#include <LiquidCrystal_I2C.h>

const byte   slots = 1; // no of slots in a rotating object
volatile unsigned long us, prevPulseUs, pulseUs, prevPulseUsCopy, pulseUsCopy;
unsigned long pulsePeriod, rpm;
const unsigned long slotUs = 60000000 / slots;

void isr() {
  us = micros();
  if ((us - pulseUs) > 100) {  // debounce interval, also determines max rpm
    prevPulseUs = pulseUs;
    pulseUs = us;
  }
}

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

LiquidCrystal_I2C lcd (0x27, 16, 2);

void setup() {
  attachInterrupt(digitalPinToInterrupt(2), isr, RISING);
  lcd.init();
  lcd.backlight();
  //lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("___TACHOMETER___");
  lcd.setCursor(0, 1);
  lcd.print("RPM: ");
}

void loop() {
  if (timeOut(250)) {
    cli(); //stop interrupts
    prevPulseUsCopy = prevPulseUs;
    pulseUsCopy = pulseUs;
    sei(); //allow interrupts
    pulsePeriod = pulseUsCopy - prevPulseUsCopy;
    if (pulsePeriod) rpm = slotUs / pulsePeriod ;
    lcd.setCursor(5, 1);
    lcd.print(rpm);
  }
}