bridge

name	bridge
description	Arduino Bridge — software RPC layer that connects Python on the MPU (Linux side) with Arduino sketches on the MCU (STM32U585). Read this BEFORE writing any project that combines a Python program with an Arduino sketch, or that asks "how do I send data from my sketch to the Linux side?". The Uno Q is two processors on one board; Bridge is how they talk.

Bridge — Inter-Processor RPC on the Uno Q

The Arduino Uno Q is two computers on one board:

• MPU (Qualcomm QRB2210, 4× Cortex-A53 @ 2.0 GHz, Debian Linux) — runs Python, web servers, AI models, OpenCV.
• MCU (STMicroelectronics STM32U585, Cortex-M33 @ 160 MHz, Zephyr + Arduino Core) — runs .ino sketches, controls every Arduino header pin (digital I/O, PWM, ADC, I²C, SPI, UART, CAN).

Bridge is Arduino's software-based Remote Procedure Call (RPC) layer that lets either side call functions on the other:

Python (Linux side)                Arduino sketch (MCU side)
─────────────────                  ─────────────────────────
from bridge import ...             #include <Bridge.h>
result = bridge.call("read_A0")    Bridge.begin();
print(result)                      // expose a service the Python can call

Without Bridge, users treat the Uno Q like a classic Arduino and never use the Linux side. The whole point of this board is the combination.

When to use

Use Bridge when a project needs both sides:

Lesson scenario	Why Bridge
Python program reads a sensor wired to A0	MCU owns the pin; Python calls MCU via Bridge
Arduino sketch wants to fetch weather data from the internet	MCU has no Wi-Fi; MCU calls MPU via Bridge, MPU hits the network
AI model on Linux decides when to blink an LED	Python runs the model, calls MCU to drive the LED
Sketch streams accelerometer data to a web dashboard	MCU samples 200 Hz, MPU serves the HTTP/WebSocket UI
Voice assistant on the board	Mic + speech-to-text on Linux; servo control via MCU

Don't use Bridge when only one side is involved:

• Pure GPIO/PWM/sensor work → write a normal Arduino sketch, no Linux.
• Pure web scraping / AI inference / file processing → write a Python program, no MCU.

Quick rules

1. Bridge runs over a physical transport. USB CDC (virtual serial port), UART, or SPI. The default in App Lab is USB CDC.
2. Either side can expose a service. The other side calls it by name with arguments and receives a structured response.
3. Either side can push notifications (one-way events) for asynchronous things — button presses, sensor thresholds.
4. Bridge.begin() must be called in setup() before any RPC call from the MCU side. Forgetting this leaves both sides waiting forever.
5. Type-safe API. Arguments and return values are typed; the wire format handles serialization. You don't write Serial.print glue code.
6. Latency is ~1–5 ms per call over USB CDC. Fast enough for control loops at 100 Hz, not for hard real-time deadlines under 1 ms (do that in the sketch alone).

How a project is structured (Arduino App Lab)

An App in App Lab combines:

my-app/
├── main.py              ← runs on the Linux MPU
├── sketch.ino           ← runs on the STM32U585 MCU
└── bricks/              ← optional pre-packaged services (AI models, web UIs)

When the user presses Run in App Lab:

1. App Lab compiles the .ino and flashes the MCU (via the same OpenOCD path the arduino tool uses, sketch partition 0x8100000).
2. App Lab starts main.py on the Linux side.
3. Both processes initialize Bridge — Bridge.begin() on the MCU, from bridge import ... on the Python side.
4. They start exchanging messages.

Reference files

• references/python-side.md — how to call MCU services from Python, set up Bridge clients, handle notifications.
• references/mcu-side.md — how to expose services from an .ino sketch and push notifications back to Linux.
• references/examples.md — three complete worked examples (sensor relay, AI-controlled LED, button-triggered web request).

Pitfalls

• Confusing MCU GPIO with MPU GPIO. The Arduino headers (D0–D21, A0–A5) are MCU pins. Linux can NOT import RPi.GPIO-style drive them; it must call the MCU via Bridge.
• Treating the Uno Q like a Raspberry Pi. Linux on the QRB2210 has its own GPIO at 1.8 V on JMISC, but those are mostly system-reserved (camera, audio, MPU SoC functions). The user-facing pins belong to the MCU.
• Forgetting Bridge.begin(). Both sides will hang silently. Always include it in setup().
• Hot-plugging USB during a Bridge call. USB CDC drops; the call returns an error. Catch it on both sides.
• Confusing Bridge with Serial. Serial.print writes to USART1 (or USB CDC depending on configuration); Bridge has its own framing on top. Use Bridge.print / bridge.log() to send debug strings through the framed channel.

See also

• The MPU/MCU split is documented in uno-q-hardware/SKILL.md.
• For uploading sketches without Bridge involvement (single-side .ino only), see sketch-patterns/references/upload.md.
• For the App Lab workflow that wraps Bridge, see arduino-app-lab/SKILL.md.