const byte interruptPin = 13;
const unsigned long sampleTime = 2000;
int rpmMaximum = 0;

void setup() {

  Serial.begin(115200);
  pinMode(interruptPin, INPUT);

}

int getRPM()
{
  int count = 0;
  boolean countFlag = LOW;
  unsigned long currentTime = 0;
  unsigned long startTime = millis();
  while (currentTime <= sampleTime)
  {
    if (digitalRead(interruptPin) == HIGH)
    {
      countFlag = HIGH;
    }
    if (digitalRead(interruptPin) == LOW && countFlag == HIGH)
    {
      count++;
      countFlag=LOW;
    }
    currentTime = millis() - startTime;
  }
  Serial.println(count/2);
  int countRpm = int(60000/float(sampleTime))*int(count/2);
  return countRpm;
}

void loop() {
  delay(100);
  int rpm = getRPM();
  Serial.print("An interrupt has occurred. Total: ");
  Serial.println(rpm);

}