#define LED_PIN 4
#define BUTTON_PIN 15

volatile bool ledState = false;  // state of LED (volatile because it's changed in ISR)

// Interrupt Service Routine (ISR)
void IRAM_ATTR handleButtonPress() {
  ledState = !ledState;          // toggle LED
  digitalWrite(LED_PIN, ledState);
}

void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);  // enable internal pull-up

  // Attach interrupt
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), handleButtonPress, FALLING);

  Serial.begin(115200);
  Serial.println("Button interrupt example started.");
}

void loop() {
  // Main loop does nothing; LED is controlled by ISR
}