const int buttonPin = 2;     // INT0
const int ledInterrupt = 12; // LED toggled by interrupt
const int ledMillis = 8;     // LED blinking in main loop

volatile bool ledState = false;
unsigned long previousMillis = 0;
const long interval = 500;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledInterrupt, OUTPUT);
  pinMode(ledMillis, OUTPUT);

  attachInterrupt(digitalPinToInterrupt(buttonPin), toggleLED, FALLING);
  Serial.begin(9600);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    digitalWrite(ledMillis, !digitalRead(ledMillis));
    Serial.println("Main loop LED toggled");
  }
}

void toggleLED() {
  ledState = !ledState;
  digitalWrite(ledInterrupt, ledState);
}