int button = 4;
int counter = 0;

hw_timer_t *Timer0_Cfg = NULL;

void IRAM_ATTR interrupt_routine() {
  Serial.println(counter++);
}

void setup() {
  Serial.begin(115200);
  Timer0_Cfg = timerBegin(1, 80, true);
  timerAttachInterrupt(Timer0_Cfg, &interrupt_routine, true);
  timerAlarmWrite(Timer0_Cfg, 500000, true);
  timerAlarmEnable(Timer0_Cfg);

  pinMode(button, INPUT_PULLUP);
  esp_sleep_enable_ext0_wakeup(GPIO_NUM_4, HIGH);
}


void loop() {
  if (millis() > 5000) {
    Serial.println("Sleep...");
    delay(500);
    Serial.flush();
    esp_deep_sleep_start();
  }
}