Sensor MCU (RP2040)
Overview
The RP2040 Pico runs CircuitPython firmware (code.py) and serves as the environmental sensor interface. It communicates with the Pi Zero FSW over UART and manages:
- BNO055 9-DOF IMU (accelerometer, gyroscope, magnetometer)
- BMP280 barometric pressure / temperature / altitude sensor
- Piezo buzzer for morse code output
Hardware Connections
I2C Bus
| Pin | Function |
|---|---|
| GP27 | SCL |
| GP26 | SDA |
I2C Devices:
| Device | Address | Function |
|---|---|---|
| BNO055 | 0x28 (default) | 9-DOF IMU |
| BMP280 | 0x76 | Pressure, temperature, altitude |
UART
| Pin | Function |
|---|---|
| GP0 | RX (from Pi Zero TX) |
| GP1 | TX (to Pi Zero RX) |
UART Parameters: 9600 baud, 8N1
Buzzer
| Pin | Function |
|---|---|
| GP15 | Digital output to piezo buzzer |
Command Protocol
The RP2040 receives commands as uppercase ASCII strings terminated with \n. Responses are Python list literals terminated with \n.
Sensor Commands
| Command | Response Format | Description |
|---|---|---|
GET_GYRO |
[x, y, z] |
Gyroscope (°/s) |
GET_ACCEL |
[x, y, z] |
Accelerometer (m/s²) |
GET_MAG |
[x, y, z] |
Magnetometer (µT) |
GET_GRAVITY |
[x, y, z] |
Gravity vector (m/s²) |
GET_EULER |
[yaw, roll, pitch] |
Euler angles (°) |
GET_QUATERNION |
[w, x, y, z] |
Quaternion orientation |
GET_BME |
[temp, pressure, altitude] |
BME280 readings |
GET_TEMP |
[temp] |
BNO055 die temperature (°C) |
POLL |
[16 float values] |
All sensor data concatenated |
System Commands
| Command | Response | Description |
|---|---|---|
MORSE <message> |
OK |
Play morse code on buzzer |
RESET |
— | Soft reset MCU (microcontroller.reset()) |
Response Parsing
The Pi Zero FSW parses RP2040 responses using ast.literal_eval():
# RP2040 sends: [1.23, -0.45, 0.78]\n
# Pi Zero receives and parses:
response = ast.literal_eval("[1.23, -0.45, 0.78]")
# result: [1.23, -0.45, 0.78] (Python list of floats)
This approach safely evaluates Python literals without executing arbitrary code.
Morse Code
The buzzer on GP15 supports morse code playback with the following timing:
| Element | Duration |
|---|---|
| Dot | 0.1s |
| Dash | 0.3s |
| Symbol space | 0.1s |
| Letter space | 0.3s |
| Word space | 0.7s |
On boot, the MCU plays "Tempest" in morse code as a startup indicator.
BNO055 Sensor Modes
The BNO055 provides multiple data types from its integrated sensors:
| Data Type | Source | Values |
|---|---|---|
| Gyroscope | Angular rate sensor | X, Y, Z rotation rates |
| Accelerometer | Linear acceleration sensor | X, Y, Z acceleration |
| Magnetometer | Geomagnetic sensor | X, Y, Z field strength |
| Gravity | Sensor fusion | X, Y, Z gravity components |
| Euler | Sensor fusion | Heading, Roll, Pitch |
| Quaternion | Sensor fusion | W, X, Y, Z |
| Temperature | On-die sensor | Integer °C |
First-Read Behavior
The first sensor read after MCU startup may return stale data from the UART buffer. The FSW performs a throwaway POLL command during initialization to flush this stale data. Subsequent reads return current values.
