powerpods/main/README.md

# Powerpod firmware

ESP32-S3 firmware for Powerpod nodes. Master and slave devices run the **same binary**; role and ESP-NOW network are selected at boot via DIP switches and an I2C IO expander.

## System overview

```
                    ┌─────────────┐
                    │     PC      │
                    │  (goTool)   │
                    └──────┬──────┘
                           │ UART1 (921600)
                           │ framed commands + protobuf
                    ┌──────▼──────┐
                    │   MASTER    │
                    │   ESP32     │
                    └──────┬──────┘
                           │ ESP-NOW (WiFi channel = network ID)
              ┌────────────┼────────────┐
              │            │            │
       ┌──────▼──────┐ ┌───▼────┐ ┌─────▼─────┐
       │   SLAVE     │ │ SLAVE  │ │   SLAVE   │
       └─────────────┘ └────────┘ └───────────┘
```

| Role   | UART to PC | ESP-NOW |
|--------|------------|---------|
| Master | Yes — command handler, protobuf replies | Broadcasts discover; collects slave info |
| Slave  | No         | Responds to discover with slave info |

Master keeps a **client registry** (`client_registry.c`) of slaves seen via ESP-NOW. The PC can query it with the `CLIENT_INFO` UART command.

## Boot configuration

Read in `app_main()` before subsystems start. Stored in `app_config_t` (`app_config.h`).

| Setting   | Source | Notes |
|-----------|--------|--------|
| `master`  | GPIO `DIP_MASTER` (pin 4) | Low = master, high = slave |
| `network` | I2C IO expander `0x20`, port bits (nibble reversed) | Value 1–8 → ESP-NOW WiFi channel 1–8 |
| `running_partition` | OTA API | Active partition label |

Pins (`powerpod.h`):

| Signal | GPIO |
|--------|------|
| DIP master | 4 |
| I2C SCL | 5 |
| I2C SDA | 6 |
| UART TX | 3 |
| UART RX | 2 |
| LED ring | 7 |
| BMA456 INT | 10 |
| Button (Taster) | 12 |
| LiPo sense 1 (ADC) | 1 |
| LiPo sense 2 (ADC) | 12 (skipped if same as button) |

> **TODO:** GPIO assignments above are provisional; confirm pinning against the real board before release.

Startup order:

1. Read DIP + IO expander → `app_config`
2. **I2C bus** — IO expander `0x20`; optional **BMA456H** (`init_bma456`, same bus)
3. `esp_now_comm_init(&app_config)` — WiFi + ESP-NOW
4. `led_ring_init()`
5. `board_input_init()` — button press logs, LiPo ADC logs every **10 s**
6. **Master only:** command queue, UART, registered commands (e.g. VERSION)

## BMA456 accelerometer (`bosch456.c`)

Powerpod uses the Bosch **BMA456H** (hearable) variant, not the generic `bma456w` examples in the vendor tree.

| Item | Value |
|------|--------|
| Project wrapper | `main/bosch456.c`, `main/bosch456.h` |
| Vendor component | `components/bma456` — only `bma4.c` + `bma456h.c` are linked |
| I2C | Shared bus with IO expander (SCL/SDA GPIO 5/6), address **0x18**, **100 kHz** |
| Interrupt | **GPIO 10**, active high, tap events (single / double / triple) |
| Polling | FreeRTOS task `bma456_poll`, **10 Hz** accel read |

**Boot:** `init_bma456(bus_handle)` runs on **master and slave** after the IO expander. If the sensor is missing or init fails, firmware logs `BMA456 init skipped` and continues (`bma456_is_ready() == false`).

**Accel logging:** Samples are printed only when any axis changes by more than the **deadzone** (raw LSB) since the last logged sample (default **100**). This is a **software** filter on top of the sensor; it does not change BMA456 hardware thresholds.

**Persistence:** The local deadzone is stored in the **`nvs`** partition (namespace `powerpod`, key `accel_dz`) via `pod_settings.c`. Each node (master or slave) keeps its own value across reboot. Loaded at boot after `init_bma456()`; saved when set locally (UART `client_id = 0`, `all_clients` on master, or ESP-NOW deadzone on a slave).

**Configuration paths:**

| Path | Effect |
|------|--------|
| UART `ACCEL_DEADZONE` with `client_id = 0` | Set + save local deadzone |
| ESP-NOW `SET_ACCEL_DEADZONE` | Set + save on the receiving slave |
| `make gotool-deadzone-set DEADZONE=… CLIENT=0` | Host shortcut for local deadzone |

**Logs:** `[BMA456] ACC X=… Y=… Z=…` when deadzone exceeded; `[BMA456] tap: single|double|triple` on interrupt.

Regenerate nanopb only when changing protos; sensor code has no code generation step.

## ESP-NOW discovery

Implementation: `esp_now_comm.c` / `esp_now_comm.h`.

WiFi is brought up in STA mode (no AP association). Channel = `app_config.network` (clamped to 1–13).

### Air protocol (nanopb)

Schema: `proto/esp_now_messages.proto`. Encode/decode: `esp_now_proto.c`. The ESP-NOW payload is a single encoded `EspNowMessage` (no extra framing).

| `EspNowMessageType` | Direction | `oneof` payload |
|---------------------|-----------|-----------------|
| `ESPNOW_DISCOVER` | Master → broadcast `FF:FF:FF:FF:FF:FF` | `EspNowDiscover` (`network`) |
| `ESPNOW_SLAVE_INFO` | Slave → master | `EspNowSlavePresence` |
| `ESPNOW_HEARTBEAT` | Slave → master | `EspNowSlavePresence` (same fields) |
| `ESPNOW_SET_ACCEL_DEADZONE` | Master → slave | `EspNowAccelDeadzone` (`deadzone` LSB) |
| `ESPNOW_UNICAST_TEST` | Master → slave | `EspNowUnicastTest` (`seq`) |
| `ESPNOW_FIND_ME` | Master → slave | `EspNowFindMe` (`client_id` filter) — LED locate sequence |
| `ESPNOW_RESTART` | Master → slave | `EspNowRestart` (`client_id` filter) — reboot slave |
| `ESPNOW_ACCEL_SAMPLE` | Slave → master | `EspNowAccelSample` (`slave_id`, `x`, `y`, `z` raw LSB) — ~every 16 ms |
| `ESPNOW_OTA_START` | Master → slave (unicast) | `EspNowOtaStart` (`total_size`) |
| `ESPNOW_OTA_PAYLOAD` | Master → slave | `EspNowOtaPayload` (`seq`, up to 200 B `data`) |
| `ESPNOW_OTA_END` | Master → slave | `EspNowOtaEnd` |
| `ESPNOW_OTA_STATUS` | Slave → master | `EspNowOtaStatus` (same status codes as UART OTA) |

### ESP-NOW OTA (master → slaves)

Triggered automatically after a successful UART `OTA_END` on the master (or manually via UART `OTA_START_ESPNOW` if an image is already **staged**). Implementation: `ota_espnow.c`.

| Step | Master → slave | Slave → master |
|------|----------------|----------------|
| 1 | `ESPNOW_OTA_START` + `total_size` | `ESPNOW_OTA_STATUS` preparing, then **ready** |
| 2 | `ESPNOW_OTA_PAYLOAD` (**≤200 B**, shared `seq`) | **block_ack** after each **4096 B** written to flash |
| 3 | `ESPNOW_OTA_END` | **success** or **failed** (+ `bytes_written`) |

Master reads the staged partition with `esp_partition_read` (same image just written via UART). Only **available** registry slaves are updated. The last transfer block may be **under 4096 B** — no block_ack is waited for that block; slaves flush the remainder on `ESPNOW_OTA_END`.

Status codes match UART `OtaStatusPayload` (`1`…`5`). After success, master and slaves have the boot partition set — **reboot all nodes** to run the new firmware.

`EspNowSlavePresence`: `network`, `mac` (6 bytes), `version`, `slave_id`, `available`, `used`.

**Master:** task `espnow_disc` sends `DISCOVER` every **500 ms** on the configured network. Logs `slave joined id=… mac=… ver=…` when a new slave is seen (up to 16 entries). Task `espnow_mon` runs every **1 s** and marks a client **inactive** (`available = false`) if no `SLAVE_INFO` or `HEARTBEAT` was received for **3 s** (three missed 1 s heartbeats). A later heartbeat sets `available` true again and logs reactivation.

**Slave:** on first matching `DISCOVER`, logs `joined network N, master …`, sends `SLAVE_INFO` once, then sends `HEARTBEAT` to the master every **1 s**. While joined, periodic discovers from the same master refresh a “master alive” timer; if no discover arrives for **5 s**, the slave treats the master as lost, clears join state, and will register again on the next discover (reconnect). Discover from a different master is ignored while already joined.

Monitor via USB-JTAG (`/dev/ttyACM0`) while using a USB-serial adapter on **GPIO2/3** (`/dev/ttyUSB0`) for UART — they are different interfaces.

## UART (master only)

Hardware: **UART1**, **921600** baud, **TX = GPIO3**, **RX = GPIO2** (adapter TX → ESP RX, adapter RX → ESP TX).

### Frame format

| Field | Value |
|-------|--------|
| Start | `0xAA` |
| Length | 1 byte, payload size 1–252 |
| Payload | `length` bytes |
| Checksum | XOR of all payload bytes |
| Stop | `0xCC` |

### Command handler payload

| Offset | Meaning |
|--------|---------|
| 0 | Command ID (`MessageType` / `msg_id`) |
| 1… | Arguments (handler receives bytes after ID only) |

Example VERSION request: single-byte payload `03` → frame `AA 01 03 03 CC`.

Logging:

- `[UART] received message cmd=0x… len=…`
- `[CMDH] trigger command VERSION (0x03)` (or other name from `message_type_name()`)

## Command handler

Generic dispatch for host commands (UART today; `msg_post()` for in-firmware sources later).

```
UART  →  generic_msg_t queue  →  vCmdDispatcherTask  →  registered handler
```

| API | Description |
|-----|-------------|
| `init_cmdHandler(queue)` | Start dispatcher task (priority 5) |
| `msg_register_handler(id, cb)` | Register callback; max 32 handlers |
| `msg_post(id, data, len)` | Enqueue from firmware (e.g. future ESP-NOW → PC path) |

```c
typedef void (*msg_callback_t)(const uint8_t *data, size_t len);
```

Init order on master:

```c
cmd_queue = xQueueCreate(10, sizeof(generic_msg_t));
init_cmdHandler(cmd_queue);
init_uart(cmd_queue);
cmd_version_register();
```

## Protobuf (`proto/uart_messages.proto`)

Host and master speak nanopb-encoded `UartMessage` inside UART frames (byte 0 = `MessageType`, bytes 1… = encoded message).

| ID | Name | Status |
|----|------|--------|
| 3 | `VERSION` | Implemented (`cmd/cmd_version.c`) |
| 4 | `CLIENT_INFO` | Implemented (`cmd/cmd_client_info.c`) — slave list from registry |
| 5 | `CLIENT_INPUT` | Planned |
| 6 | `ACCEL_DEADZONE` | Implemented (`cmd/cmd_accel_deadzone.c`) — get/set accel filter LSB |
| 7 | `ESPNOW_UNICAST_TEST` | Implemented (`cmd/cmd_espnow_unicast_test.c`) |
| 8 | `LED_RING` | Implemented (`cmd/cmd_led_ring.c`) — ring progress bar (0–100 %, RGB, intensity) |
| 16 | `OTA_START` | Implemented (`cmd/cmd_ota.c`) — begin UART OTA on inactive slot |
| 17 | `OTA_PAYLOAD` | Implemented — up to 200 B per frame; device buffers 4 KiB |
| 18 | `OTA_END` | Implemented — flush, `esp_ota_end`, push image to slaves via ESP-NOW, set boot |
| 19 | `OTA_STATUS` | Device → host (prepare/ready/block ACK/success/failed) |
| 20 | `OTA_START_ESPNOW` | Implemented — re-distribute staged image to slaves only |
| 21 | `OTA_SLAVE_PROGRESS` | Implemented (`cmd/cmd_ota_slave_progress.c`) — query per-slave ESP-NOW OTA progress |
| 22 | `FIND_ME` | Implemented (`cmd/cmd_espnow_find_me.c`) — `client_id=0` local ring, `>0` ESP-NOW to slave |
| 23 | `RESTART` | Implemented (`cmd/cmd_restart.c`) — `client_id=0` reboot master, `>0` ESP-NOW reboot slave |
| 24 | `ACCEL_SNAPSHOT` | Implemented (`cmd/cmd_accel_snapshot.c`) — cached slave accel from ESP-NOW stream |

Regenerate C code:

```bash
make proto_generate
# or individually:
make proto_generate_uart
make proto_generate_espnow
```

After generation, ensure `main/proto/*.pb.c` includes use `#include "…pb.h"` (not `main/proto/…`).

Build embeds `POWERPOD_GIT_HASH` via `git rev-parse` in `main/CMakeLists.txt`.

### VERSION command

**Request:** framed payload `03` only.

**Response:** payload `03` + nanopb `UartMessage`:

- `type = VERSION`
- `version_response.version` — `POWERPOD_FW_VERSION`
- `version_response.git_hash` — build git hash string
- `version_response.running_partition` — active OTA label (`ota_0` / `ota_1`)

Encoding: `uart_send_uart_message()` in `uart_proto.c`.

At boot, firmware logs the running partition and OTA slot index (A/B).

### UART OTA (A/B)

**UART upload is master-only.** Slaves receive the same image afterwards via [ESP-NOW OTA](#esp-now-ota-master--slaves).

Inactive app partition is selected with `esp_ota_get_next_update_partition()`; `esp_ota_begin` erases it (can take ~30 s — host should wait).

| Step | Host → master | Master → host |
|------|---------------|---------------|
| 1 | `OTA_START` + `total_size` | `OTA_STATUS` preparing, then **ready** (+ `target_slot` 0/1) |
| 2 | `OTA_PAYLOAD` chunks (**≤200 B**, `seq` optional) | `OTA_STATUS` **block_ack** only after each **4096 B** written to flash |
| 3 | `OTA_END` | Stages image, runs ESP-NOW OTA to all available slaves, sets boot partition, then `OTA_STATUS` **success** or **failed** |

`OTA_END` can take a long time on the wire (slave flash + ESP-NOW); the host should use a generous read timeout.

During OTA the LED ring shows progress at ~5 % brightness: **blue** while the image is written (UART on master, ESP-NOW on slaves), **green** on the master while it forwards the image to slaves over ESP-NOW. On **success** the ring gives one short **green** blink; on **failure** one **red** blink and ESP-NOW distribution is not started (failed UART upload / `OTA_END` validation).

`OTA_START_ESPNOW` (type `20`): re-run ESP-NOW distribution from the last staged image without a new UART upload (no-op if nothing staged).

Implementation: `ota_uart.c` (4 KiB buffer, `esp_ota_write`), `ota_espnow.c`, `cmd/cmd_ota.c`.

Host upload:

```bash
go run . -port /dev/ttyUSB0 ota build/powerpod.bin
```

`OtaStatusPayload.status`: `1` preparing, `2` ready, `3` block_ack, `4` success, `5` failed, `6` distributing (`bytes_written` = progress, `target_slot` = slave count).

### OTA_SLAVE_PROGRESS command

**Request:** framed `15` (`0x15`) + optional `ota_slave_progress_request` (`client_id`; `0` = all slaves in the current/last distribution session).

**Response:** `ota_slave_progress_response`:

| Field | Meaning |
|-------|---------|
| `active` | ESP-NOW distribution running |
| `total_bytes` | Image size |
| `aggregate_bytes` | Overall bytes sent to all slaves |
| `slave_count` | Number of slaves in session |
| `slaves[]` | Per slave: `client_id`, `bytes_written`, `total_bytes`, `status`, `error` |

Per-slave `status`: `0` idle, `1` preparing, `2` ready, `3` block_ack/distributing, `4` success, `5` failed.

```bash
go run . -port /dev/ttyUSB0 ota-progress
go run . -port /dev/ttyUSB0 ota-progress -client 16
```

### ACCEL_DEADZONE command

Sets the **software** deadzone used by `bosch456.c` when logging accel (see [BMA456 accelerometer](#bma456-accelerometer-bosch456c)). Default **100** LSB.

**Request:** framed `06` + nanopb `UartMessage` with `accel_deadzone_request`:

| Field | Meaning |
|-------|---------|
| `write` | `false` = read, `true` = write |
| `deadzone` | Threshold in LSB (write) |
| `client_id` | `0` = local sensor on this node; `>0` = slave id (master) |
| `all_clients` | Master: ESP-NOW unicast to every registered slave |

**Response:** `accel_deadzone_response` with applied `deadzone`, `success`, and `slaves_updated` (ESP-NOW count).

### ACCEL_SNAPSHOT command

Read **cached** accelerometer samples on the **master** (one entry per registered slave). Slaves send `ESPNOW_ACCEL_SAMPLE` to the master every **16 ms** (`esp_now_comm.c`); the master stores the latest value per client in `client_registry.c`.

**Request:** framed `18` (`0x18`) + optional `accel_snapshot_request` (`client_id`: `0` = all slaves, `>0` = one id).

**Response:** `accel_snapshot_response.samples[]`:

| Field | Meaning |
|-------|---------|
| `client_id` | Slave id (registry) |
| `valid` | At least one ESP-NOW sample received since boot |
| `x`, `y`, `z` | Raw BMA456 LSB (±2g) |
| `age_ms` | Ms since last sample from that slave |

Host:

```bash
go run . -port /dev/ttyUSB0 accel
```

External API (`serve -api-addr :8081`) polls this command every 16 ms and streams JSON over WebSocket.

### ESPNOW_UNICAST_TEST command

Minimal master→slave ESP-NOW unicast check (no BMA456). Use this before debugging `ACCEL_DEADZONE` unicast.

**Request:** framed `07` + `espnow_unicast_test_request` (`client_id`, `seq`).

**Response:** `espnow_unicast_test_response` (`success`, `seq`).

**Firmware logs:** master `unicast TEST to … seq=N`; slave `UNICAST TEST OK from master … seq=N`.

### FIND_ME command

Locate a pod: the LED ring blinks **3× red, 3× green, 3× blue** at full brightness.

**Request:** framed `22` + `espnow_find_me_request` (`client_id`: `0` = master only, `>0` = ESP-NOW unicast to that slave).

**Response:** `espnow_find_me_response` (`success`, `client_id`).

```bash
go run . -port /dev/ttyUSB0 find-me
go run . -port /dev/ttyUSB0 find-me -client 16
```

### RESTART command

Reboot the master (`client_id=0`) or one slave via ESP-NOW (`client_id` = registry id). The device sends the UART response, then restarts after ~150 ms.

**Request:** framed `23` + `restart_request`  
**Response:** `restart_response` (`success`, `client_id`)

```bash
go run . -port /dev/ttyUSB0 restart
go run . -port /dev/ttyUSB0 restart -client 16
```

### LED_RING command

Control the 95-LED ring from the host. The firmware **does not** animate digits locally; only UART updates the display.

**Request:** framed `08` + `led_ring_progress_request`:

| Field | Meaning |
|-------|---------|
| `mode` | `0` = clear, `1` = progress bar, `2` = digit, `3` = blink full ring, `4` = find-me (R/G/B ×3 @ full brightness) |
| `progress` | 0–100 (% of ring lit, mode `1`) |
| `digit` | 0–10 (mode `2`, same segment maps as built-in digits) |
| `r`, `g`, `b` | Color 0–255 |
| `intensity` | Brightness 0–255 (scaled into RGB; `0` → firmware default ~5 %) |
| `blink_ms`, `blink_count` | Pulse length and count (mode `3`; defaults 350 ms, 1) |

**Response:** `led_ring_progress_response` (`success`, `mode`, `progress`, `digit`).

```bash
go run . -port /dev/ttyUSB0 led-ring -mode progress -progress 75 -g 80 -b 255
go run . -port /dev/ttyUSB0 led-ring -mode digit -digit 7 -r 255 -g 200
go run . -port /dev/ttyUSB0 led-ring -mode clear
go run . -port /dev/ttyUSB0 led-ring -mode blink -g 255 -blink-count 2
go run . -port /dev/ttyUSB0 find-me
go run . -port /dev/ttyUSB0 find-me -client 16
go run . -port /dev/ttyUSB0 led-ring -mode find-me
```

### CLIENT_INFO command

**Request:** framed payload `04` only (`MessageType.CLIENT_INFO`).

**Response:** payload `04` + nanopb `UartMessage` with `client_info_response.clients` — one `ClientInfo` per registered slave (from ESP-NOW `SLAVE_INFO`).

Fields per client: `id`, `mac`, `version`, `available`, `used`, `last_ping`, `last_success_ping` — **milliseconds since** the last packet / last successful heartbeat (computed when `CLIENT_INFO` is answered; typically 0–1000 while the slave is heartbeating every 1 s).

## Client registry

| API | Description |
|-----|-------------|
| `client_registry_init()` | Clear all slots (called from `esp_now_comm_init`) |
| `client_registry_upsert(mac, id, version, …)` | Add or refresh client; updates ping timestamps |
| `client_registry_heartbeat(mac, id, version, …)` | Same as upsert for heartbeats; reactivates inactive clients |
| `client_registry_check_timeouts(timeout_ms)` | Mark stale clients inactive (master monitor task) |
| `client_registry_count()` / `client_registry_at(i)` | Iterate for UART encoding |

Slaves register when the master receives `SLAVE_INFO` on the matching network; `HEARTBEAT` keeps them marked available. The registry **MAC is always the ESP-NOW source address** (`recv_info.src_addr`), not the optional `mac` bytes in the protobuf (used only on the wire for debugging).

`slave_id` is the sender’s WiFi STA address last octet (`mac[5]`); it can collide across devices — use `gotool clients` and match the full MAC.

## Host tool (`goTool/`)

Go CLI to test UART from a PC connected to the **master** only.

```bash
cd goTool
go mod tidy
go run . -port /dev/ttyUSB0 version
go run . -port /dev/ttyUSB0 clients
```

| Flag | Default | Description |
|------|---------|-------------|
| `-port` | (required) | Serial device, e.g. `/dev/ttyUSB0` |
| `-baud` | `921600` | Must match `UART_BAUD_RATE` |

| Command | Description |
|---------|-------------|
| `version` | Firmware version and git hash |
| `clients` | Registered slaves from master client registry |

Regenerate Go protobuf:

```bash
protoc --go_out=./pb --go_opt=paths=source_relative \
  --go_opt=Muart_messages.proto=powerpod/gotool/pb \
  -I ../main/proto ../main/proto/uart_messages.proto
```

See `goTool/README.md` for tool-only notes.

## Build and flash

```bash
source ~/esp/esp-idf/export.sh   # or export.fish in fish
cd /path/to/powerpod
idf.py build
idf.py -p /dev/ttyUSB0 flash monitor   # USB-JTAG / console
```

Target: ESP32-S3. Close serial monitor on the UART adapter port before running `goTool` on the same device.

## Source files

| File | Role |
|------|------|
| `powerpod.c` | `app_main`, DIP/network config, init order |
| `powerpod.h` | Pin defines |
| `app_config.h` | `app_config_t` |
| `esp_now_comm.c/h` | WiFi, ESP-NOW, discover / slave info / OTA send |
| `ota_uart.c/h` | Shared 4 KiB OTA flash buffer (UART + ESP-NOW) |
| `ota_espnow.c/h` | Master: distribute staged image to slaves |
| `cmd/cmd_ota.c/h` | UART OTA command handlers (master only) |
| `uart.c/h` | Framed UART RX/TX |
| `uart_proto.c/h` | Encode/send `UartMessage` |
| `cmd/cmd_handler.c/h` | Command queue and dispatch |
| `uart_cmd.c/h` | Shared UART decode/send helpers for handlers |
| `cmd/cmd_version.c/h` | VERSION handler |
| `cmd/cmd_client_info.c/h` | CLIENT_INFO handler |
| `client_registry.c/h` | Registered slave table |
| `bosch456.c/h` | BMA456H I2C driver, accel poll, on-demand read, tap INT, deadzone filter |
| `cmd/cmd_accel_snapshot.c` | UART `ACCEL_SNAPSHOT` — cached slave accel |
| `board_input.c/h` | Taster GPIO12, LiPo ADC on GPIO1 / GPIO12 |
| `pod_settings.c/h` | NVS persistence (accel deadzone, …) |
| `led_ring.c/h` | LED ring (digit display, progress bar) |
| `cmd/cmd_led_ring.c` | UART `LED_RING` progress command |
| `proto/uart_messages.proto` | UART protocol schema |
| `proto/esp_now_messages.proto` | ESP-NOW protocol schema |
| `esp_now_proto.c/h` | Encode/decode `EspNowMessage` |
| `proto/*.pb.c/h` | Generated nanopb |
| `CMakeLists.txt` | Sources, `esp_wifi`, drivers, git hash |

## Adding a new feature (UART → ESP-NOW)

End-to-end walkthrough (protobuf, master handler, ESP-NOW unicast to slaves, goTool, dashboard) with **Find me** as the worked example:

**[docs/adding-a-feature.md](../docs/adding-a-feature.md)**

Short checklist:

1. Add or extend messages in `uart_messages.proto` (and `esp_now_messages.proto` if slaves are involved); run `make proto_generate` and `make gotool-proto`.
2. Implement device logic in a shared module (e.g. `led_ring.c`), not only in the UART handler.
3. Create `cmd/cmd_*.c`, register with `uart_cmd_register()`; decode with `uart_cmd_decode()` / `UART_CMD_REQ()`; reply with `uart_cmd_init_response()` + `uart_cmd_send()`.
4. Master → slave: `esp_now_comm_send_*()` + slave branch in `espnow_recv_cb`.
5. Extend `goTool` (CLI, optional `/api/…` and web UI).

For ESP-NOW-driven PC updates later: map slave state to `ClientInfo` and send `CLIENT_INFO` over UART from the master.