firmware: add XIAO C6 sensor node

docs/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- Added standalone XIAO ESP32-C6 BNO086/MAX-M10S sensor-node firmware and a versioned RS485 telemetry contract without changing Cargo `/api/v1` or JBD/NimBLE behavior.
+
+## Unreleased
+
 ### Added
 - Cargo ESP local OTA firmware upload page at `/ota`.
 - Cargo ESP OTA API endpoint at `POST /api/v1/system/ota`.

docs/README.md

@@ -53,5 +53,6 @@ The LILYGO T-Relay-S3 6-way board has been ordered as a future Cargo ESP candida
 Current development remains on the 2-channel relay board while the API and docs are prepared for output-count-agnostic operation.
 
 - `api-v1-audit.md` - Audit of legacy API paths vs `/api/v1` namespace.
+- `SENSOR_NODE_PROTOCOL.md` - XIAO ESP32-C6 BNO086/MAX-M10S RS485 telemetry contract.
 
 - `waveshare-5b-bringup-checklist.md` - Bench bring-up checklist for the Waveshare ESP32-S3 Touch LCD 5B dashboard.

docs/SENSOR_NODE_PROTOCOL.md

@@ -0,0 +1,17 @@
+# XIAO ESP32-C6 Sensor Node Protocol
+
+The XIAO sensor node reads the BNO086 and MAX-M10S and publishes vehicle telemetry over half-duplex RS485 at 115200 8-N-1, one JSON object per line. It owns no control state. Cargo remains the controller, HTTP server, and `/api/v1` source of truth; JBD/NimBLE behavior is unchanged.
+
+The expansion-board pins are D2 driver enable, D4 TX, and D5 RX. Only physical bus ends should be terminated. Confirm A/B polarity because vendor labels vary.
+
+## Schema version 1
+
+`sensor_status` includes `node_id`, `firmware_version`, `uptime_ms`, and:
+
+- `imu`: `online`, `valid`, `accuracy`, quaternion, yaw, pitch, and roll in degrees.
+- `gps`: `online`, `fix`, satellites, latitude, longitude, altitude meters, speed kph, course degrees, and structured UTC.
+
+Consumers must check `valid` or `fix` before displaying measurements and ignore unknown fields. The native breakout coordinate frame is reported initially; installation-specific axis remapping should follow physical calibration.
+
+This link does not replace dashboard-to-Cargo WiFi/HTTP. A future dashboard receiver may display this telemetry without moving relay, BMS, alarm, or configuration authority away from Cargo.
+

firmware/xiao-esp32c6-sensor-node/README.md

@@ -0,0 +1,15 @@
+# XIAO ESP32-C6 Sensor Node
+
+This Arduino sketch reads a BNO086 IMU and MAX-M10S GPS and publishes versioned, newline-delimited JSON over the Seeed RS485 expansion board. It produces vehicle telemetry only; Cargo remains the controller and `/api/v1` authority.
+
+| Function | XIAO pin |
+| --- | --- |
+| RS485 DE / TX / RX | D2 / D4 / D5 |
+| BNO086 SDA / SCL | D7 / D8 |
+| GPS TX to XIAO RX | D9 |
+| GPS RX from XIAO TX | D10 |
+
+Power both sensors from 3.3V with shared ground. Required Arduino libraries are Adafruit BNO08x, TinyGPSPlus, and ArduinoJson 6.x. Enable **USB CDC On Boot** for diagnostics. GPS uses hardware UART0 and RS485 uses hardware UART1.
+
+The node sends `sensor_status` at 115200 baud every 200 ms. See `docs/SENSOR_NODE_PROTOCOL.md`. The initial firmware is transmit-only; add a dashboard receiver after bench-testing the node, RS485 polarity, and sensor axes.
+

firmware/xiao-esp32c6-sensor-node/gps.cpp

@@ -0,0 +1,19 @@
+#include <TinyGPSPlus.h>
+#include "gps.h"
+#include "sensor_node_config.h"
+namespace { TinyGPSPlus parser; }
+
+void initGps(HardwareSerial& serial) { serial.begin(GPS_BAUD, SERIAL_8N1, GPS_RX_PIN, GPS_TX_PIN); }
+void updateGps(HardwareSerial& serial, GpsState& s) {
+    while (serial.available()) { s.lastByteMs = millis(); parser.encode(serial.read()); }
+    s.online = s.lastByteMs && millis() - s.lastByteMs <= SENSOR_STALE_MS;
+    s.fix = parser.location.isValid() && parser.location.age() <= SENSOR_STALE_MS;
+    s.satellites = parser.satellites.isValid() ? parser.satellites.value() : 0;
+    if (parser.location.isValid()) { s.latitude = parser.location.lat(); s.longitude = parser.location.lng(); }
+    if (parser.altitude.isValid()) s.altitudeMeters = parser.altitude.meters();
+    if (parser.speed.isValid()) s.speedKph = parser.speed.kmph();
+    if (parser.course.isValid()) s.courseDegrees = parser.course.deg();
+    if (parser.date.isValid()) { s.year = parser.date.year(); s.month = parser.date.month(); s.day = parser.date.day(); }
+    if (parser.time.isValid()) { s.hour = parser.time.hour(); s.minute = parser.time.minute(); s.second = parser.time.second(); }
+}
+

firmware/xiao-esp32c6-sensor-node/gps.h

@@ -0,0 +1,6 @@
+#pragma once
+#include <HardwareSerial.h>
+#include "sensor_state.h"
+void initGps(HardwareSerial& serial);
+void updateGps(HardwareSerial& serial, GpsState& state);
+

firmware/xiao-esp32c6-sensor-node/imu.cpp

@@ -0,0 +1,39 @@
+#include <Adafruit_BNO08x.h>
+#include <Wire.h>
+#include <math.h>
+#include "imu.h"
+#include "sensor_node_config.h"
+
+namespace {
+Adafruit_BNO08x bno086(-1);
+sh2_SensorValue_t value;
+bool initialized = false;
+float degrees(float r) { return r * 180.0f / PI; }
+void updateEuler(ImuState& s) {
+    float sinr = 2 * (s.real * s.i + s.j * s.k);
+    float cosr = 1 - 2 * (s.i * s.i + s.j * s.j);
+    s.rollDegrees = degrees(atan2f(sinr, cosr));
+    float sinp = 2 * (s.real * s.j - s.k * s.i);
+    s.pitchDegrees = degrees(fabsf(sinp) >= 1 ? copysignf(PI / 2, sinp) : asinf(sinp));
+    float siny = 2 * (s.real * s.k + s.i * s.j);
+    float cosy = 1 - 2 * (s.j * s.j + s.k * s.k);
+    s.yawDegrees = degrees(atan2f(siny, cosy));
+}
+}
+
+bool initImu() {
+    Wire.begin(BNO086_SDA_PIN, BNO086_SCL_PIN);
+    initialized = bno086.begin_I2C(BNO086_I2C_ADDRESS, &Wire);
+    if (initialized) initialized = bno086.enableReport(SH2_ROTATION_VECTOR, IMU_REPORT_INTERVAL_US);
+    return initialized;
+}
+
+void updateImu(ImuState& s) {
+    s.online = initialized;
+    if (!initialized || !bno086.getSensorEvent(&value) || value.sensorId != SH2_ROTATION_VECTOR) return;
+    s.i = value.un.rotationVector.i; s.j = value.un.rotationVector.j;
+    s.k = value.un.rotationVector.k; s.real = value.un.rotationVector.real;
+    s.accuracy = value.status; s.valid = true; s.lastUpdateMs = millis();
+    updateEuler(s);
+}
+

firmware/xiao-esp32c6-sensor-node/imu.h

@@ -0,0 +1,5 @@
+#pragma once
+#include "sensor_state.h"
+bool initImu();
+void updateImu(ImuState& state);
+

firmware/xiao-esp32c6-sensor-node/rs485_transport.cpp

@@ -0,0 +1,30 @@
+#include <ArduinoJson.h>
+#include "rs485_transport.h"
+#include "sensor_node_config.h"
+
+void initRs485(HardwareSerial& serial) {
+    pinMode(RS485_DE_PIN, OUTPUT); digitalWrite(RS485_DE_PIN, LOW);
+    serial.begin(RS485_BAUD, SERIAL_8N1, RS485_RX_PIN, RS485_TX_PIN);
+}
+
+void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsState& gps) {
+    StaticJsonDocument<1024> doc;
+    doc["type"] = "sensor_status"; doc["schema_version"] = SENSOR_NODE_SCHEMA_VERSION;
+    doc["node_id"] = SENSOR_NODE_NAME; doc["firmware_version"] = SENSOR_NODE_FIRMWARE_VERSION;
+    doc["uptime_ms"] = millis();
+    JsonObject i = doc.createNestedObject("imu");
+    i["online"] = imu.online; i["valid"] = imu.valid && millis() - imu.lastUpdateMs <= SENSOR_STALE_MS;
+    i["accuracy"] = imu.accuracy;
+    JsonObject q = i.createNestedObject("quaternion"); q["i"] = imu.i; q["j"] = imu.j; q["k"] = imu.k; q["real"] = imu.real;
+    i["yaw_degrees"] = imu.yawDegrees; i["pitch_degrees"] = imu.pitchDegrees; i["roll_degrees"] = imu.rollDegrees;
+    JsonObject g = doc.createNestedObject("gps");
+    g["online"] = gps.online; g["fix"] = gps.fix; g["satellites"] = gps.satellites;
+    g["latitude"] = gps.latitude; g["longitude"] = gps.longitude; g["altitude_meters"] = gps.altitudeMeters;
+    g["speed_kph"] = gps.speedKph; g["course_degrees"] = gps.courseDegrees;
+    JsonObject utc = g.createNestedObject("utc"); utc["year"] = gps.year; utc["month"] = gps.month; utc["day"] = gps.day;
+    utc["hour"] = gps.hour; utc["minute"] = gps.minute; utc["second"] = gps.second;
+    digitalWrite(RS485_DE_PIN, HIGH); delayMicroseconds(20);
+    serializeJson(doc, serial); serial.write('\n'); serial.flush();
+    digitalWrite(RS485_DE_PIN, LOW);
+}
+

firmware/xiao-esp32c6-sensor-node/rs485_transport.h

@@ -0,0 +1,6 @@
+#pragma once
+#include <HardwareSerial.h>
+#include "sensor_state.h"
+void initRs485(HardwareSerial& serial);
+void publishSensorStatus(HardwareSerial& serial, const ImuState& imu, const GpsState& gps);
+

firmware/xiao-esp32c6-sensor-node/sensor_node_config.h

@@ -0,0 +1,20 @@
+#pragma once
+#include <Arduino.h>
+
+#define SENSOR_NODE_NAME "xiao_c6_sensor_node"
+#define SENSOR_NODE_FIRMWARE_VERSION "0.1.0"
+#define SENSOR_NODE_SCHEMA_VERSION 1
+#define RS485_DE_PIN D2
+#define RS485_TX_PIN D4
+#define RS485_RX_PIN D5
+#define BNO086_SDA_PIN D7
+#define BNO086_SCL_PIN D8
+#define GPS_RX_PIN D9
+#define GPS_TX_PIN D10
+#define BNO086_I2C_ADDRESS 0x4A
+#define GPS_BAUD 9600
+#define RS485_BAUD 115200
+#define IMU_REPORT_INTERVAL_US 20000
+#define TELEMETRY_INTERVAL_MS 200
+#define SENSOR_STALE_MS 2000
+

firmware/xiao-esp32c6-sensor-node/sensor_state.h

@@ -0,0 +1,21 @@
+#pragma once
+#include <Arduino.h>
+
+struct ImuState {
+    bool online = false, valid = false;
+    uint8_t accuracy = 0;
+    float i = 0, j = 0, k = 0, real = 1;
+    float yawDegrees = 0, pitchDegrees = 0, rollDegrees = 0;
+    unsigned long lastUpdateMs = 0;
+};
+
+struct GpsState {
+    bool online = false, fix = false;
+    uint32_t satellites = 0;
+    double latitude = 0, longitude = 0, altitudeMeters = 0;
+    double speedKph = 0, courseDegrees = 0;
+    uint16_t year = 0;
+    uint8_t month = 0, day = 0, hour = 0, minute = 0, second = 0;
+    unsigned long lastByteMs = 0;
+};
+

firmware/xiao-esp32c6-sensor-node/xiao-esp32c6-sensor-node.ino

@@ -0,0 +1,28 @@
+#include <Arduino.h>
+#include "gps.h"
+#include "imu.h"
+#include "rs485_transport.h"
+#include "sensor_node_config.h"
+
+HardwareSerial gpsSerial(0), rs485Serial(1);
+ImuState imuState;
+GpsState gpsState;
+unsigned long lastTelemetryMs = 0;
+
+void setup() {
+    Serial.begin(115200); delay(250);
+    Serial.println("XIAO ESP32-C6 sensor node booting");
+    initGps(gpsSerial); initRs485(rs485Serial);
+    imuState.online = initImu();
+    Serial.println(imuState.online ? "BNO086 ready" : "BNO086 not detected");
+}
+
+void loop() {
+    updateGps(gpsSerial, gpsState); updateImu(imuState);
+    unsigned long now = millis();
+    if (now - lastTelemetryMs >= TELEMETRY_INTERVAL_MS) {
+        publishSensorStatus(rs485Serial, imuState, gpsState); lastTelemetryMs = now;
+    }
+    delay(1);
+}
+

run_tests.py

@@ -4,6 +4,7 @@ import sys
 
 COMMANDS = [
     [sys.executable, "-m", "pytest", "tests/test_http_api_contract.py"],
+    [sys.executable, "-m", "pytest", "tests/test_xiao_sensor_node_contract.py"],
 ]
 
 

tests/test_xiao_sensor_node_contract.py

@@ -0,0 +1,27 @@
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+NODE = ROOT / "firmware" / "xiao-esp32c6-sensor-node"
+def source(name): return (NODE / name).read_text()
+
+def test_wiring_matches_assembly():
+    config = source("sensor_node_config.h")
+    expected = {"RS485_DE_PIN":"D2", "RS485_TX_PIN":"D4", "RS485_RX_PIN":"D5",
+                "BNO086_SDA_PIN":"D7", "BNO086_SCL_PIN":"D8", "GPS_RX_PIN":"D9", "GPS_TX_PIN":"D10"}
+    for name, pin in expected.items(): assert re.search(rf"#define\s+{name}\s+{pin}\b", config)
+
+def test_transport_contract():
+    text = source("rs485_transport.cpp")
+    for token in ['doc["type"] = "sensor_status"', 'doc["schema_version"]', 'createNestedObject("imu")', 'createNestedObject("gps")', "serial.write('\\n')", "serial.flush()"]:
+        assert token in text
+
+def test_loop_is_non_blocking():
+    loop = source("xiao-esp32c6-sensor-node.ino").split("void loop()", 1)[1]
+    assert "updateGps" in loop and "updateImu" in loop and "TELEMETRY_INTERVAL_MS" in loop
+    assert not re.search(r"delay\((?:[2-9]|[1-9]\d+)\)", loop)
+
+def test_node_does_not_take_cargo_or_ble_authority():
+    combined = "\n".join(p.read_text() for p in NODE.glob("*.*"))
+    assert "NimBLE" not in combined and "JBD" not in combined and "server.on(" not in combined
+