as pointed out in thread: atmega32u4 use Timer as souce for a counter

I tried to use the generated clock of timer 4 as input for a counter1 and counter1 should fire the compare match event after 42 clock cycles.

But I'm not able to reach the desired behavior. Here is a little test code:

#include "Arduino.h"

#define CLOCKPIN 12
#define DEBUGPIN 19

//#define USE_A

// generate 1 MHz from 8 MHz
#define CLOCKCYCLE 3

#ifdef USE_A

void setup()

    // set up pins
    pinMode(CLOCKPIN, OUTPUT);
    pinMode(DEBUGPIN, OUTPUT);

//--------------------- generate clock -------------------
    // reset timer4
    OCR4C = 0;
    OCR4D = 0;

    TCCR4A = 0x00;
    TCCR4B = 0x00;
    TCCR4C = 0x00;
    TCCR4D = 0x00;


    // Toggle OC4D on Compare Match | enable PWM
    TCCR4C = _BV(COM4D0) | _BV(PWM4D);

    // Clear Timer on Compare Match
    TCCR4D = _BV(WGM41);

    // set lock bit for sync update
    TCCR4E = _BV(TLOCK4);

    // Set compare value

    // No prescaling
    TCCR4B = _BV(CS10);

//--------------------- setup counter -------------------

     //reset timer
    TCNT1 = 0;

    // disable
    TCCR1A = 0;
    TCCR1B = 0;

    // clear interrupt flags
    TIFR1 = _BV(OCF1A) | _BV(OCF1B);

    // trigger on falling edge of T1
    TCCR1B = _BV(CS12) | _BV(CS11) | _BV(CS10) | _BV(WGM12);

#ifdef USE_A
    // interrupt on every 42. cycle
    OCR1A = 42;

    // turn on compare A interrupt.
    TIMSK1 = _BV(OCIE1A);
    // interrupt on every 42. cycle
    OCR1B = 42;

    // turn on compare B interrupt.
    TIMSK1 = _BV(OCIE1B);

loop is empty.

With define USE_A I get the following waveform: USE_A the interrupt is fired every ~66200µs sounds like the 16bit counter overflow.

With define USE_A commented out I got the following wired result: USE_B 219µs clock on 1180µs clock of. the only idea I have for this is a reset or something.

Does anybody have an idea whats wrong with my counter code?


Changed the missing flag and ISR call. Now does USE_A look like this: USE_A1 Top should now set to 42 and reset to 0 after interrupt occured? But currently it takes nearly twice the time...


1 Answer 1


When you comment out the USE_A line, it enables the compare interrupt for channel B instead of channel A. As a result the compare match interrupt vector is now TIMER1_COMPB_vect.

However you have not defined an interrupt handler for this which means when the compare match occurs the interrupt call results in exception that leads to the program restarting from the reset vector, hence you see it reset at the same rate as when A was used.

In order to get a compare match to occur every 42 cycles, you need to use "CTC" mode. There are two variants to this - the first uses OCR1A as the compare match, and the other uses ICR1 as the compare match. When in the correct mode a match with one of these will automatically clear the counter.

However, you have set TCCR1A and TCCR1B to 0 which in turn will set all of the WGM1x bits to be zero. This selects the "Normal" mode of operation. In this mode a compare match with either OCR1A or OCR1B will trigger the TIMER1_COMPA_vect and TIMER1_COMPB_vect respectively, but won't reset the timer meaning that it has to count all the way to the top (65535) and overflow between each compare - so the interrupts occur every 65536 cycles regardless of what you set the OCR1x registers to.

In order to use OCR1A as the top and get it to interrupt every 42 clocks, you need to set the correct mode. The modes are listed in Table 14-4 in the datasheet. Mode 4 is CTC with OCR1A as the top value. This implies you need to set the WGM12 bit in the TCCR1B register to be a 1.

With that bit set, and OCR1A set to 42, you should then get an interrupt occurring every 42 counts.

  • \$\begingroup\$ Oh yes your right forgot to copy the WGM12 flag to this stripdown... \$\endgroup\$
    – Steve
    Commented Jul 18, 2016 at 9:44

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