#include <LiquidCrystal.h>
LiquidCrystal myLCD(13, 12, 11, 10, 9, 8);
#define key 21
int loopCounter, intCounter;
boolean keyState, prevKeyState;

void setup() {
  myLCD.begin(16, 2);
  pinMode(key, INPUT_PULLUP);

noInterrupts();

//int intNum = digitalPinToInterrupt(key);
//attachInterrupt(intNum, ExtInt, FALLING);

bitSet(EICRA, ISC01);
bitSet(EIMSK, INT0);

interrupts();

}

void display() {
  myLCD.setCursor(0,0);
  myLCD.print("loopCounter=");
  myLCD.print(loopCounter);
  myLCD.setCursor(0,1);
  myLCD.print(" intCounter=");
  myLCD.print(intCounter);
  delay(200);
}

void ExtInt(){
  intCounter++;
}

ISR(INT0_vect){
  intCounter++;
}

void loopCount() {
  keyState = digitalRead(key);
  if ((keyState == LOW) && (prevKeyState == HIGH)){
    loopCounter++;
  }
  prevKeyState = keyState;
}

void loop() {
  loopCount();
  display();
}