Embedded-Hacking/drivers/0x07_i2c_rust/README.md

0x07 I2C Rust Driver

This repository contains a Bare-Metal Rust driver for the I2C (Inter-Integrated Circuit) peripheral on the RP2350 (and RP2040) microcontrollers.

It includes:

• A thin demo (src/main.rs) that continuously scans the I2C bus and prints out a nicely formatted hex grid of all connected devices over UART.
• A reusable library module (src/i2c.rs) providing a hardware-agnostic i2c_lib with helper math and string formatting functions to neatly layout the scan grid.
• Board initialization logic (src/board.rs).

🚀 Getting Started from Scratch

If you're starting with a fresh machine, follow these exact steps to install the toolchain, build the code, and flash it to your microcontroller.

1. Install Rust

First, install rustup (the Rust toolchain installer) if you haven't already:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Note: Restart your terminal or run source $HOME/.cargo/env after this finishes.

Ensure your Rust compiler is up to date:

rustup update

2. Install the Target Architecture

This project is configured for the RP2350 (ARM Cortex-M33). We need to install the cross-compilation target for it:

rustup target add thumbv8m.main-none-eabihf

(If you were targeting the RP2040, you would use thumbv6m-none-eabi instead).

3. Install Build Tools

You will need a few extra tools to help link and format the firmware for the RP-series chips.

Install flip-link (adds zero-cost stack overflow protection):

cargo install flip-link

Install picotool (used by cargo run to flash the chip):

• macOS: brew install picotool
• Linux/Windows: Follow the official Raspberry Pi documentation to install picotool or build it from source.

4. Building the Code

To compile the code for the microcontroller, simply run:

cargo build

To build a highly optimized release version (smaller and faster):

cargo build --release

5. Flashing to the Microcontroller

This project is pre-configured in .cargo/config.toml to use picotool as the custom runner.

To flash the code:

1. Hold down the BOOTSEL button on your RP2350 board.
2. Plug it into your computer via USB (or press the RUN/RESET button while holding BOOTSEL).
3. Run the following command:

cargo run --release

cargo will compile the code and automatically use picotool to upload the .elf file directly to your board and start executing it!

6. Testing on the Host

Because the I2C grid string formatting logic is separated into a reusable math library without touching hardware registers, you can run the unit tests natively on your computer!

However, because this project sets a default bare-metal target (thumbv8m.main-none-eabihf) in .cargo/config.toml, running a plain cargo test will fail because the standard library doesn't exist on the microcontroller. You must explicitly tell Cargo to compile the tests for your host computer's processor architecture:

Mac (Apple Silicon):

cargo test --lib --target aarch64-apple-darwin

Linux (Intel/AMD 64-bit):

cargo test --lib --target x86_64-unknown-linux-gnu

Windows (64-bit):

cargo test --lib --target x86_64-pc-windows-msvc

🧠 Code Walkthrough

This section explains exactly how the code works, where the entry point is, and traces the flow of execution as if you were stepping through it line-by-line.

1. The Entry Point (src/main.rs)

Unlike a standard computer program, bare-metal microcontrollers do not have an operating system to call main(). Instead, we use the #[entry] macro from the HAL (Hardware Abstraction Layer) to define the very first function that runs after the chip boots up.

• main() -> !: This is the absolute start of our code. It takes ownership of all the hardware peripherals (hal::pac::Peripherals::take().unwrap()) and immediately passes them into board::run(...). The -> ! means this function never returns (because embedded devices run in an infinite loop).

2. Board Initialization (src/board.rs)

Once execution enters board.rs, we initialize the system clocks, pins, UART (for logging), SysTick (for delay), and the I2C peripheral (for scanning).

• run(...): The master setup function. Calls the helper initialization functions below, prints an announcement over UART, and enters the infinite I2C scanning loop.
• init_clocks(...): Wakes up the external 12 MHz crystal (XOSC) and configures the PLLs (Phase-Locked Loops) to drive the system clock at its maximum speed.
• init_pins(...): Takes control of physical pins across IO_BANK0.
• init_uart(...): Configures the hardware UART0 peripheral to operate at a standard 115200 baud rate with an 8N1 configuration for debug printing.
• init_i2c(...): Initializes the hardware I2C1 peripheral, reconfigures GPIO 2 and GPIO 3 as SDA and SCL lines (with internal pull-ups enabled), and sets the clock speed to 100 kHz (standard mode).
• scan_loop(...): Loops infinitely, printing the table header, iterating through all 128 possible addresses to probe them, and sleeping for SCAN_DELAY_MS (5 seconds).
• scan_addresses(...): Loops from 0x00 to 0x7F, ignoring reserved addresses, and uses probe_addr to see if a device ACKs a 1-byte read.
• probe_addr(...): The core interaction. Attempts to read exactly 1 byte from a target 7-bit address. If the HAL's i2c.read() succeeds without error, we know a device exists at that address.

3. The Reusable I2C String Formatting Library (src/i2c.rs)

Because formatting a 16x8 hex grid in standard Rust would usually require alloc, String, and format!() (which we do not have), this module provides a lightweight, zero-allocation way to generate the table bytes.

• format_scan_header(...): Injects the table column headers (0 1 2 3 ... F).
• format_scan_entry(...): Determines if the current address is at the beginning of a row (adds XX: prefix) or at the end (adds \r\n).
• cell_bytes(...): Given a specific address and whether a device was found there, returns a fixed 3-byte array like 51 (found), -- (not found), or (reserved address block).
• is_reserved(...): True for addresses strictly less than 0x08 and strictly greater than 0x77 (these ranges are globally reserved by the I2C spec).