#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()
{

}