# How it works

[← Back to the README](../README.md)

1. **ESP32 boot.** The ESP32 connects to WiFi and starts ArduinoOTA. After a crash it
   runs only OTA for 15 seconds, and after repeated crashes it goes to safe mode. Bad OTA
   firmware is rolled back automatically, see
   [Keeping OTA alive](../src/esp32/README.md#keeping-ota-alive). It then switches the display relay on, waits for
   the Nano's `READY` message (at most 3 s), shows "Connecting...", starts NTP and
   connects to MQTT.
2. **Clock.** Once the time is synced, the ESP32 sends a new frame whenever the screen
   changes, mostly once a minute:
   - **08:00–22:00:** the page: the chosen page (by default the built-in Clock page,
     Pikachu with a thought bubble showing `HH:MM`, yawning from 17:00), or what the
     [schedule and rules](web-ui.md#schedule) pick.
   - **22:00–23:30:** the clock is only shown during the minutes :00, :15, :30 and :45.
     30 seconds after each of those minutes, the relay turns the display off.
   - **23:31–08:00:** the screen is cleared (or shows the off image chosen in the web UI)
     and power is cut after 30 seconds.
3. **Home Assistant**: switches, dropdowns and sensors over MQTT, see
   [Home Assistant](home-assistant.md).
4. **Nano.** The Nano reads commands from the serial line and updates its frame buffer.
   On `D` it flips only the dots that changed. Every 2 minutes it redraws the whole screen
   to fix dots that did not flip.

The serial protocol is described in [src/nano/README.md](../src/nano/README.md).