#define NUM_SAMPLES 10

volatile unsigned long duration = 0;
volatile unsigned int pulseCount = 0;
volatile unsigned long previousMicros = 0;


void setup() {
  Serial.begin(19200);
  attachInterrupt(digitalPinToInterrupt(2), myIntHandler, RISING);
}


void loop() {
  unsigned long _duration;
  unsigned long _pulseCount;
  float Freq_Hz;
  float Freq_RPM;

  // bufferize to avoid glitches
  _duration = duration;
  _pulseCount = pulseCount;

  if (_pulseCount == NUM_SAMPLES) {

    // clear counters
    duration = 0;
    pulseCount = 0;

    Freq_Hz = _pulseCount * 1e6 / float(_duration); // frequency in Hz
    Freq_RPM = Freq_Hz * 60; // frequency in RPM

    Serial.print("freq: ");
    Serial.println(Freq_RPM);
  }
}


void myIntHandler() {
  unsigned long currentMicros = micros();

  duration += currentMicros - previousMicros;
  previousMicros = currentMicros;
  pulseCount++;
}