TIMER2_COMPA_vect

Anstract

The TIMER2_COMPA_vect is an interrupt handler which is called when the timer/counter2(TCNT2) and compare register(OCR2A) becomes the same value. It is used to control the output of tone().

Source Code

The TIMER2_COMPA_vect is defined in hardware/arduino/avr/cores/arduino/Tone.cpp as below.

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volatile long timer2_toggle_count;
volatile uint8_t *timer2_pin_port;
volatile uint8_t timer2_pin_mask;
 
ISR(TIMER2_COMPA_vect)
{
 
  if (timer2_toggle_count != 0)
  {
    // toggle the pin
    *timer2_pin_port ^= timer2_pin_mask;
 
    if (timer2_toggle_count > 0)
      timer2_toggle_count--;
  }
  else
  {
    // need to call noTone() so that the tone_pins[] entry is reset, so the
    // timer gets initialized next time we call tone().
    // XXX: this assumes timer 2 is always the first one used.
    noTone(tone_pins[0]);
//    disableTimer(2);
//    *timer2_pin_port &= ~(timer2_pin_mask);  // keep pin low after stop
  }
}

The timer2_toggle_count is a variable which holds how long the tone() outputs HIGH. The timer2_pin_port holds the port(digitalWrite()) corresponding to the output pin of tone(). The timer2_pin_mask holds bit mask corresponding to the port.

The ISR() is a macro to define an interrupt handler.

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if (timer2_toggle_count != 0)
{
  // toggle the pin
  *timer2_pin_port ^= timer2_pin_mask;
 
  if (timer2_toggle_count > 0)
    timer2_toggle_count--;
}

If the timer2_toggle_count is not 0, it inverts the output of timer2_pin_mask of timer2_pin_port. This toggles ON and OFF of the output pin at each interrupt. As the output is inverted constant period, the duty ratio is 50%.

If the time2_toggle_count is greater than 0, it is decremented.

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else
  {
    // need to call noTone() so that the tone_pins[] entry is reset, so the
    // timer gets initialized next time we call tone().
    // XXX: this assumes timer 2 is always the first one used.
    noTone(tone_pins[0]);
//    disableTimer(2);
//    *timer2_pin_port &= ~(timer2_pin_mask);  // keep pin low after stop
  }
}

If the timer2_toggle_count becomes 0, the noTone() is called to turn off the tone.

Version

Arduino AVR Boards 1.8.6

Last Update

March 21, 2023