# 0x09 DHT11 Rust Driver

This repository contains a Bare-Metal Rust driver for the **DHT11 Temperature and Humidity Sensor** on the **RP2350** (and RP2040) microcontrollers, implemented strictly using a GPIO pin and a hardware timer (no specialized hardware peripheral like PIO or PWM).

It includes:
- A demo (`src/main.rs`) that continuously reads the sensor every 2 seconds and outputs the results over UART.
- A reusable library module (`src/dht11.rs`) providing a hardware-agnostic `dht11_lib` containing the pure-logic checksum validation, data parsing, and string formatting routines.
- 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 DHT11 data parsing, checksum validation, and 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):**
```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 delays), and a hardware timer (for microsecond precision measurement).

*   **`run(...)`**: The master setup function. Calls the helper initialization functions below, prints an initialization message over UART, and enters the infinite polling loop.
*   **`poll_sensor(...)`**: Called repeatedly in the main loop. It invokes the read sequence, formats the result, writes it to UART, and blocks for 2 seconds (the DHT11's minimum polling interval).
*   **`read_sensor(...)`**: The high-level function for a single DHT11 read. It delegates to `acquire_data()` to handle the timing protocol, and then uses the `dht11_lib` to parse the resulting 5-byte array into temperature and humidity.
*   **`acquire_data(...)`**: Executes the strict protocol: pulls the pin low for 18ms (`send_start_signal()`), waits for the sensor's acknowledgment (`wait_response()`), and reads 40 bits into an array (`read_40_bits()`).
*   **`measure_bit_duration(...)`**: Crucial for decoding the data. It waits for the sensor to pull the pin high, takes a timestamp, waits for it to pull low, takes another timestamp, and returns the duration. 
*   **`read_bit(...)`**: Takes the measured pulse duration and calls the hardware-agnostic `accumulate_bit()` function in the `dht11_lib` to pack the 1 or 0 into the byte array.
*   **`gpio_drive(...)` & `gpio_release(...)`**: Manually flips the `GPIO 4` pin direction (output/input) via the `SIO` (Single-Cycle IO) registers to support the bidirectional 1-wire protocol.

### 3. The Reusable DHT11 Library (`src/dht11.rs`)
Because manipulating bits across byte boundaries and formatting floats without `std::fmt` can be error-prone, this module handles all pure-logic data manipulation.

*   **`accumulate_bit(...)`**: Shifts the current byte left and inserts a `1` if the measured pulse duration exceeds the 40-microsecond threshold.
*   **`validate_checksum(...)`**: Adds the first four bytes (humidity int, humidity frac, temp int, temp frac) and ensures the lowest 8 bits match the fifth byte (the checksum).
*   **`parse_humidity(...)` & `parse_temperature(...)`**: Converts the specific byte pairs into standard floating-point numbers.
*   **`format_reading(...)` & `format_error(...)`**: Efficiently constructs the final ASCII string without allocating memory, allowing the `board.rs` code to pipe the raw bytes directly to UART.