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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:
# let t =
std::thread::spawn(move || {
    my_device.do_something_on_the_bus();
});
# t.join().unwrap();

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`
NA	`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).
ADC	`AdcProxy`	`.acquire_adc()`

License

shared-bus 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.