Embedded-Hacking/drivers/0x06_adc_rust

0x06 ADC Rust Driver

This repository contains a Bare-Metal Rust driver for the ADC (Analog-to-Digital Converter) peripheral on the RP2350 (and RP2040) microcontrollers.

It includes:

• A thin demo (src/main.rs) that continuously reads the analog voltage on GPIO 26 and the internal chip temperature, printing both over UART.
• A reusable library module (src/adc.rs) providing a hardware-agnostic adc_lib with helper math to map raw ADC counts to millivolts and degrees Celsius.
• 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 ADC mathematical conversion 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):

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 ADC (for reading analog inputs).

• run(...): The master setup function. Calls the helper initialization functions below, prints an announcement over UART, and enters the infinite ADC polling 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_adc(...): Initializes the hardware ADC peripheral, allocates GPIO 26 as an analog input pin, and claims the internal chip temperature sensor.
• adc_loop(...): Loops infinitely, reading the latest voltage and temperature, formatting the line, transmitting it over UART, and sleeping for POLL_MS (500ms).
• read_adc(...): Queries the hardware ADC peripheral for the current 12-bit analog counts for both GPIO 26 and the internal temp sensor, then passes those raw counts to the adc_lib math helpers to convert them to millivolts (mV) and degrees Celsius.
• format_adc_line(...): Safely constructs a formatted string like ADC0: 1650 mV | Chip temp: 34.5 C\r\n without using the heap or standard library formatting mechanisms, relying instead on custom integer-to-ascii division algorithms (write_mv_digits, write_temp, etc.).

3. The Reusable ADC Math Library (src/adc.rs)

The hardware ADC returns a raw 12-bit number between 0 and 4095. This number represents a ratio of the measured voltage compared to the ADC reference voltage (which is 3.3V, or 3300mV).

• raw_to_mv(...): Linearly maps a 12-bit ADC result (0..=4095) to a voltage in millivolts (0..=3300).
• raw_to_celsius(...): Takes a 12-bit reading from the internal temperature sensor (Channel 4) and applies the datasheet-specified polynomial equation to yield the core temperature in degrees Celsius as a floating-point number.