#include <LiquidCrystal_I2C.h>

#define MILLIS_MAX 4294967295  // = (2^32 - 1)
#define PIN_BUTTON 32

#define DISPLAY_I2C_ADDR 0x27
#define DISPLAY_WIDTH 20
#define DISPLAY_HEIGHT 4

const unsigned short debounceDelay = 50;  // [ms]
const unsigned short displayTime = 2000;  // [ms]

// volatile since manipulated by an ISR
volatile unsigned long currentMillis;
volatile unsigned long lastDebounceMillis;   // the last time the interrupt was triggered
volatile unsigned long millisAtButtonPushed;
volatile boolean LCDOn;
bool refreshNeeded = false;

LiquidCrystal_I2C lcd( DISPLAY_I2C_ADDR, DISPLAY_WIDTH, DISPLAY_HEIGHT );

unsigned long getMillisDelta( unsigned long millisStart, unsigned long millisEnd ) {
  if ( millisEnd < millisStart ) return MILLIS_MAX - millisStart + millisEnd + 1;
  else return millisEnd - millisStart;
}

void IRAM_ATTR isr_falling_edge() {
  currentMillis = millis();
  if ( getMillisDelta(lastDebounceMillis, currentMillis) > debounceDelay ) {
    millisAtButtonPushed = currentMillis;
    LCDOn = true;
  }
}

void setup() {
  Serial.begin( 115200 );

  lcd.begin( DISPLAY_WIDTH, DISPLAY_HEIGHT );
  lcd.init();
  lcd.backlight();
  lcd.println("Hello!!");

  pinMode( PIN_BUTTON, INPUT_PULLUP );
  attachInterrupt( PIN_BUTTON, isr_falling_edge, FALLING );

  delay(500);
  lcd.noDisplay();
  lcd.noBacklight();
  LCDOn = false;
  refreshNeeded = true;
}

void loop() {
  currentMillis = millis();
  if ( LCDOn ) {
    if (refreshNeeded) {
      lcd.init();
      lcd.backlight();
      lcd.println("Now you see me..");
      refreshNeeded = false;
    }
    Serial.println( "LCD should be on..." );
    if ( getMillisDelta(millisAtButtonPushed, currentMillis) > displayTime ) {
      lcd.noDisplay();
      lcd.noBacklight();
      LCDOn = false;
      refreshNeeded = true;
    }
  }
  else Serial.println( "LCD should be off..." );
  delay(1000);
}