firmware: diagnose BNO086 I2C address

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

@@ -11,5 +11,6 @@ This Arduino sketch reads a BNO086 IMU and MAX-M10S GPS and publishes versioned,
 
 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.
+At startup the firmware scans D7/D8 and accepts either valid BNO08x address, `0x4A` or `0x4B`. If neither appears in the scan, verify 3.3V/GND, swap-check SDA and SCL, and confirm the breakout's PS0/PS1 jumpers are configured for I2C.
 
+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/imu.cpp

@@ -9,6 +9,26 @@ Adafruit_BNO08x bno086(-1);
 sh2_SensorValue_t value;
 bool initialized = false;
 float radiansToDegrees(float r) { return r * 180.0f / PI; }
+bool addressResponds(uint8_t address) {
+    Wire.beginTransmission(address);
+    return Wire.endTransmission() == 0;
+}
+
+uint8_t findBnoAddress() {
+    Serial.println("Scanning I2C bus on D7 SDA / D8 SCL...");
+    uint8_t bnoAddress = 0;
+    for (uint8_t address = 1; address < 127; address++) {
+        if (!addressResponds(address)) continue;
+        Serial.print("I2C device found at 0x");
+        if (address < 0x10) Serial.print('0');
+        Serial.println(address, HEX);
+        if (address == BNO086_I2C_ADDRESS || address == BNO086_I2C_ADDRESS_ALTERNATE) {
+            bnoAddress = address;
+        }
+    }
+    return bnoAddress;
+}
+
 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);
@@ -23,7 +43,18 @@ void updateEuler(ImuState& s) {
 
 bool initImu() {
     Wire.begin(BNO086_SDA_PIN, BNO086_SCL_PIN);
-    initialized = bno086.begin_I2C(BNO086_I2C_ADDRESS, &Wire);
+    Wire.setClock(BNO086_I2C_CLOCK_HZ);
+    delay(100);
+
+    uint8_t address = findBnoAddress();
+    if (address == 0) {
+        Serial.println("No BNO086 found at 0x4A or 0x4B");
+        return false;
+    }
+
+    Serial.print("Starting BNO086 at 0x");
+    Serial.println(address, HEX);
+    initialized = bno086.begin_I2C(address, &Wire);
     if (initialized) initialized = bno086.enableReport(SH2_ROTATION_VECTOR, IMU_REPORT_INTERVAL_US);
     return initialized;
 }

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

@@ -12,9 +12,10 @@
 #define GPS_RX_PIN D9
 #define GPS_TX_PIN D10
 #define BNO086_I2C_ADDRESS 0x4A
+#define BNO086_I2C_ADDRESS_ALTERNATE 0x4B
+#define BNO086_I2C_CLOCK_HZ 100000
 #define GPS_BAUD 9600
 #define RS485_BAUD 115200
 #define IMU_REPORT_INTERVAL_US 20000
 #define TELEMETRY_INTERVAL_MS 200
 #define SENSOR_STALE_MS 2000
-

tests/test_xiao_sensor_node_contract.py

@@ -26,6 +26,14 @@ def test_imu_does_not_redeclare_arduino_degrees_macro():
     assert "float degrees(" not in imu
     assert "radiansToDegrees" in imu
 
+def test_imu_scans_both_bno08x_i2c_addresses():
+    config = source("sensor_node_config.h")
+    imu = source("imu.cpp")
+    assert "BNO086_I2C_ADDRESS 0x4A" in config
+    assert "BNO086_I2C_ADDRESS_ALTERNATE 0x4B" in config
+    assert "findBnoAddress()" in imu
+    assert "Wire.endTransmission()" in imu
+
 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