const int LED_PIN = 13;
const int INTERRUPT_PIN = 2;
volatile bool ledState = LOW;
double i;

void setup() {
    Serial.begin(9600);

    pinMode(LED_PIN, OUTPUT);
    pinMode(INTERRUPT_PIN, INPUT_PULLUP);
    
    attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN), myISR, FALLING); 
    // trigger when button pressed, but not when released.

    Serial.println("start");
}

void loop() {
    digitalWrite(LED_PIN, ledState);
     i=i+1;
    //Serial.println(i);
    //Serial.println(ledState);
    delay(10);
    if (i==1000) i=0;

}

void myISR() {
    ledState = !ledState;
    // note: 
    // LOW == false == 0, 
    // HIGH == true == 1, 
    // so inverting the boolean is the same as switching between LOW and HIGH.
     Serial.print("x");
}