#define LED_PIN 2
#define LED2_PIN 4

bool timerFlag = false;

void IRAM_ATTR isrBlinkLED(){
  digitalWrite(LED2_PIN, !digitalRead(LED2_PIN));

  timerFlag = true;
}

void setup() {
  // put your setup code here, to run once:
  pinMode(LED_PIN, OUTPUT);
  pinMode(LED2_PIN, OUTPUT);
  digitalWrite(LED_PIN, LOW);


  hw_timer_t *myTimer = timerBegin(1000000);

  timerAttachInterrupt(myTimer, isrBlinkLED);
  timerAlarm(myTimer, 500000, true, true);
}

void loop() {
  // put your main code here, to run repeatedly:
  while (!timerFlag);
  timerFlag = false;
  digitalWrite(LED_PIN, LOW);
  while (!timerFlag);
  digitalWrite(LED_PIN, HIGH);
}