= Sharing peripherals between tasks

Often times, more than one task needs access to the same resource (pin, communication interface, etc.). The following example shows how to use the on-board LED on a Raspberry Pi Pico board by two tasks simultaneously. 

[,rust]
----
use defmt::*;
use embassy_executor::Spawner;
use embassy_rp::gpio;
use embassy_sync::blocking_mutex::raw::ThreadModeRawMutex;
use embassy_sync::mutex::Mutex;
use embassy_time::{Duration, Ticker};
use gpio::{AnyPin, Level, Output};
use {defmt_rtt as _, panic_probe as _};

type LedType = Mutex<ThreadModeRawMutex, Option<Output<'static, AnyPin>>>;
static LED: LedType = Mutex::new(None);

#[embassy_executor::main]
async fn main(spawner: Spawner) {
    let p = embassy_rp::init(Default::default());
    // set the content of the global LED reference to the real LED pin
    let led = Output::new(AnyPin::from(p.PIN_25), Level::High);
    // inner scope is so that once the mutex is written to, the MutexGuard is dropped, thus the
    // Mutex is released
    {
        *(LED.lock().await) = Some(led);
    }
    let dt = 100 * 1_000_000;
    let k = 1.003;

    unwrap!(spawner.spawn(toggle(&LED, Duration::from_nanos(dt))));
    unwrap!(spawner.spawn(toggle_slightly_slower(
        &LED,
        Duration::from_nanos((dt as f64 * k) as u64)
    )));
}

async fn toggle_led(led: &'static LedType, delay: Duration) {
    let mut ticker = Ticker::every(delay);
    loop {
        {
            let mut led_unlocked = led.lock().await;
            if let Some(pin_ref) = led_unlocked.as_mut() {
                pin_ref.toggle();
            }
        }
        ticker.next().await;
    }
}
#[embassy_executor::task]
async fn toggle(led: &'static LedType, delay: Duration) {
    toggle_led(led, delay).await
}

#[embassy_executor::task]
async fn toggle_slightly_slower(led: &'static LedType, delay: Duration) {
    toggle_led(led, delay).await
}
----

The structure facilitating access to the resource is the defined `LedType`.

== Why so complicated

Unwrapping the layers gives insight into why each one is needed.

=== `Mutex<RawMutexType, T>`

The mutex is there so if one task gets the resource first and begins modifying it, all other tasks wanting to write will have to wait (the `led.lock().await` will return immediately if no task has locked the mutex, and will block if it is accessed somewhere else). 

=== `Option<T>`

The `LED` variable needs to be defined outside the main task as references accepted by tasks need to be `'static`. However, if it is outside the main task, it cannot be initialised to point to any pin, as the pins themselves are not initialised. Thus, it is set to `None`. 

=== `Output<AnyPin>`

To indicate that the pin will be set to an Output. The `AnyPin` could have been `embassy_rp::peripherals::PIN_25`, however this option lets the `toggle_led` function be more generic.