Crate shared_bus

Expand description

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:

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.

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`
`shared_bus::XtensaMutex` (`spin::Mutex` in critical section)	`BusManagerXtensa`	`new_xtensa!()`	`xtensa`
None (Automatically Managed)	`BusManagerAtomicCheck`	`new_atomic_check!()`	`cortex-m`

Supported buses and hardware blocks

Currently, the following buses/blocks can be shared with shared-bus:

Bus/Block	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).
ADC	`AdcProxy`	`.acquire_adc()`