#define led  21
 //static const int led_pin = LED_BUILTIN;
 static hw_timer_t * timer = NULL;
 // /*************************************/
 /* ISR function*/
 void IRAM_ATTR OnTimer(){
  // Toggle BUILTIN LED
     int pin_stat = digitalRead(led);
     digitalWrite(led, !pin_stat);
  }
void setup() {
     Serial.begin(9600);
    delay(1000);
    // Configure LED pin mode
     pinMode(led, OUTPUT);
     // Create and start time using timeBegin(num. divider, upCounting)
     timer  = timerBegin(0,80, true );
     // Provide ISR to timer
     timerAttachInterrupt(timer, &OnTimer, true);
     timerAlarmWrite(timer,1000000, true );
     // Allow ISR to trigger
      delay(1000);
      Serial.print("----Timer Tick Interrupt setup----");
      delay(1000);
    timerAlarmEnable(timer);  
    Serial.println("last line of setupt function");
  }
 void loop() {

  }