const byte   pulsesPerRev = 8;
const byte   debounceInterval = 200;

volatile unsigned long us, prevPulseUs, pulseUs, prevPulseUsCopy, pulseUsCopy;
unsigned long pulsePeriod, rpm;
const unsigned long slotUs = 60000000 / pulsesPerRev;

const byte rpm_in = 2;

void rpm_interrupt() {
  us = micros();
  if ((us - pulseUs) > debounceInterval) {  //also determines max rpm
    prevPulseUs = pulseUs;
    pulseUs = us;
  }
}

bool processInterval(unsigned long ms) {
  volatile unsigned long prevMs, now;
  now = millis();
  if ((now - prevMs) >= ms) {
    prevMs = now;
    return true;
  }
  return false;
}

void setup() {
  Serial.begin(115200);

  pinMode(rpm_in, INPUT); //Interrupt
  attachInterrupt(digitalPinToInterrupt(rpm_in), rpm_interrupt, FALLING);
}

void loop() {
  if (processInterval(400)) {
    noInterrupts();
    prevPulseUsCopy = prevPulseUs;
    pulseUsCopy = pulseUs;
    interrupts();
    pulsePeriod = pulseUsCopy - prevPulseUsCopy;
    if (pulsePeriod) rpm = slotUs / pulsePeriod ;
    Serial.print("RPM = ");
    Serial.println(rpm);
  }
}