volatile int rpm = 0; // viimatimõõdetud RPM väärtus.
volatile int revCount = 0; // loendab pulsse viimasest arvutamisest
volatile unsigned long lastTime = 0; // viimase arvutamise aeg

void setup() {
   attachInterrupt(RPM_INPUT_INTERRUPT, rpmInterrupt, RISING);
}
void rpmInterrupt() {
   revCount++;
   if (revCount == RPM_COUNT_COLLECT) {
      unsigned long thisTime = micros();
      rpm = (double) revCount
            / (((double) thisTime - (double) lastTime) / (double) 60000000);
      revCount = 0;
      lastTime = thisTime;
   }
}