#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,50000, true);
  timerAlarmEnable(Timer0_Cfg);
}
void loop()
{

}