firmware: recover slow BNO086 startup

docs/CHANGELOG.md

@@ -7,6 +7,7 @@
 - Prevented echoed or oversized RS485 frames from starving the XIAO sensor loop.
 - Silently discard malformed partial RS485 echoes instead of emitting recursive `invalid_json` responses.
 - Replaced sensor auto-detection with the verified installation settings: BNO086 address `0x4B` and MAX-M10S UART at 38400 baud.
+- Added fixed-address BNO086 power-up retries, runtime recovery, and report restoration after sensor resets.
 
 ## Unreleased
 

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

@@ -12,6 +12,7 @@ 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 at the verified 38400 baud; RS485 uses hardware UART1.
 
 The BNO086 uses the verified `0x4B` I2C address on D7/D8. If initialization fails, verify 3.3V/GND, SDA/SCL wiring, and the breakout's PS0/PS1 configuration.
+Firmware retries the fixed address during power-up and periodically retries afterward, so a slow BNO086 power-up does not require cycling the XIAO. If the BNO086 resets at runtime, rotation-vector reporting is enabled again automatically.
 
 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

@@ -8,7 +8,14 @@ namespace {
 Adafruit_BNO08x bno086(-1);
 sh2_SensorValue_t value;
 bool initialized = false;
+unsigned long lastInitAttemptMs = 0;
 float radiansToDegrees(float r) { return r * 180.0f / PI; }
+bool tryInitImu() {
+    lastInitAttemptMs = millis();
+    initialized = bno086.begin_I2C(BNO086_I2C_ADDRESS, &Wire);
+    if (initialized) initialized = bno086.enableReport(SH2_ROTATION_VECTOR, IMU_REPORT_INTERVAL_US);
+    return initialized;
+}
 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);
@@ -27,13 +34,29 @@ bool initImu() {
     delay(100);
     Serial.print("Starting BNO086 at 0x");
     Serial.println(BNO086_I2C_ADDRESS, HEX);
-    initialized = bno086.begin_I2C(BNO086_I2C_ADDRESS, &Wire);
+    for (int attempt = 1; attempt <= BNO086_INIT_ATTEMPTS; attempt++) {
-    if (initialized) initialized = bno086.enableReport(SH2_ROTATION_VECTOR, IMU_REPORT_INTERVAL_US);
+        if (tryInitImu()) return true;
-    return initialized;
+        Serial.print("BNO086 init attempt failed: ");
+        Serial.println(attempt);
+        delay(BNO086_INIT_RETRY_DELAY_MS);
+    }
+    return false;
+}
+
+void maintainImu(ImuState& s) {
+    if (initialized || millis() - lastInitAttemptMs < BNO086_RUNTIME_RETRY_MS) return;
+    Serial.println("Retrying BNO086 initialization");
+    s.online = tryInitImu();
+    if (s.online) Serial.println("BNO086 recovered");
 }
 
 void updateImu(ImuState& s) {
     s.online = initialized;
+    if (initialized && bno086.wasReset()) {
+        initialized = bno086.enableReport(SH2_ROTATION_VECTOR, IMU_REPORT_INTERVAL_US);
+        s.online = initialized;
+        if (!initialized) return;
+    }
     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;

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

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

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

@@ -13,6 +13,9 @@
 #define GPS_TX_PIN D10
 #define BNO086_I2C_ADDRESS 0x4B
 #define BNO086_I2C_CLOCK_HZ 100000
+#define BNO086_INIT_ATTEMPTS 8
+#define BNO086_INIT_RETRY_DELAY_MS 250
+#define BNO086_RUNTIME_RETRY_MS 5000
 #define GPS_BAUD 38400
 #define RS485_BAUD 115200
 #define IMU_REPORT_INTERVAL_US 20000

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

@@ -20,7 +20,7 @@ void setup() {
 }
 
 void loop() {
-    updateGps(gpsSerial, gpsState); updateImu(imuState);
+    updateGps(gpsSerial, gpsState); maintainImu(imuState); updateImu(imuState);
     pollRs485Commands(rs485Serial, imuState);
     String calibrationResult;
     if (updateLevelCalibration(imuState, calibrationResult)) {

tests/test_xiao_sensor_node_contract.py

@@ -35,6 +35,9 @@ def test_imu_uses_verified_i2c_address():
     imu = source("imu.cpp")
     assert "BNO086_I2C_ADDRESS 0x4B" in config
     assert "findBnoAddress" not in imu
+    assert "BNO086_INIT_ATTEMPTS" in imu
+    assert "BNO086_RUNTIME_RETRY_MS" in imu
+    assert "bno086.wasReset()" in imu
 
 def test_gps_uses_verified_uart_baud():
     config = source("sensor_node_config.h")