I am creating pwm from an arduino pin with 50% duty cycle. I just want to know how can i detect its every falling edge using timer interrupt? I don't want to connect that PWM pin to an external interrupt pin. Please help me.

2 Answers 2


You can define an interrupt service routine (ISR) for each Timer and PWM pin that it handles. These are known as TIMERn_COMPA_vect, TIMERn_COMPB_vect and TIMERn_COMPC_vect. The Mega has six Timers (n=0..5).

// Timer/Counter compare match A

Check the PWM configuration for how the pin is set, cleared or toggled.


  • 1
    Besides creating the ISR(... you also need to enable that interrupt (e.g. TIMSK1 |= _BV(OCIE1A))
    – Gerben
    Feb 27, 2016 at 17:11

Referring to the datasheet of ATMega328P: If you want to generate an interrupt every falling edge you must know if the Pin is inverse mode or not. As you can see from the timing diagram the Pin stays high until the compare unit matches (current value of the TCNCT == value stored in OCRn ) and then goes low in the normal mode or works as opposite in the inverting mode.

This could be regulated by changing the bits COMnx in the TCCRn register. If, for example: COM0 = 1 and COM1 = 1 the table says: Set OC0A on Compare Match, clear OC0A at BOTTOM, (inverting mode).

Now with these settings the falling edge happens when the timer overflow, so the ISR must occur when the timer reach its maximum value.

PWM timing diagram

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