#define LED 12
hw_timer_t *My_timer = NULL;
int ledstate = 0;
//-------------
void IRAM_ATTR onTimer(){
  ledstate = !ledstate;
digitalWrite(LED, ledstate);
}
//-----------
void setup() {
pinMode(LED, OUTPUT);
My_timer = timerBegin(0, 80, true);
timerAttachInterrupt(My_timer, &onTimer, true);
timerAlarmWrite(My_timer, 2500000, true);
timerAlarmEnable(My_timer);
}
//---------------------------------------------
void loop(){
  delay(10);
}