Interrupt for measuring time

Projects Programming
2

You can use an interrupt triggered when the pin goes from low to high (voltage) and then, measure time.

unsigned long lastTime;
unsigned long Period;

void setup()
{
  attachInterrupt(0, measureFrec, RISING);  // Arduino Pin 2
}

void measureFrec()
{
  unsigned long time;
  
  time = millis();  // Beware overflow!!
  Period = time - lastTime;
  lastTime = time;
}