Move the Nano link to Serial2, update READMEs

- ESP32 talks to the Nano on UART2 (GPIO 17 TX / GPIO 16 RX); Serial (USB)
  is now free for debug output and boot messages no longer reach the Nano
- README: wiring table, actual schedule (clock from 08:00), TODO section
  for the rewiring

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -68,21 +68,34 @@ pio device monitor -e nano       # serial monitor
 
 In VS Code you can also pick the environment in the PlatformIO status bar.
 
-> **Tip:** The Nano and the ESP32 share a serial line. Disconnect the ESP32 TX → Nano RX
-> wire before you flash the Nano over USB, or the upload may fail.
+> **Tip:** The ESP32 → Nano wire goes to the Nano's RX pin, which the Nano's USB chip
+> also uses. Disconnect that wire before you flash the Nano over USB, or the upload may
+> fail.
+
+## Wiring between ESP32 and Nano
+
+The ESP32 talks to the Nano on **UART2** (`Serial2`, 115200 baud). The ESP32's own
+`Serial` (UART0, USB) is free for debug output.
+
+| ESP32 | Nano | Notes |
+|-------|------|-------|
+| GPIO 17 (TX2) | D0 (RX) | Display commands |
+| GPIO 16 (RX2) | D1 (TX) | `READY` and `ERR` replies. Use a 5 V → 3.3 V voltage divider (e.g. 1 kΩ / 2 kΩ), because the ESP32 is not 5 V tolerant. Optional: without it, the ESP waits a fixed 3 s after power-on. |
+| GND | GND | Common ground |
+| GPIO 13 | Relay input | Relay powers the panels and the Nano (`LOW` = on) |
 
 ## How it works
 
 1. **ESP32 boot.** The ESP32 connects to WiFi and starts ArduinoOTA. After a crash or
    unexpected reset it stays in an *OTA panic window* for 15 seconds, so you can flash a
-   fix before the main code runs again. It then switches the display relay on, starts NTP
-   and connects to MQTT.
+   fix before the main code runs again. It then switches the display relay on, waits for
+   the Nano's `READY` message (at most 3 s), starts NTP and connects to MQTT.
 2. **Clock loop (every second).**
-   - **07:00–22:00:** Pikachu with a thought bubble showing `HH:MM`. A yawning Pikachu is
+   - **08:00–22:00:** Pikachu with a thought bubble showing `HH:MM`. A yawning Pikachu is
      shown from 17:00.
-   - **22:00–23:30:** the clock is only drawn on the quarter hour. In between, the relay
-     turns the display off.
-   - **Night:** the screen is cleared and power is cut after 30 seconds.
+   - **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 and power is cut after 30 seconds.
 3. **Home Assistant switches** (MQTT discovery):
    - `FlipDot Display Clock`: off shows a Nintendo logo instead of the clock.
    - `FlipDot Display Power`: off clears the display and cuts power through the relay.
@@ -91,3 +104,15 @@ In VS Code you can also pick the environment in the PlatformIO status bar.
    to fix dots that did not flip.
 
 The serial protocol is described in [src/nano/README.md](src/nano/README.md).
+
+## TODO
+
+- [ ] **Rewire the Nano link to UART2.** The firmware now uses `Serial2` (GPIO 16/17)
+  instead of `Serial` (GPIO 1/3). Move the ESP32 TX wire from GPIO 1 to **GPIO 17** before
+  you flash this firmware, or the display gets no data. See
+  [Wiring between ESP32 and Nano](#wiring-between-esp32-and-nano).
+- [ ] **Add the reply wire:** Nano D1 (TX) → voltage divider → ESP32 GPIO 16 (RX2). With it,
+  the ESP sends the first frame as soon as the Nano is ready, instead of after 3 s.
+- [ ] **Check the boot timing.** Build the Nano with `MEASURE_REFRESH` to see how long a
+  full redraw takes. If the reply wire isn't fitted, check that 3 s
+  (`NANO_BOOT_TIMEOUT`) is enough for the Nano to boot.