Migrating from embedded-hal 0.2.x to 1.0.0

Overview and reasoning
Trait organization
Trait unification
Removed traits
SPI Bus/device separation
Fallibility
SPI transfer return type
Error type bounds
GPIO traits now require &mut self
Prelude
Removed blanket implementations
Cargo Features
Companion crates
Supporting both 0.2 and 1.0 in the same HAL
embedded-hal-compat

Overview and reasoning

There have been a lot of changes in embedded_hal between versions 0.2.x and 1.0.0. We understand the significance of embedded-hal in the Rust embedded ecosystem and thus intend to release a version that stays compatible for a long time.

The main difference between embedded-hal 0.2 and 1.0 is the project is now focused on a single goal: traits for writing drivers that work on any HAL.

In embedded-hal 0.2, the traits had dual goals:

Standardize the API of HAL crates, so HAL crate authors get guidance on how to design their APIs and end users writing code directly against one HAL get a familiar API.
Allowing writing generic drivers using the traits, so they work on top of any HAL crate.

For embedded-hal 1.0, we decided to drop the first goal, targeting only the second. The reasons are:

Standardizing HAL APIs is difficult, because hardware out there has wildly different sets of capabilities. Modeling all capabilities required many different variants of the traits, and required "customization points" like associated types, significantly increasing complexity.
There is a tension between both goals. "Customization points" like associated types make the traits hard to use in generic HAL-independent drivers.
The second goal delivers much more value. Being able to use any driver together with any HAL crate, out of the box, and across the entire Rust Embedded ecosystem, is just plain awesome.

This refocusing on drivers is the root cause of many of the changes between embedded-hal 0.2 and 1.0:

Associated type compatibility
Trait fragmentation
Bus/device separation
Fallibility
Execution model support

Trait organization

All traits have been organized in modules for each peripheral. For example embedded_hal::spi and embedded_hal::i2c. We only foresee having blocking traits in embedded-hal. We have put the traits for different execution models into separate crates. Notably embedded-hal-async and embedded-hal-nb. See companion crates. This allows for a separate and more tailored evolution.

Execution-model-independent definitions have been moved into the peripheral's module. For example, SPI Phase is now defined in embedded_hal::spi::Phase.

Trait unification

Previously, there were multiple traits for the same peripheral, for different sets of capabilities. The reasoning was different hardware supports a different set of features, so by making the traits granular each HAL implementation can implement only the features supported by the hardware.

However, this has proven to be troublesome for generic drivers, in cases where a driver expects to use one trait, but the HAL crate implements only other traits. To avoid this fragmentation and ensure interoperability for generic code, these have now been unified.

I2C: Read, Write, WriteIter, WriteIterRead, WriteRead, Transactional, TransactionalIter have now been unified into a single I2c trait.
SPI: Write WriteIter, Transfer, Transactional have been unified into SpiBus.
GPIO: ToggleableOutputPin has been merged into StatefulOutputPin.
Delays: DelayMs, DelayUs has been unified into DelayNs (and precision extended to nanoseconds).

HAL implementation crates should implement the full functionality of the traits. If a feature is not supported natively by the hardware, it should be polyfilled/emulated in software. In no case should "not supported" errors be returned. This ensures maximum compatibility.

Removed traits

These traits have been removed in the 1.0.0 release, with no replacement for now:

adc::OneShot
adc::Channel
capture::Capture
digital::IoPin
pwm::Pwm
qei::Qei
timer::Cancel
timer::CountDown
timer::Periodic
watchdog::Disable
watchdog::Enable
watchdog::Watchdog

Please find a general roadmap with further guidance here about whether and how to get these traits back in a future release.

If you are a generic driver author and need one of them, we would like to hear from you. Please add your use case to the appropriate issue for the trait affected.

HAL implementation crates are encouraged to provide their own APIs for functionality for the removed traits, and not implement any traits. This will allow the APIs to more closely match the hardware capabilities, and allow users to continue to use them.

Unconstrained associated types

Traits defined in embedded-hal pursue creating an interface for interoperability between generic code (be it generic user code, generic application code, generic device drivers, etc.). When a trait has an unconstrained associated type (for example type Time;), it is not possible to write generic code around it. Each side (implementer and user) need to specify which type the associated type will be. If the types match, the both parts can work together, however, this is not truly generic code.

For example, if somebody creates a device driver that receives a CountDown struct, it needs to specify what its Time type should be. If they choose a type coming from fugit, somebody else cannot use this driver if the HAL implementation for the MCU they are using only provides CountDown with Time types defined in embedded-time. It is also not possible for the user to implement CountDown for Time types defined by fugit in a straight-forward way due to the orphan rule. In summary, it is not possible for anybody to start a countdown for a certain duration in a generic way, without it being tied to a particular time implementation and thus forcing everybody to use that one. This means all these traits don't fulfill the "allow writing generic drivers" goal.

At the moment no solution for this has been found so we have decided to remove such traits hoping that a solution may be found and we can add them back in a future 1.x release.

Impractical traits

The digital::IoPin trait and the adc traits have been deemed impractical for use and have thus been removed. Please feel free to comment on the appropriate issue if you need any of these traits and propose a solution.

Serial traits

The blocking::serial::Write trait has been removed in favor of the embedded-io traits, also maintained within the embedded-hal repository.

RNG traits

The rng module and its traits have been removed in favor of the rand_core traits.

CAN traits

The can module and its traits have been removed in favor of the embedded-can traits.

SPI Bus/device separation

The SPI traits have been unified into a single SpiBus trait. However, to allow sharing an SPI bus, and hardware control of the CS pin, 1.0 adds the SpiDevice trait.

The short summary is:

SpiBus represents an entire SPI bus (with SCK, MOSI, MISO) pins
SpiDevice represents a single device on an SPI bus, selected by a CS pin.

See the SPI documentation for more details.

When upgrading code to embedded-hal 1.0, it is critical to implement/use the right trait depending on the underlying situation.

For HAL implementation crates

If you previously implemented the SPI traits, and did not manage a CS pin automatically, you should now implement SpiBus, which is the equivalent in 1.0.
Optionally, if the API does manage a CS pin automatically, you may implement SpiDevice.
- This is required if the underlying API requires it to manage the CS pin, like spidev on Linux.

Do not implement SpiBus and SpiDevice on the same struct, since this is never correct. When there's no CS pin being controlled you must implement only SpiBus, and when there is, implement only SpiDevice. If you want to offer both APIs, implement them on separate structs so the user has to choose one or the other.

For driver crates

If your device has SCK, MOSI, MISO, CS pins: use SpiDevice.
- Do NOT take the CS pin as a separate OutputPin, the SpiDevice will manage it for you. Taking the CS pin separately will make your driver not work on shared buses.
If your device only has SCK, MOSI, MISO: use SpiBus.
- This means bus sharing won't be supported, but there's no way to share without a CS pin anyway.
If you're using SPI to bitbang non-SPI protocols (for example, WS2812 smart LEDs), use SpiBus.

For end users

You will most likely find the HAL crate you're using implements SpiBus, and the driver you want to use requires SpiDevice. To convert from SpiBus to SpiDevice, wrap it with a embedded_hal_bus::spi::ExclusiveDevice, together with the CS pin:

use embedded_hal_bus::spi::{ExclusiveDevice, NoDelay};

// Create the SPI from the HAL. This implements SpiBus, not SpiDevice!
let spi_bus = my_hal::spi::Spi::new(...);
// Create the CS. This must implement OutputPin.
let cs = my_hal::gpio::Output::new(...);

// Combine the SPI bus and the CS pin into a SPI device. This now does implement SpiDevice!
let spi_device = ExclusiveDevice::new(spi_bus, cs, NoDelay);

// Now you can create your driver with it!
let driver = my_driver::Driver::new(spi_device, ...);

If you want multiple drivers to share the same SPI bus, embedded_hal_bus::spi has a few options depending on the kind of mutex you want to use. This is now built-in to embedded-hal, using external crates like shared-bus is discouraged.

For example, you can use RefCellDevice when you don't need drivers to be Send.

use core::cell::RefCell;
use embedded_hal_bus::spi::{RefCellDevice, NoDelay};

// Create the SPI bus and CS pins.
let spi_bus = my_hal::spi::Spi::new(...);
let cs1 = my_hal::gpio::Output::new(...);
let cs2 = my_hal::gpio::Output::new(...);

// Wrap the bus with a RefCell.
let spi_bus = RefCell::new(spi_bus);

// Combine references to the SPI bus with a CS pin to get a SpiDevice for one device on the bus.
let device1 = RefCellDevice::new(&spi_bus, cs1, NoDelay);
let device2 = RefCellDevice::new(&spi_bus, cs2, NoDelay);

// Now you can create drivers. They will transparently talk each to its own device, sharing the same bus.
let driver1 = my_driver::Driver::new(device1, ...);
let driver2 = my_driver::Driver::new(device2, ...);

Fallibility

All trait methods are now fallible so that they can be used in any possible situation. However, HAL implementations can also provide infallible versions of the methods.

For example, an implementation similar to the one below would allow to use the GPIO pins as OutputPins in any generic driver or implementation-agnostic code (by importing the OutputPin trait), as well as using the infallible methods in non-generic code. This avoids the need to use unwrap() the results in many cases and results in more succinct code.

It should be noted that given this implementation, importing the OutputPin trait can result in ambiguous calls, so please remove the trait imports if you do not need them.

use core::convert::Infallible;
use embedded_hal::digital::blocking::OutputPin;

struct HalImplGpioPin;

impl OutputPin for HalImplGpioPin {
  type Error = Infallible;

  fn set_high(&mut self) -> Result<(), Self::Error> {
    // ...
    Ok(())
  }

  fn set_low(&mut self) -> Result<(), Self::Error> {
    // ...
    Ok(())
  }
}

impl HalImplGpioPin {
  fn set_high(&mut self) {
    // ...
  }

  fn set_low(&mut self) {
    // ...
  }
}

SPI transfer return type

Previously the transfer() method in SPI returned a slice of the output data. This slice is the same as the output buffer which is passed to the method, though, thus redundant and potentially confusing. The transfer() method now returns Result<(), Self::Error>. If you were using this return value, adapting the code should be straight forward by simply using the reception buffer which is passed.

See an example:

let tx_data = [1, 2, 3, 4];
let mut rx_data = [0; 4];
let data = spi.transfer(&tx_data, &mut rx_data)?;
println!("{:?}", data);
// There is no need to do `let data = `, since we already have the data in `rx_data`.
// Do this instead:
spi.transfer(&tx_data, &mut rx_data)?;
println!("{:?}", rx_data);

Error type bounds

All associated error types are now required to implement core::fmt::Debug. Usually it is enough to add a #[derive(Debug)] clause to your error types. For example:

+ #[derive(Debug)]
pub enum MyError {
  InvalidInputData,
  // ...
}

Additionally, for the I2C and SPI communication interfaces we have added a dedicated mechanism which allows for two crucial requirements:

Generic code like drivers can interpret and act on errors if they want to.
HAL implementations can have arbitrarily-precise error types.

This works in the following way:

For each interface, embedded-hal defines an ErrorKind enum type with all sensible error variants as well as an Error trait featuring a method that converts the type into that ErrorKind.

embedded-hal still allows for implementation-defined error types associated to each trait, but requires these to implement the appropriate Error trait, thus providing a mapping to a defined set of error variants.

With this mechanism, HAL implementations can continue to define their own error types which can carry as much information as they want. On the other hand it is now possible for generic code to inspect those errors and act on common errors like I2Cs NACK.

Furthermore, implementation-specific code can access the original error type and retrieve any information contained.

An example of a driver which looks for I2C NACK errors and returns its own DeviceBusy or Comm error wrapping the original one could be as follows:

const address = 0x1D;

fn set_some_parameter(&mut self) -> Result<(), Self::Error> {
  const data = [0, 1];
  match self.i2c.write(address, &data) {
    Err(e) => match e.kind() {
      ErrorKind::NoAcknowledge(_) => Err(Self::Error::DeviceBusy(e)),
      _ => Err(Self::Error::Comm(e)) // wrap and return any other error
    },
    Ok(_) => Ok(())
  }
}

GPIO traits now require `&mut self`

Methods on InputPin and State now take &mut self instead of &self, to allow implementations to have mutable state or access exclusive resources.

For HAL implementors: You should not need to do any changes, since &mut self is strictly more permissive for implementations.

For ease of use, you might want to provide inherent methods that take &self if the hardware permits it. In this case, you might need to do *self to call them from the trait methods. Otherwise Rust will resolve the method call to the trait method, causing infinite recursion.

struct HalPin;

impl HalPin {
    fn is_high(&self) -> bool {
        true
    }
    
    fn is_low(&self) -> bool {
        true
    }
}

impl InputPin for HalPin {
    fn is_high(&mut self) -> Result<bool, Self::Error> {
        // Needs `*self` so that the inherent `is_high` is picked.
        Ok((*self).is_high())
    }
    
    fn is_low(&mut self) -> Result<bool, Self::Error> {
        Ok((*self).is_low())
    }
}

For driver authors: If your driver does not need sharing input pins, you should be able to upgrade without any changes. If you do need to share input pins, the recommended solution is wrapping them with a RefCell.

Note that if you need to share multiple objects, you should prefer using a single RefCell wherever possible to reduce RAM usage. Make an "inner" struct with all the objects that need sharing, and wrap it in a single RefCell. Below is an example skeleton of a keypad driver using row/column multiplexing, sharing multiple InputPins and OutputPins with a single RefCell:

use core::cell::RefCell;

use embedded_hal::digital::{ErrorType, InputPin, OutputPin};

pub struct Keypad<O: OutputPin, I: InputPin, const NCOLS: usize, const NROWS: usize> {
    inner: RefCell<KeypadInner<O, I, NCOLS, NROWS>>,
}

struct KeypadInner<O: OutputPin, I: InputPin, const NCOLS: usize, const NROWS: usize> {
    cols: [O; NCOLS],
    rows: [I; NROWS],
}

pub struct KeypadInput<'a, O: OutputPin, I: InputPin, const NCOLS: usize, const NROWS: usize> {
    inner: &'a RefCell<KeypadInner<O, I, NCOLS, NROWS>>,
    row: usize,
    col: usize,
}

impl<'a, O: OutputPin, I: InputPin, const NCOLS: usize, const NROWS: usize> ErrorType for KeypadInput<'a, O, I, NCOLS, NROWS> {
    type Error = core::convert::Infallible;
}

impl<'a, O: OutputPin, I: InputPin, const NCOLS: usize, const NROWS: usize> InputPin for KeypadInput<'a, O, I, NCOLS, NROWS> {
    fn is_high(&mut self) -> Result<bool, Self::Error> {
        Ok(!self.is_low()?)
    }

    fn is_low(&mut self) -> Result<bool, Self::Error> {
        let inner = &mut *self.inner.borrow_mut();
        let row = &mut inner.rows[self.row];
        let col = &mut inner.cols[self.col];

        // using unwrap for demo purposes, you should propagate errors up instead.
        col.set_low().unwrap();
        let out = row.is_low().unwrap();
        col.set_high().unwrap();

        Ok(out)
    }
}

Prelude

The prelude has been removed because it could make method calls ambiguous, since the method names are now the same across traits. To overcome this, please import the traits you wish to use individually.

If you run into ambiguous method calls, you can disambiguate using the fully-qualified syntax (the error message from the compiler should already tell you how it should look in your case) or tweak your trait imports or code to limit the scope of the trait imports and thus avoid the ambiguity. Please note that it is also possible to import traits inside a function.

Removed blanket implementations

There were several blanket implementations of blocking traits using the non-blocking traits as a base.

Since the non-blocking traits have been extracted into the separate crate embedded-hal-nb, these have been removed.

Cargo features

The unproven feature has been removed and the traits have been marked as proven. In the past, managing unproven features, and having "sort of breaking" changes has been a struggling point. Also, people tended to adopt unproven features quickly, but the features would take a very long time to stabilize.

Instead, we would like to push experimentation OUT of the embedded-hal crate, allowing people to experiment externally, and merge when some kind of feasibility had been proven.

Companion crates

The embedded-hal project now spans several crates, where some functionality has been moved out from the main embedded-hal crate to separate crates as detailed above.

Different crates are released independently. The main embedded-hal-* trait crates have reached 1.0 maturity, others will become 1.0 as time passes.

Here is the full listing of crates:

Crate	crates.io	Docs
embedded-hal			Core traits, blocking version
embedded-hal-async			Core traits, async version
embedded-hal-nb			Core traits, polling version using the `nb` crate
embedded-hal-bus			Utilities for sharing SPI and I2C buses
embedded-can			Controller Area Network (CAN) traits
embedded-io			I/O traits (read, write, seek, etc.), blocking and nonblocking version.
embedded-io-async			I/O traits, async version
embedded-io-adapters			Adapters between the `embedded-io` and `embedded-io-async` traits and other IO traits (`std`, `tokio`, `futures`...)

Supporting both 0.2 and 1.0 in the same HAL

It is not recommended to support 0.2 version of the HAL. Many of the traits are not fit for purpose which is already discussed in this document.

If you wish to support both the 0.2 and 1.0 release it is recommended to support embedded-hal version 0.2 behind a feature flag named embedded-hal-02.

The way you do it is adding a dependency on both versions in Cargo.toml like this:

[dependencies]
embedded-hal-02 = { package = "embedded-hal", version = "0.2.7", features = ["unproven"], optional = true }
embedded-hal-1 = { package = "embedded-hal", version = "1.0" }

This allows you to refer to the v0.2 traits under the embedded_hal_02 name, and the v1.0 traits under embedded_hal_1. Implement both versions on the same struct. For example, for an input pin:

/// The HAL's input pin struct
struct Input {...}

/// Implement the v0.2 traits on the struct.
#[cfg(feature = "embedded-hal-02")]
impl embedded_hal_02::digital::v2::InputPin for Input {
    type Error = Infallible;

    fn is_high(&self) -> Result<bool, Self::Error> {
        ...
    }

    fn is_low(&self) -> Result<bool, Self::Error> {
        ...
    }
}

/// ... and implement the v1.0 traits on the *same* struct.
impl embedded_hal_1::digital::ErrorType for Input {
    type Error = Infallible;
}

impl embedded_hal_1::digital::InputPin for Input {
    fn is_high(&mut self) -> Result<bool, Self::Error> {
        ...
    }

    fn is_low(&mut self) -> Result<bool, Self::Error> {
        ...
    }
}

`embedded-hal-compat`

For HAL implementation crates that haven't been updated yet, embedded-hal-compat provides shims to support interoperability between embedded-hal v0.2 and v1.0.

This allows using a driver requiring v1.0 with a HAL crate implementing only v0.2 or vice-versa, (generally) without alteration. See the docs for examples.

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