#define ledA 5

hw_timer_t*timerLed = NULL;

void IRAM_ATTR parpadeo() {
  digitalWrite(ledA, !digitalRead(ledA));
}

void setup() {
  Serial.begin(115200);
  pinMode(ledA, OUTPUT);

  timerLed = timerBegin(0, 80, true);
  timerAttachInterrupt(timerLed, &parpadeo, true);
  timerAlarmWrite(timerLed, 500000, true); //500ms
  timerAlarmEnable(timerLed);
}

void loop() {

  delay(10); // this speeds up the simulation
}