Embassy

Embassy is a project to make async/await a first-class option for embedded development. For more information and instructions to get started, go to https://embassy.dev.

Traits and types

embassy provides a set of traits and types specifically designed for async usage.

• embassy::io: AsyncBufRead, AsyncWrite. Traits for byte-stream IO, essentially no_std compatible versions of futures::io.
• embassy::traits::flash: Flash device trait.
• embassy::time: Clock and Alarm traits. Std-like Duration and Instant.
• More traits for SPI, I2C, UART async HAL coming soon.

Executor

The embassy::executor module provides an async/await executor designed for embedded usage.

• No alloc, no heap needed. Task futures are statically allocated.
• No "fixed capacity" data structures, executor works with 1 or 1000 tasks without needing config/tuning.
• Integrated timer queue: sleeping is easy, just do Timer::after(Duration::from_secs(1)).await;.
• No busy-loop polling: CPU sleeps when there's no work to do, using interrupts or WFE/SEV.
• Efficient polling: a wake will only poll the woken task, not all of them.
• Fair: a task can't monopolize CPU time even if it's constantly being woken. All other tasks get a chance to run before a given task gets polled for the second time.
• Creating multiple executor instances is supported, to run tasks with multiple priority levels. This allows higher-priority tasks to preempt lower-priority tasks.

Utils

embassy::util contains some lightweight async/await utilities, mainly helpful for async driver development (signaling a task that an interrupt has occured, for example).

HALs

Hardware Absraction Layers with asynchronous behaviors are provided for a variety of platforms. For example, the embassy-nrf crate contains implementations for nRF 52 series SoCs.

• uarte: UARTE driver implementing AsyncBufRead and AsyncWrite.

• qspi: QSPI driver implementing Flash.

• gpiote: GPIOTE driver. Allows awaiting GPIO pin changes. Great for reading buttons or receiving interrupts from external chips.

• saadc: SAADC driver. Provides a full implementation of the one-shot sampling for analog channels.

• rtc: RTC driver implementing Clock and Alarm, for use with embassy::executor.

Examples

Examples are found in the examples/ folder seperated by the chip manufacturer they are designed to run on. For example:

• examples/nrf are designed to run on the nrf52840-dk board (PCA10056) but should be easily adaptable to other nRF52 chips and boards.
• examples/rp are for the RP2040 chip.
• examples/stm32 are designed for the STM32F429ZI chip but should be easily adaptable to other STM32F4xx chips.
• examples/std are designed to run locally on your pc.

Running examples

• Setup git submodules (needed for STM32 examples)

git submodule init
git submodule update

• Install probe-run with defmt support.

cargo install probe-run

• Change directory to the sample's base directory. For example:

cd examples/nrf

• Run the example

For example:

cargo run --bin blinky

Developing Embassy with Rust Analyzer based editors

The Rust Analyzer is used by Visual Studio Code and others. Given the multiple targets that Embassy serves, there is no Cargo workspace file. Instead, the Rust Analyzer must be told of the target project to work with. In the case of Visual Studio Code, please refer to the .vscode/settings.json file's rust-analyzer.linkedProjectssetting.

Minimum supported Rust version (MSRV)

Required nightly version is specified in the rust-toolchain.toml file. Nightly is required for:

• generic_associated_types: for trait funcs returning futures.
• type_alias_impl_trait: for trait funcs returning futures implemented with async{} blocks, and for static-executor.

Stable support is a non-goal until these features get stabilized.

Documentation

Embassy documentation is located in the docs/ folder. The documentation is built in embassy-book and published to https://embassy.dev by CI.

Why the name?

EMBedded ASYnc! :)

License

This work is licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.