a compare match occurs with either OCR1A or ICR1, and then TCNT1 is cleared." \$\endgroup\$
    – bielu000
    Commented Jul 3, 2018 at 4:06

1 Answer 1


You did not set CTC mode, WGM12 is in TCCR1B


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