int interrupt = 2;

volatile unsigned int teller, teller_copy;

volatile unsigned long current_micros;
volatile unsigned long old_micros;

volatile unsigned long current_millis;
volatile unsigned long old_millis;

volatile bool state;

unsigned long time1;
unsigned long rpm;
unsigned long frequentie;
float periode;

void setup() {
  analogWrite(3, 225);
  pinMode(interrupt, INPUT);
  attachInterrupt(digitalPinToInterrupt(interrupt), toerental, FALLING);
  Serial.begin(115200);

}

void loop() {
  current_millis = millis();

  if (current_millis - old_millis >= 1000) {

    cli(); //stop interrupts
    teller_copy = teller;
    teller = 0;
    sei(); //allow interrupts

    frequentie = teller_copy / 2;
    rpm = frequentie * 60ul;

    Serial.print(teller_copy);
    Serial.print(" ");
    Serial.print(frequentie);
    Serial.print(" ");
    Serial.println(rpm);

    old_millis = current_millis;
  }
}

void toerental() {

  if (micros() - old_micros > 500) {

    old_micros = micros();
    teller++;
  }
}