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Deep sleep

Before deep sleeping, you will want to ensure that all messages are sent, including property values of your HomieNode and the $online → false. To do that, you can call Homie.prepareToSleep(). But the event MQTT_READY is emitted before your HomieNodes loop() method is called. Therefore you need to postpone the call to Homie.prepareToSleep() until the loop() method of all HomieNodes is called and their properties are submitted over MQTT. For that the additional Timer is necessary. Then Homie.prepareToSleep() will disconnect everything cleanly, so that you can call Homie.doDeepSleep().

#include <Homie.h>
#include "Timer.h"

Timer t;

void prepareSleep() {
  Homie.prepareToSleep();
}

void onHomieEvent(const HomieEvent& event) {
  switch(event.type) {
    case HomieEventType::MQTT_READY:
      Homie.getLogger() << "MQTT connected, preparing for deep sleep after 100ms..." << endl;
      t.after(100, prepareSleep);
      break;
    case HomieEventType::READY_TO_SLEEP:
      Homie.getLogger() << "Ready to sleep" << endl;
      Homie.doDeepSleep(60000000); // 60 sec
      break;
  }
}

void setup() {
  Serial.begin(115200);
  Serial << endl << endl;
  Homie.onEvent(onHomieEvent);
  Homie.setup();
}

void loop() {
  Homie.loop();
  t.update();
}

Be aware that you have to connect the pin D0 to the pin RST, otherwise the board won't wake up.