Count down timer for Arduino sketch

Question

I wrote an Arduino sketch a little while ago, and I am trying to add functionality to the sketch. Basically I want a count down timer that closes a solenoid cut off valve after 30 seconds has passed.

Answer 1

You can do that using timers and interrupts, but some more informations is needed (which board, which processor).

Note: F_CPU is already defined if you are using arduino libraries (#define F_CPU 20000000U)

Note 2: You may want to use another timer than TIMER0 since it is use to track time on arduino

#define GMilliSecondPeriod F_CPU / 1000

unsigned int gNextOCR = 0;
volatile unsigned long gMillis = 0;
bool valveOpened = false;

// This interruption will be called every 1ms
ISR(TIMER2_COMPA_vect)
{
  if(valve_open){
    gMillis++;
    if(gMillis >= 30000){
      close_valve();
      gMillis = 0;
    }
  }  

  gNextOCR += GMilliSecondPeriod;
  OCR2A = gNextOCR >> 8; // smart way to handle millis, they will always be average of what they should be
}

// Just call this function within your setup
void setupTime(){
  TCCR2B |= _BV(CS02);
  TIMSK2 |= _BV(OCIE0A);

  sei(); // enable interupts
}