Document that the relay also powers the Nano

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

CLAUDE.md

@@ -29,6 +29,10 @@ See README.md for the overview.
   s/S/F/D/P/B/R/T will be run as a command.
 - `src/esp32/secrets.h` holds the real credentials and is gitignored. Never commit it.
   Add new secrets to `secrets.example.h` as placeholders.
+- The relay (ESP32 GPIO 13, `LOW` = on) powers both the panels and the Nano. Each time it
+  switches on, the Nano cold-boots: bootloader, then `begin()` flips every dot on and off.
+  Any data the ESP sends during that time is lost. The Nano keeps no state over a power
+  cycle except the flip time in EEPROM.
 - Hardware is not available here. Builds can be checked, but runtime behaviour can't.
 
 ## Nano serial parser (`src/nano/main.cpp`)

README.md

@@ -9,14 +9,17 @@ The system uses two microcontrollers:
  ┌──────────────┐   WiFi / MQTT   ┌────────────────┐   UART 115200   ┌──────────────┐   GPIO   ┌──────────────┐
  │ Home         │ ◄─────────────► │ ESP32          │ ──────────────► │ Arduino Nano │ ───────► │ Brose        │
  │ Assistant    │                 │ (controller)   │  text commands  │ (driver)     │          │ flipdot      │
- └──────────────┘                 └───────┬────────┘                 └──────────────┘          │ panels 84x19 │
+ └──────────────┘                 └───────┬────────┘                 └──────▲───────┘          │ panels 84x19 │
-                                          │ GPIO 13                                            └──────▲───────┘
+                                          │ GPIO 13                                 │                  └──────▲───────┘
-                                          ▼                                                           │
+                                          ▼                                         │     power               │
-                                   ┌─────────────┐                     display power                  │
+                                   ┌─────────────┐                                  │                         │
-                                   │ Relay       │ ───────────────────────────────────────────────────┘
+                                   │ Relay       │ ─────────────────────────────────┴─────────────────────────┘
                                    └─────────────┘
 ```
 
+The relay powers both the panels **and the Nano**. When the relay is off, the Nano is off
+too, and it boots again each time the relay switches on. Only the ESP32 runs all the time.
+
 | Part | Folder | What it does |
 |------|--------|--------------|
 | **Arduino Nano** | [src/nano](src/nano/README.md) | Drives the panel's row and column electronics directly. Keeps a frame buffer and flips only the dots that changed. Takes simple text commands over serial. |

src/esp32/README.md

@@ -25,7 +25,8 @@ BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent over `S
 - **Home Assistant** through MQTT (`home-assistant-integration` library) with two switches:
   - `FlipDotDisplayClock`: clock on, or the Nintendo logo when off
   - `FlipDotDisplayPower`: turns the display off through the relay
-- **Relay on GPIO 13** switches the display's power supply. `LOW` turns it on.
+- **Relay on GPIO 13** switches the power for both the panels and the Nano. `LOW` turns it
+  on. After switching on, the ESP waits 3 s so the Nano can boot before frames are sent.
 - **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
   off at night or when the display is powered off.