Arduino additions

arduino/IMU.md

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+# WT61 IMU Quick Reference
+
+6-axis IMU (accelerometer + gyroscope) with onboard angle calculation via Kalman filter.
+
+## Serial Configuration
+
+| Setting | Factory Default | Our Config |
+|---------|-----------------|------------|
+| Baud rate | 115200 | 9600 |
+| Output rate | 100Hz | 20Hz |
+
+**Config command** (send at 115200 to switch to 9600/20Hz):
+```
+0xFF 0xAA 0x64
+```
+This is saved to flash - persists across power cycles.
+
+## Wiring (ATmega328P / AltSoftSerial)
+
+| WT61 Pin | Arduino Pin | Notes |
+|----------|-------------|-------|
+| TX | 8 (RX) | AltSoftSerial fixed pin |
+| RX | 9 (TX) | Only needed for config commands |
+| VCC | 5V | |
+| GND | GND | |
+
+## Packet Structure
+
+11 bytes per packet, continuous stream (3 packet types interleaved):
+
+```
+Byte 0:    0x55 (header)
+Byte 1:    Packet type
+Bytes 2-3: Value 0 (int16_t, little-endian)
+Bytes 4-5: Value 1
+Bytes 6-7: Value 2
+Bytes 8-9: Temperature (usually ignored)
+Byte 10:   Checksum (sum of bytes 0-9, lower 8 bits)
+```
+
+### Packet Types
+
+| Type | Byte 1 | V0 | V1 | V2 |
+|------|--------|----|----|-----|
+| Acceleration | 0x51 | ax | ay | az |
+| Gyroscope | 0x52 | gx | gy | gz |
+| Angle | 0x53 | roll | pitch | yaw |
+
+## Scale Factors
+
+| Measurement | Formula | Range |
+|-------------|---------|-------|
+| Acceleration | `raw / 32768.0 * 16.0` | +/-16g |
+| Gyroscope | `raw / 32768.0 * 2000.0` | +/-2000 deg/s |
+| Angle | `raw / 32768.0 * 180.0` | +/-180 deg |
+| Temperature | `raw / 340.0 + 36.25` | Celsius |
+
+## Known Quirks
+
+- **Boot time**: Module needs ~200-500ms after power-on before sending valid data
+- **Config at wrong baud**: Commands sent at wrong baud rate are ignored (garbled bytes) - this is actually useful for idempotent config-on-boot
+- **AltSoftSerial at 115200**: Technically out of spec for 16MHz AVR, but TX-only bursts of a few bytes work fine. Don't try sustained RX at that rate.
+
+## Commands
+
+All commands are 3 bytes: `0xFF 0xAA <data>`
+
+| Data Byte | Function | Notes |
+|-----------|----------|-------|
+| 0x52 | Zero Z-axis angle | Resets yaw to 0 |
+| 0x67 | Accelerometer calibration | Keep module level, zeros X/Y |
+| 0x60 | Toggle sleep mode | Toggles between standby and active |
+| 0x61 | Serial mode | Enable UART, disable I2C |
+| 0x62 | I2C mode | Enable I2C, disable UART |
+| 0x63 | 115200 baud / 100Hz | Factory default |
+| 0x64 | 9600 baud / 20Hz | Our config |
+| 0x65 | Horizontal mounting | Module placed flat |
+| 0x66 | Vertical mounting | Module placed upright |