Embedded-Hacking/drivers/0x0b_spi_rust/README.md

0x0b SPI Rust Driver

This repository contains a Bare-Metal Rust driver for the SPI (Serial Peripheral Interface) bus on the RP2350 (and RP2040) microcontrollers.

It includes:

• A demo (src/main.rs) that executes an SPI loopback transfer (MOSI wired to MISO) and logs the results over UART.
• A reusable library module (src/spi.rs) providing a hardware-agnostic spi_lib containing formatting helpers for UART logging.
• 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 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 SPI peripheral.

• run(...): The master setup function. Calls the helper initialization functions below, prints an initialization banner over UART, and enters the infinite loop executing SPI transfers.
• init_spi(...): Configures the hardware SPI0 peripheral. Reconfigures MISO, MOSI, and SCK for SPI function, and configures CS as a software-controlled SIO output. Sets the SPI speed to 1 MHz in Mode 0 (CPOL=0, CPHA=0).
• loopback_transfer(...): Called repeatedly in the main loop. It allocates an RX buffer, calls execute_transfer(), formats the TX and RX buffers as ASCII strings, and streams them over UART so you can verify the loopback on your console.
• execute_transfer(...): The core hardware interaction. Asserts chip select (low), blocks while executing a full-duplex transfer using the embedded-hal SPI trait (spi_dev.transfer()), and de-asserts chip select (high).

3. The Reusable SPI Library (src/spi.rs)

While board.rs handles the hardware registers, spi.rs abstracts away the string formatting logic required to cleanly log the SPI transfers over UART without requiring dynamic memory allocation.

• clear_rx_buffer(...): Clears the receive buffer before the next loopback transfer to ensure no stale data remains.
• format_tx_line(...) / format_rx_line(...): Formats the byte arrays into TX: ... and RX: ... strings terminating in CRLF.
• copy_c_string(...): A safe helper that copies bytes up to the first NUL terminator, allowing us to safely log C-style strings over UART.