#define LED_PIN PA5
#define BUTTON_PIN PA0

volatile bool blinkEnable = false;

void buttonISR() {
  blinkEnable = !blinkEnable;   // Toggle blinking
}

void setup(void) {
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);

  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN),
                  buttonISR,
                  FALLING);   // Button press
}

void loop(void) {
  if (blinkEnable) {
    digitalWrite(LED_PIN, HIGH);
    delay(300);
    digitalWrite(LED_PIN, LOW);
    delay(300);
  } else {
    digitalWrite(LED_PIN, LOW); // LED OFF
  }
}