Module serpente::sercom::v2::spi::impl_ehal

Implement [embedded_hal] traits for Spi structs

As noted in the spi module documentation, the embedded-hal trait implementations vary by both Size and Capability. Each implementation is optimized to take advantage of all information known at compile-time, so it is importatnt to carefully read the documentation in this module.

Variations by Size

SAMD11 and SAMD21 chips do not have 32-bit extension mode, so their transaction Size can only vary by the CharSize. Both options, EightBit and NineBit, are AtomicSizes, because each can be completed with a single read/write of the DATA register. Consequently, each can implement both the blocking and non-blocking embedded HAL traits. These traits are implemented for the Word type of the corresponding CharSize. For example, an Spi struct with a NineBit CharSize would implement spi::FullDuplex<u16>.

Note that embedded HAL does not offer a way to transmit slices in a non-blocking fashion, but this can be done using DMA or using interrupts and the spi_future module.

Variations by Capability

The implementations in this module also seek to optimize as much as possible based on the Capability of the Spi struct. They follow a few general rules:

• Tx structs can never receive data, so their corresponding trait implementations never read the DATA register and can never return an Error::Overflow.
• Rx structs in a MasterMode must initiate all transactions, so their implementations of non-blocking traits must track the state of on-going transactions.
• Duplex structs must always read as many bytes as they send, even when implementing Write-only traits, to ensure they never introduce an Error::Overflow.

Notes on individual embedded HAL traits

spi::FullDuplex

spi::FullDuplex is only implemented for structs with Duplex Capability. Although the embedded HAL documentation assumes a MasterMode, this module also implements it for the Slave OpMode.

serial::Read

serial::Read is only implemented for structs with Rx Capability. When in a MasterMode, it initiates and tracks the state of the on-going transactions. But this is not required when acting as a Slave.

serial::Write

serial::Write is only implemented for structs with Tx Capability. These implementations never read the DATA register and ignore all instances of Error::Overflow.

blocking::serial::Write

This trait uses the blocking::serial::write::Default implementation for implementers of serial::Write.

blocking::spi traits

These traits are implemented following all of the rules outlined above for the different Size and Capability options.