Embedded-Hacking/drivers/0x0e_watchdog_rust/README.md

# 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:
```bash
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:
```bash
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:
```bash
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):
```bash
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:
```bash
cargo build
```

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

```bash
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):**
```bash
cargo test --lib --target aarch64-apple-darwin
```

**Linux (Intel/AMD 64-bit):**
```bash
cargo test --lib --target x86_64-unknown-linux-gnu
```

**Windows (64-bit):**
```bash
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`).