Embassy
embassy-nrf

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Embassy nRF HAL

HALs implement safe, idiomatic Rust APIs to use the hardware capabilities, so raw register manipulation is not needed.

The Embassy nRF HAL targets the Nordic Semiconductor nRF family of hardware. The HAL implements both blocking and async APIs for many peripherals. The benefit of using the async APIs is that the HAL takes care of waiting for peripherals to complete operations in low power mod and handling interrupts, so that applications can focus on more important matters.

EasyDMA considerations

On nRF chips, peripherals can use the so called EasyDMA feature to offload the task of interacting with peripherals. It takes care of sending/receiving data over a variety of bus protocols (TWI/I2C, UART, SPI). However, EasyDMA requires the buffers used to transmit and receive data to reside in RAM. Unfortunately, Rust slices will not always do so. The following example using the SPI peripheral shows a common situation where this might happen:

// As we pass a slice to the function whose contents will not ever change,
// the compiler writes it into the flash and thus the pointer to it will
// reference static memory. Since EasyDMA requires slices to reside in RAM,
// this function call will fail.
let result = spim.write_from_ram(&[1, 2, 3]);
assert_eq!(result, Err(Error::BufferNotInRAM));

// The data is still static and located in flash. However, since we are assigning
// it to a variable, the compiler will load it into memory. Passing a reference to the
// variable will yield a pointer that references dynamic memory, thus making EasyDMA happy.
// This function call succeeds.
let data = [1, 2, 3];
let result = spim.write_from_ram(&data);
assert!(result.is_ok());

Each peripheral struct which uses EasyDMA (Spim, Uarte, Twim) has two variants of their mutating functions:

  • Functions with the suffix (e.g. write_from_ram, transfer_from_ram) will return an error if the passed slice does not reside in RAM.
  • Functions without the suffix (e.g. write, transfer) will check whether the data is in RAM and copy it into memory prior to transmission.

Since copying incurs a overhead, you are given the option to choose from _from_ram variants which will fail and notify you, or the more convenient versions without the suffix which are potentially a little bit more inefficient. Be aware that this overhead is not only in terms of instruction count but also in terms of memory usage as the methods without the suffix will be allocating a statically sized buffer (up to 512 bytes for the nRF52840).

Note that the methods that read data like read and transfer_in_place do not have the corresponding _from_ram variants as mutable slices always reside in RAM.

Minimum supported Rust version (MSRV)

Embassy is guaranteed to compile on the latest stable Rust version at the time of release. It might compile with older versions but that may change in any new patch release.

License

This work is licensed under either of

at your option.

Modules

  • Async buffered UART driver.
  • Configuration options used when initializing the HAL.
  • General purpose input/output (GPIO) driver.
  • GPIO task/event (GPIOTE) driver.
  • Interrupt definitions.
  • Non-Volatile Memory Controller (NVMC, AKA internal flash) driver.
  • Peripheral Access Crate
  • Pulse Density Modulation (PDM) mirophone driver.
  • Types for the peripheral singletons.
  • Programmable Peripheral Interconnect (PPI/DPPI) driver.
  • Pulse Width Modulation (PWM) driver.
  • Quadrature decoder (QDEC) driver.
  • Quad Serial Peripheral Interface (QSPI) flash driver.
  • Successive Approximation Analog-to-Digital Converter (SAADC) driver.
  • Serial Peripheral Instance in master mode (SPIM) driver.
  • Serial Peripheral Instance in slave mode (SPIS) driver.
  • Timer driver.
  • I2C-compatible Two Wire Interface in master mode (TWIM) driver.
  • I2C-compatible Two Wire Interface in slave mode (TWIM) driver.
  • Universal Asynchronous Receiver Transmitter (UART) driver.
  • Universal Serial Bus (USB) driver.

Macros

Structs

Constants

  • The maximum buffer size that the EasyDMA can send/recv in one operation.
  • Number available in the NVIC for configuring priority

Traits

  • Trait for any type that can be used as a peripheral of type P.

Functions

  • Initialize peripherals with the provided configuration. This should only be called once at startup.

Attribute Macros