#include <LiquidCrystal_I2C.h>

const byte   wings = 4; // no of wings of rotating object, for disc object use 1 with white tape on one side
volatile unsigned long prevPulseUs, pulseUs, prevPulseUsCopy, pulseUsCopy;
unsigned long pulsePeriod, rpm;

void isr() { //interrupt service routine
  prevPulseUs = pulseUs;
  pulseUs = micros();
}

bool timeOut(unsigned long ms) { //ms timer
  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() {
  pinMode(3, OUTPUT);  // jumper to pin 2 for pwm test
  analogWrite(3, 1);   // test pwm @ 490 Hz = 29400 rpm / wings

  lcd.init(); //initialize LCD
  lcd.backlight(); //turn on the backlight on LCD
  attachInterrupt(digitalPinToInterrupt(2), isr, RISING); //attaching the interrupt
}

void loop() {

  if (timeOut(1000)) {
    cli(); //stop interrupts
    prevPulseUsCopy = prevPulseUs;
    pulseUsCopy = pulseUs;
    sei(); //allow interrupts
    pulsePeriod = pulseUsCopy - prevPulseUsCopy;
    if (pulsePeriod < 600) pulsePeriod = 600; // upper limit 120000 / wings
    if (!pulseUsCopy || !prevPulseUsCopy) pulsePeriod = 10000000; // startup 0 rpm 

    rpm = 1000000 / pulsePeriod * 60 / wings; //calculates rpm
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("___TACHOMETER___");
    lcd.setCursor(0, 1);
    lcd.print(rpm);
    lcd.print(" RPM");
    lcd.print(" ");
  }
}