Multitasking_Timer.ino

#define LED1 LED_BUILTIN
#define LED2 23

hw_timer_t *timer = NULL;

void onTimer(){
  digitalWrite(LED2, !digitalRead(LED2));
}

void setup() {
  pinMode(LED1, OUTPUT);
  pinMode(LED2, OUTPUT);
  
  timer = timerBegin(0, 80, true);
  timerAttachInterrupt(timer, &onTimer, true);
  timerAlarmWrite(timer, 300000, true);
  timerAlarmEnable(timer);
}

void loop() {
  digitalWrite(LED1, !digitalRead(LED1));
  delay(500);
}