#define LED 21
hw_timer_t *Timer0_Cfg = NULL;

void IRAM_ATTR TIMER0_ISR()
{
    digitalWrite(LED, !digitalRead(LED));
}

void setup()
{
    pinMode(LED, OUTPUT);
    Timer0_Cfg = timerBegin(0, 80, true);
    timerAttachInterrupt(Timer0_Cfg, &TIMER0_ISR, true);
    timerAlarmWrite(Timer0_Cfg, 500000, true);
    timerAlarmEnable(Timer0_Cfg);
}

void loop()
{
    // Do nothing...
}