Expand description

A Hardware Abstraction Layer (HAL) for embedded systems

NOTE This HAL is still is active development. Expect the traits presented here to be tweaked, split or be replaced wholesale before being stabilized, i.e. before hitting the 1.0.0 release.

NOTE If you want to use an alpha release of the 1.0.0 version, use an exact version specifier in your Cargo.toml like: embedded-hal = "=1.0.0-alpha.2".

Design goals


  • Must erase device specific details. Neither register, register blocks or magic values should appear in the API.

  • Must be generic within a device and across devices. The API to use a serial interface must be the same regardless of whether the implementation uses the USART1 or UART4 peripheral of a device or the UART0 peripheral of another device.

  • Where possible must not be tied to a specific asynchronous model. The API should be usable in blocking mode, with the futures model, with an async/await model or with a callback model. (cf. the nb crate)

  • Must be minimal, and thus easy to implement and zero cost, yet highly composable. People that want higher level abstraction should prefer to use this HAL rather than re-implement register manipulation code.

  • Serve as a foundation for building an ecosystem of platform agnostic drivers. Here driver means a library crate that lets a target platform interface an external device like a digital sensor or a wireless transceiver. The advantage of this system is that by writing the driver as a generic library on top of embedded-hal driver authors can support any number of target platforms (e.g. Cortex-M microcontrollers, AVR microcontrollers, embedded Linux, etc.). The advantage for application developers is that by adopting embedded-hal they can unlock all these drivers for their platform.

  • Trait methods must be fallible so that they can be used in any possible situation. Nevertheless, HAL implementations can additionally provide infallible versions of the same methods if they can never fail in their platform. This way, generic code can use the fallible abstractions provided here but platform-specific code can avoid fallibility-related boilerplate if possible.

Out of scope

  • Initialization and configuration stuff like “ensure this serial interface and that SPI interface are not using the same pins”. The HAL will focus on doing I/O.

Reference implementation

The stm32f1xx-hal crate contains a reference implementation of this HAL.

Platform agnostic drivers

You can find platform agnostic drivers built on top of embedded-hal on crates.io by searching for the embedded-hal keyword.

If you are writing a platform agnostic driver yourself you are highly encouraged to add the embedded-hal keyword to your crate before publishing it!

Detailed design


The HAL is specified as traits to allow generic programming. These traits make use of the nb crate (please go read that crate documentation before continuing) to abstract over the asynchronous model and to also provide a blocking operation mode.

Here’s how a HAL trait may look like:

use embedded_hal::nb;

/// A serial interface
pub trait Serial {
    /// Error type associated to this serial interface
    type Error: core::fmt::Debug;

    /// Reads a single byte
    fn read(&mut self) -> nb::Result<u8, Self::Error>;

    /// Writes a single byte
    fn write(&mut self, byte: u8) -> nb::Result<(), Self::Error>;

The nb::Result enum is used to add a WouldBlock variant to the errors of the serial interface. As explained in the documentation of the nb crate this single API, when paired with the macros in the nb crate, can operate in a blocking manner, or be adapted to other asynchronous execution schemes.

Some traits, like the one shown below, may expose possibly blocking APIs that can’t fail. In those cases nb::Result<_, Infallible> is used.

use embedded_hal::nb;

use ::core::convert::Infallible;

/// A count down timer
pub trait CountDown {
    // ..

    /// "waits" until the count down is over
    fn wait(&mut self) -> nb::Result<(), Infallible>;

Suggested implementation

The HAL traits should be implemented for device crates generated via svd2rust to maximize code reuse.

Shown below is an implementation of some of the HAL traits for the stm32f1xx-hal crate. This single implementation will work for any microcontroller in the STM32F1xx family.

// crate: stm32f1xx-hal
// An implementation of the `embedded-hal` traits for STM32F1xx microcontrollers

use embedded_hal as hal;
use hal::nb;

// device crate
use stm32f1::stm32f103::USART1;

/// A serial interface
// NOTE generic over the USART peripheral
pub struct Serial<USART> { usart: USART }

// convenience type alias
pub type Serial1 = Serial<USART1>;

impl hal::serial::ErrorType for Serial<USART1> {
    type Error = hal::serial::ErrorKind;

impl hal::serial::nb::Read<u8> for Serial<USART1> {
    fn read(&mut self) -> nb::Result<u8, Self::Error> {
        // read the status register
        let isr = self.usart.sr.read();

        if isr.ore().bit_is_set() {
            // Error: Buffer overrun
        // omitted: checks for other errors
        else if isr.rxne().bit_is_set() {
            // Data available: read the data register
            Ok(self.usart.dr.read().bits() as u8)
        } else {
            // No data available yet

impl hal::serial::nb::Write<u8> for Serial<USART1> {
    fn write(&mut self, byte: u8) -> nb::Result<(), Self::Error> {
        // Similar to the `read` implementation

    fn flush(&mut self) -> nb::Result<(), Self::Error> {
        // Similar to the `read` implementation

Intended usage

Thanks to the nb crate the HAL API can be used in a blocking manner with the block! macro or with futures.

Blocking mode

An example of writing a string over the serial interface in a blocking fashion:

use crate::stm32f1xx_hal::Serial1;
use embedded_hal::serial::nb::Write;
use embedded_hal::nb::block;

let mut serial: Serial1 = {
    // ..

for byte in b"Hello, world!" {
    // NOTE `block!` blocks until `serial.write()` completes and returns
    // `Result<(), Error>`

Generic programming and higher level abstractions

The core of the HAL has been kept minimal on purpose to encourage building generic higher level abstractions on top of it. Some higher level abstractions that pick an asynchronous model or that have blocking behavior and that are deemed useful to build other abstractions can be found in the blocking module.

Some examples:

NOTE All the functions shown below could have been written as trait methods with default implementation to allow specialization, but they have been written as functions to keep things simple.

  • Write a whole buffer to a serial device in blocking a fashion.
use embedded_hal as hal;
use hal::nb::block;
use hal::serial::nb::Write;

fn write_all<S>(serial: &mut S, buffer: &[u8]) -> Result<(), S::Error>
    S: hal::serial::nb::Write<u8>
    for &byte in buffer {

  • Buffered serial interface with periodic flushing in interrupt handler
use embedded_hal as hal;
use hal::nb;

use hal::serial::{ErrorKind, nb::Write};

fn flush<S>(serial: &mut S, cb: &mut CircularBuffer)
    S: hal::serial::nb::Write<u8, Error = ErrorKind>,
    loop {
        if let Some(byte) = cb.peek() {
            match serial.write(*byte) {
                Err(nb::Error::Other(_)) => unreachable!(),
                Err(nb::Error::WouldBlock) => return,
                Ok(()) => {}, // keep flushing data


// The stuff below could be in some other crate

/// Global singleton
pub struct BufferedSerial1;

// NOTE private
static BUFFER1: Mutex<CircularBuffer> = {
    // ..
static SERIAL1: Mutex<Serial1> = {
    // ..

impl BufferedSerial1 {
    pub fn write(&self, byte: u8) {

    pub fn write_all(&self, bytes: &[u8]) {
        let mut buffer = BUFFER1.lock();
        for byte in bytes {
            buffer.push(*byte).expect("buffer overrun");
        // omitted: pend / enable interrupt_handler

fn interrupt_handler() {
    let mut serial = SERIAL1.lock();
    let mut buffer = BUFFER1.lock();

    flush(&mut *serial, &mut buffer);


pub use nb;


Controller Area Network


Digital I/O

Implementation of core::fmt::Write for the HAL’s serial::Write.

Blocking I2C API

Serial traits

SPI traits