#define LED 21

hw_timer_t *Timer0 = NULL;

void IRAM_ATTR Timer0_ISR()
{
  digitalWrite(LED, !digitalRead(LED));
}

void setup()
{
  pinMode(LED, OUTPUT);
  Timer0 = timerBegin(0, 80, true);
  timerAttachInterrupt(Timer0, Timer0_ISR, true);
  timerAlarmWrite(Timer0, 500000, true);
  timerAlarmEnable(Timer0);
}

void loop()
{
}