Arduino frameworks

- AltSoftSerial UART with WT61 IMU
- UART with Pi on TX0/RX0 (and USB)

arduino/README.md

@@ -11,7 +11,58 @@ Sensor interface running on Arduino Nano, communicating with Pi via UART.
 ## Current Capabilities
 
 - Battery voltage monitoring (voltage divider on A0)
-- Serial output at 9600 baud, 1Hz update rate
+- WT61 IMU/gyro via AltSoftSerial (9-axis: accel, gyro, euler angles)
+- Duplex UART to Pi at 115200 baud, 10Hz telemetry output
+- Simple text-based protocol for easy debugging
+
+## Dependencies
+
+Install via Arduino Library Manager:
+- **AltSoftSerial** by Paul Stoffregen - for WT61 IMU serial
+
+## Pin Assignments
+
+| Pin | Function |
+|-----|----------|
+| A0 | Battery voltage (via divider) |
+| D0 (RX) | Pi UART RX ← Arduino TX |
+| D1 (TX) | Pi UART TX → Arduino RX |
+| D8 | WT61 IMU RX (AltSoftSerial) |
+| D9 | WT61 IMU TX (unused, AltSoftSerial fixed pin) |
+| D13 | Status LED (heartbeat) |
+
+## Hardware
+
+- **MCU**: Arduino Nano (ATmega328P)
+- **Pi Connection**: UART at 115200 baud (TX→RX, RX→TX, common GND)
+- **IMU**: WT61 module at 9600 baud, 20Hz output
+- **Voltage sensing**: Resistor divider (100k/47k) scaled for 0-20V input
+
+## Protocol
+
+Simple text lines, one per sensor reading:
+```
+V_bat: 12.45
+Ax: 0.02
+Ay: -0.01
+Az: 1.00
+Gx: 0.50
+Gy: -0.25
+Gz: 0.10
+Roll: 2.35
+Pitch: -1.20
+Yaw: 45.80
+```
+
+If IMU data is stale (no valid packets for 200ms):
+```
+IMU: STALE
+```
+
+Commands from Pi are echoed back:
+```
+ACK: PING
+```
 
 ## Planned
 
@@ -19,18 +70,3 @@ Sensor interface running on Arduino Nano, communicating with Pi via UART.
 - Engine temperature (thermocouple/NTC)
 - Gear position indicator
 - Turn signal / high beam status
-
-## Hardware
-
-- **MCU**: Arduino Nano (ATmega328P)
-- **Connection**: UART to Pi GPIO (TX→RX, RX→TX, common GND)
-- **Voltage sensing**: Resistor divider scaled for 0-20V input range
-
-## Protocol
-
-Simple text-based for now:
-```
-V_bat: 12.45V
-```
-
-Future: structured binary or JSON for multiple sensors.