bool pressed;

ISR(INT0_vect) {
  pressed = true;
}

void setup() {
  Serial.begin(9600);
  pinMode(A2, INPUT_PULLUP);
  EIMSK = 1 << INT0;  // Connect a button between GND and pin A2
}

void loop() {
  if (pressed) {
    pressed = false;
    Serial.println("Pressed!");
  }
}