Embedded-Hacking/drivers/0x04_pwm_rust/README.md

# 0x04 PWM Rust Driver

This repository contains a Bare-Metal Rust driver for Pulse Width Modulation (PWM) on the **RP2350** (and RP2040) microcontrollers.

It includes:
- A thin demo (`src/main.rs`) that slowly breathes an LED (fades it in and out) and prints the duty cycle percentage over UART.
- A reusable library module (`src/pwm.rs`) providing a hardware-agnostic `pwm_lib` with helper math to calculate clock dividers and level targets.
- 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 PWM math 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):**
```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 need to wake up the specific hardware subsystems we want to use (Clocks, Pins, UART for printing, SysTick for delay, and PWM).

*   **`run(...)`**: The master setup function. It sequentially calls the helper initialization functions below, and then kicks off the PWM sweep 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_delay(...)`**: Captures the ARM Cortex-M `SYST` (SysTick) peripheral to create a blocking delay timer.
*   **`init_pwm(...)`**: Obtains `PWM Slice 4` and its `channel_b` mapped to `GPIO 25` (the onboard LED). Sets up the clock divider and TOP value (wrap) to give us a 1 kHz frequency.
*   **`pwm_loop(...)`**: Loops infinitely, sweeping the LED duty cycle up and then down, simulating a breathing effect.
*   **`sweep_up(...)`** / **`sweep_down(...)`**: Walks through duty cycle percentages `0` to `100` and back down in steps of 5%.
*   **`apply_duty(...)`**: Computes the hardware register level using `pwm_lib`, sets the new duty cycle, formats a `Duty: XX%` string without heap allocation, transmits it, and delays 50ms.

### 3. The Reusable PWM Math Library (`src/pwm.rs`)
Instead of burying math inside initialization functions, `src/pwm.rs` extracts pure mathematical logic that's decoupled from hardware structs so we can thoroughly unit-test it natively.

*   **`calc_clk_div(...)`**: Calculates the precise floating-point clock divider required to step down the fast system clock so the counter overflows exactly `freq_hz` times per second.
*   **`clamp_percent(...)`**: Constrains duty cycle percentages between `0` and `100`.
*   **`duty_to_level(...)`**: Translates a high-level percentage (e.g., `50%`) into the raw integer counter compare-value (e.g., `5000` out of `10000`) for the hardware registers.