Why there is wdt_reset() when using WDT in interrupt mode only?

Projects Programming
1

Hello, I have a question why is there a call to wdt_reset() in the following function even though we set watchdog timer to work in interrupt mode (not the reset mode)?

Code taken from Nick Gammon website

void myWatchdogEnable() 
{
  // clear various "reset" flags
  MCUSR = 0;     
  // allow changes, disable reset
  WDTCSR = bit (WDCE) | bit (WDE);
  // set interrupt mode and an interval 
  WDTCSR = bit (WDIE) | bit (WDP3) | bit (WDP0);    // set WDIE, and 8 seconds delay
  wdt_reset();  // pat the dog

  // disable ADC
  ADCSRA = 0;  

  // ready to sleep
  set_sleep_mode (SLEEP_MODE_PWR_DOWN);  
  sleep_enable();

  // turn off brown-out enable in software
  MCUCR = bit (BODS) | bit (BODSE);
  MCUCR = bit (BODS); 
  sleep_cpu ();  

  // cancel sleep as a precaution
  sleep_disable();
}

Whereas a bit above, in Sketch J wdt_reset() is not present. What is the difference?

2

why is there a call to wdt_reset() in the following function even though we set watchdog timer to work in interrupt mode (not the reset mode)?

So that the delay (watchdog count) starts now, rather than some earlier time.

Whereas a bit above, in Sketch J wdt_reset() is not present. What is the difference?

Sketch J does not use the watchdog, thus we don't need to reset it. You only need to reset the watchdog if it is active.

3

Thank You for the answer!
I have another question, though. Is the wdt_reset essential for the following code (using both watchdog and interrupt for waking up)?

void mywatchdog (const byte interval) {  
  MCUSR = 0;                          
  WDTCSR |= 0b00011000;               
  WDTCSR =  0b01000000 | interval;    

  //wdt_reset(); //Is it needed?
  ADCSRA = 0; 
  set_sleep_mode (SLEEP_MODE_PWR_DOWN); 
  
  attachInterrupt (2, myfunction, LOW);   // allow grounding pin 0 to wake ATmega 
  sleep_mode();   
  detachInterrupt (2);
}
4

The wdt_reset() resets the watchdog timer back to zero. Without it, assuming it runs in the background, the timer might fire much sooner than you expect.