Embedded-Hacking/drivers/0x0e_watchdog_rust

0x0e Watchdog Rust Driver

This repository contains a Bare-Metal Rust driver demonstrating the Hardware Watchdog on the RP2350 (and RP2040) microcontrollers.

It includes:

• A demo (src/main.rs) that configures the hardware watchdog, checks the reset reason on boot, and then enters a loop to periodically "feed" the watchdog to prevent a reset.
• A reusable library module (src/watchdog.rs) providing a hardware-agnostic watchdog_lib containing the driver state machine and formatting helpers.
• 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 data manipulation and string formatting logic is separated into a reusable 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 hardware Watchdog peripheral.

• run(...): The master setup function. It first gets a handle to the hal::Watchdog, initializing the system clocks with it. After setting up UART, it checks the reset reason, initializes the driver state, and enters the infinite feed_loop.
• watchdog_caused_reboot(): Reads the raw WATCHDOG REASON hardware register via the PAC (Peripheral Access Crate) to determine if the timer elapsed or if a forced reboot occurred.
• watchdog_enable(...) / watchdog_feed(...): Small wrapper functions that call into the rp-hal implementation to configure the hardware timer and reset its counter, respectively.
• feed_loop(...): An infinite loop that calls watchdog_feed() to pet the dog, updates the driver state, prints a message, and sleeps for the 1000ms interval.

3. The Reusable Watchdog Library (src/watchdog.rs)

While board.rs directly manipulates the hardware Watchdog registers, watchdog.rs tracks the logical state of our watchdog process and handles string formatting for logging.

• WatchdogDriverState: A struct that acts as a state machine. It stores whether the watchdog is enabled, the configured timeout_ms, and the total feed_count. It abstracts away the global variables commonly used in C SDKs.
• enable(...) / feed(...): Methods to transition the driver state.
• format_fed(...) / format_reset_reason(...) / format_enabled(...): Helpers to generate the UART console messages.
• format_u32(...): Implements custom u32 to decimal ASCII string conversion without allocating dynamic memory, enabling core library compatibility (no_std).