#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()
{

}