Embedded-Hacking/drivers/0x01_uart_rust

0x01 UART Rust Driver

This repository contains a Bare-Metal Rust driver for the UART (Universal Asynchronous Receiver-Transmitter) peripheral on the RP2350 (and RP2040) microcontrollers.

It includes:

• A thin demo (src/main.rs) that runs an uppercase echo server over UART.
• A reusable library module (src/uart.rs) providing a hardware-agnostic UartDriver.
• 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 UART driver logic is separated into a reusable library, you can run the unit tests natively on your computer (no microcontroller required!).

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 need to wake up the specific hardware subsystems we want to use (Clocks, Pins, and the UART peripheral).

• run(...): The master setup function. It sequentially calls the helper initialization functions below, and then kicks off the echo server.
• 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_uart_pins(...): Takes control of physical pins GPIO0 and GPIO1 and configures them specifically for UART TX (Transmit) and RX (Receive) mode.
• init_uart_peri(...): Configures the hardware UART0 peripheral to operate at a standard 115200 baud rate with an 8N1 configuration (8 data bits, no parity, 1 stop bit).
• start_echo_server(...): Wraps the hardware UART peripheral in our custom UartDriver, prints a "UART ready" welcome message to the serial console, and jumps into the echo_loop.
• echo_loop(...): An infinite loop { ... } that waits for a character to arrive, reads it, and immediately transmits the uppercase version back.

3. The Reusable UART Driver (src/uart.rs)

The hardware-specific logic from board.rs uses the UartDriver struct to abstract away the messy details of sending and receiving bytes.

• UartDriver::init(...): Creates a new instance of our driver, taking ownership of the hardware UART peripheral.
• getchar(&mut self): A blocking function that waits until a byte physically arrives over the RX wire, reads it out of the hardware buffer, and returns it.
• putchar(&mut self, c: u8): A blocking function that waits until the TX wire is ready, and then writes a single byte into the hardware buffer to be transmitted.
• puts(&mut self, s: &[u8]): A convenience function that takes an array of characters (a string) and transmits them one-by-one by repeatedly calling putchar().
• to_upper(c: u8): A helper function that takes an ASCII byte and uses Rust's built-in .to_ascii_uppercase() to convert it to an uppercase letter.