Writing a Device Driver

Pico de Gallo exists for a very specific kind of loop: write a driver, plug a real part into a board on your desk, run it from your laptop, and iterate fast.

That means:

• no firmware flashing between every edit
• no SWD probe on your bench
• no clock-tree bring-up before you can read one register
• no linker scripts, BSP setup, or target-specific project scaffolding
• no throwing your driver away when you change MCUs later

We still write the driver against embedded-hal. The transport happens to be Pico de Gallo today; the same crate can ship on an RP2350, STM32, nRF, ESP, or any other target that implements the same traits.

In this part of the book we will build a driver for the TMP102, a tiny I²C temperature sensor from Texas Instruments. It is a good tutorial device: small register map, readable datasheet, and just enough detail to show where a real driver gets its shape.

We will take the chapter in the same order you would tackle the work in practice:

1. Explore the device with gallo
2. Scaffold a normal Rust crate
3. Describe the register map for code generation
4. Bridge the generated code to embedded-hal
5. Build an ergonomic public API
6. Test it against real hardware
7. Keep blocking and async users equally happy
8. Polish it for publishing

The big idea is simple:

Tip

The fastest way to write a solid embedded driver is often to not start on the microcontroller at all. Start on your laptop, with fast builds, rich tooling, and a real sensor on the wire.

By the end, we will have a driver that:

• speaks TMP102 over I²C
• uses generated register accessors for the repetitive bits
• exposes a small, type-safe API for the parts humans care about
• works with both blocking and async embedded-hal
• can be tested with a real sensor in hardware-in-the-loop

If you already know Rust and embedded-hal, this chapter should feel like a practical walkthrough rather than a Rust tutorial. We will skip the basics and spend our time on the interesting parts: where the register map comes from, how to make invalid states unrepresentable, and how Pico de Gallo changes the driver-authoring workflow.

Let’s start by interrogating the sensor directly.