Crate shared_bus

shared-bus is a crate to allow sharing bus peripherals safely between multiple devices.

In the embedded-hal ecosystem, it is convention for drivers to “own” the bus peripheral they are operating on. This implies that only one driver can have access to a certain bus. That, of course, poses an issue when multiple devices are connected to a single bus.

shared-bus solves this by giving each driver a bus-proxy to own which internally manages access to the actual bus in a safe manner. For a more in-depth introduction of the problem this crate is trying to solve, take a look at the blog post.

There are different ‘bus managers’ for different use-cases:

Sharing within a single task/thread

As long as all users of a bus are contained in a single task/thread, bus sharing is very simple. With no concurrency possible, no special synchronization is needed. This is where a BusManagerSimple should be used:

// For example:
// let i2c = I2c::i2c1(dp.I2C1, (scl, sda), 90.khz(), clocks, &mut rcc.apb1);

let bus = shared_bus::BusManagerSimple::new(i2c);

let mut proxy1 = bus.acquire_i2c();
let mut my_device = MyDevice::new(bus.acquire_i2c());

proxy1.write(0x39, &[0xc0, 0xff, 0xee]);
my_device.do_something_on_the_bus();

The BusManager::acquire_*() methods can be called as often as needed; each call will yield a new bus-proxy of the requested type.

Sharing across multiple tasks/threads

For sharing across multiple tasks/threads, synchronization is needed to ensure all bus-accesses are strictly serialized and can’t race against each other. The synchronization is handled by a platform-specific BusMutex implementation. shared-bus already contains some implementations for common targets. For each one, there is also a macro for easily creating a bus-manager with 'static lifetime, which is almost always a requirement when sharing across task/thread boundaries. As an example:

// For example:
// let i2c = I2c::i2c1(dp.I2C1, (scl, sda), 90.khz(), clocks, &mut rcc.apb1);

// The bus is a 'static reference -> it lives forever and references can be
// shared with other threads.
let bus: &'static _ = shared_bus::new_std!(SomeI2cBus = i2c).unwrap();

let mut proxy1 = bus.acquire_i2c();
let mut my_device = MyDevice::new(bus.acquire_i2c());

// We can easily move a proxy to another thread:
std::thread::spawn(move || {
    my_device.do_something_on_the_bus();
});

Those platform-specific bits are guarded by a feature that needs to be enabled. Here is an overview of what’s already available:

Mutex	Bus Manager	'static Bus Macro	Feature Name
std::sync::Mutex	BusManagerStd	new_std!()	std
cortex_m::interrupt::Mutex	BusManagerCortexM	new_cortexm!()	cortex-m
None (Automatically Managed)	BusManagerAtomicCheck	new_atomic_check!()	cortex-m

Supported Busses

Currently, the following busses can be shared with shared-bus:

Bus	Proxy Type	Acquire Method	Comments
I2C	I2cProxy	.acquire_i2c()
SPI	SpiProxy	.acquire_spi()	SPI can only be shared within a single task (See SpiProxy for details).

Macros

new_atomic_check	Construct a statically allocated bus manager.
new_cortexm	Macro for creating a Cortex-M bus manager with 'static lifetime.
new_std	Macro for creating a std-based bus manager with 'static lifetime.

Structs

AtomicCheckMutex	A simple coherency checker for sharing across multiple tasks/threads.
BusManager	“Manager” for a shared bus.
I2cProxy	Proxy type for I2C bus sharing.
NullMutex	“Dummy” mutex for sharing in a single task/thread.
SpiProxy	Proxy type for SPI bus sharing.

Traits

BusMutex

Common interface for mutex implementations.

Type Definitions

BusManagerAtomicCheck	A bus manager for safely sharing the bus when using concurrency frameworks (such as RTIC).
BusManagerCortexM	A bus manager for safely sharing between tasks on Cortex-M.
BusManagerSimple	A bus manager for sharing within a single task/thread.
BusManagerStd	A bus manager for safely sharing between threads on a platform with std support.
CortexMMutex	Alias for a Cortex-M mutex.