= Time-keeping

In an embedded program, delaying a task is one of the most common actions taken. In an event loop, delays will need to be inserted to ensure
that other tasks have a chance to run before the next iteration of the loop is called, if no other I/O is performed. Embassy provides abstractions
to delay the current task for a specified interval of time.

The interface for time-keeping in Embassy is handled by the link:https://crates.io/crates/embassy-time[embassy-time] crate. The types can be used with the internal
timer queue in link:https://crates.io/crates/embassy-executor[embassy-executor] or a custom timer queue implementation.

== Timer

The `embassy::time::Timer` type provides two timing methods.

`Timer::at` creates a future that completes at the specified `Instant`, relative to the system boot time.
`Timer::after` creates a future that completes after the specified `Duration`, relative to when the future was created.

An example of a delay is provided as follows:

TIP: Dependencies needed to run this example link:/book/dev/basic_application.html#_the_cargo_toml[can be found here].
[,rust]
----
use embassy::executor::{task, Executor};
use embassy::time::{Duration, Timer};

#[task]
/// Task that ticks periodically
async fn tick_periodic() -> ! {
    loop {
        rprintln!("tick!");
        // async sleep primitive, suspends the task for 500ms.
        Timer::after(Duration::from_millis(500)).await;
    }
}
----

== Delay

The `embassy::time::Delay` type provides an implementation of the link:https://docs.rs/embedded-hal/1.0.0/embedded_hal/delay/index.html[embedded-hal] and
link:https://docs.rs/embedded-hal-async/latest/embedded_hal_async/delay/index.html[embedded-hal-async] traits. This can be used for drivers
that expect a generic delay implementation to be provided.

An example of how this can be used:

TIP: Dependencies needed to run this example link:/book/dev/basic_application.html#_the_cargo_toml[can be found here].
[,rust]
----
use embassy::executor::{task, Executor};

#[task]
/// Task that ticks periodically
async fn tick_periodic() -> ! {
    loop {
        rprintln!("tick!");
        // async sleep primitive, suspends the task for 500ms.
        generic_delay(embassy::time::Delay).await
    }
}

async fn generic_delay<D: embedded_hal_async::delay::DelayNs>(delay: D) {
      delay.delay_ms(500).await;
}
----