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402 lines
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16 KiB
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<main>
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<h1 at/after="">Monotonic & spawn_</h1>
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<p>The understanding of time is an important concept in embedded systems, and to be able to run tasks
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based on time is essential. The framework provides the static methods
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<code>task::spawn_after(/* duration */)</code> and <code>task::spawn_at(/* specific time instant */)</code>.
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<code>spawn_after</code> is more commonly used, but in cases where it's needed to have spawns happen
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without drift or to a fixed baseline <code>spawn_at</code> is available.</p>
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<p>The <code>#[monotonic]</code> attribute, applied to a type alias definition, exists to support this.
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This type alias must point to a type which implements the <a href="https://docs.rs/rtic-monotonic"><code>rtic_monotonic::Monotonic</code></a> trait.
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This is generally some timer which handles the timing of the system.
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One or more monotonics can coexist in the same system, for example a slow timer that wakes the
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system from sleep and another which purpose is for fine grained scheduling while the
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system is awake.</p>
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<p>The attribute has one required parameter and two optional parameters, <code>binds</code>, <code>default</code> and
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<code>priority</code> respectively.
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The required parameter, <code>binds = InterruptName</code>, associates an interrupt vector to the timer's
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interrupt, while <code>default = true</code> enables a shorthand API when spawning and accessing
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time (<code>monotonics::now()</code> vs <code>monotonics::MyMono::now()</code>), and <code>priority</code> sets the priority
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of the interrupt vector.</p>
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<blockquote>
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<p>The default <code>priority</code> is the <strong>maximum priority</strong> of the system.
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If your system has a high priority task with tight scheduling requirements,
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it might be desirable to demote the <code>monotonic</code> task to a lower priority
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to reduce scheduling jitter for the high priority task.
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This however might introduce jitter and delays into scheduling via the <code>monotonic</code>,
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making it a trade-off.</p>
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</blockquote>
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<p>The monotonics are initialized in <code>#[init]</code> and returned within the <code>init::Monotonic( ... )</code> tuple.
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This activates the monotonics making it possible to use them.</p>
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<p>See the following example:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>//! examples/schedule.rs
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#![deny(unsafe_code)]
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#![deny(warnings)]
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#![deny(missing_docs)]
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#![no_main]
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#![no_std]
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965, dispatchers = [SSI0])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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use systick_monotonic::*;
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#[monotonic(binds = SysTick, default = true)]
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type MyMono = Systick<100>; // 100 Hz / 10 ms granularity
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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#[init]
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fn init(cx: init::Context) -> (Shared, Local, init::Monotonics) {
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let systick = cx.core.SYST;
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// Initialize the monotonic (SysTick rate in QEMU is 12 MHz)
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let mono = Systick::new(systick, 12_000_000);
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hprintln!("init");
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// Schedule `foo` to run 1 second in the future
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foo::spawn_after(1.secs()).unwrap();
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(
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Shared {},
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Local {},
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init::Monotonics(mono), // Give the monotonic to RTIC
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)
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}
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#[task]
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fn foo(_: foo::Context) {
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hprintln!("foo");
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// Schedule `bar` to run 2 seconds in the future (1 second after foo runs)
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bar::spawn_after(1.secs()).unwrap();
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}
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#[task]
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fn bar(_: bar::Context) {
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hprintln!("bar");
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// Schedule `baz` to run 1 seconds from now, but with a specific time instant.
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baz::spawn_at(monotonics::now() + 1.secs()).unwrap();
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}
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#[task]
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fn baz(_: baz::Context) {
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hprintln!("baz");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example schedule
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init
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foo
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bar
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baz
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</code></pre>
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<p>A key requirement of a Monotonic is that it must deal gracefully with
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hardware timer overruns.</p>
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<h2 id="canceling-or-rescheduling-a-scheduled-task"><a class="header" href="#canceling-or-rescheduling-a-scheduled-task">Canceling or rescheduling a scheduled task</a></h2>
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<p>Tasks spawned using <code>task::spawn_after</code> and <code>task::spawn_at</code> returns a <code>SpawnHandle</code>,
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which allows canceling or rescheduling of the task scheduled to run in the future.</p>
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<p>If <code>cancel</code> or <code>reschedule_at</code>/<code>reschedule_after</code> returns an <code>Err</code> it means that the operation was
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too late and that the task is already sent for execution. The following example shows this in action:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>//! examples/cancel-reschedule.rs
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#![deny(unsafe_code)]
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#![deny(warnings)]
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#![deny(missing_docs)]
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#![no_main]
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#![no_std]
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|
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965, dispatchers = [SSI0])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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use systick_monotonic::*;
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#[monotonic(binds = SysTick, default = true)]
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type MyMono = Systick<100>; // 100 Hz / 10 ms granularity
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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#[init]
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fn init(cx: init::Context) -> (Shared, Local, init::Monotonics) {
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let systick = cx.core.SYST;
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// Initialize the monotonic (SysTick rate in QEMU is 12 MHz)
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let mono = Systick::new(systick, 12_000_000);
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hprintln!("init");
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// Schedule `foo` to run 1 second in the future
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foo::spawn_after(1.secs()).unwrap();
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(
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Shared {},
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Local {},
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init::Monotonics(mono), // Give the monotonic to RTIC
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)
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}
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#[task]
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fn foo(_: foo::Context) {
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hprintln!("foo");
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// Schedule `bar` to run 2 seconds in the future (1 second after foo runs)
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let spawn_handle = baz::spawn_after(2.secs()).unwrap();
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bar::spawn_after(1.secs(), spawn_handle, false).unwrap(); // Change to true
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}
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#[task]
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fn bar(_: bar::Context, baz_handle: baz::SpawnHandle, do_reschedule: bool) {
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hprintln!("bar");
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if do_reschedule {
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// Reschedule baz 2 seconds from now, instead of the original 1 second
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// from now.
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baz_handle.reschedule_after(2.secs()).unwrap();
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// Or baz_handle.reschedule_at(/* time */)
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} else {
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// Or cancel it
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baz_handle.cancel().unwrap();
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
#[task]
|
||
|
|
fn baz(_: baz::Context) {
|
||
|
|
hprintln!("baz");
|
||
|
|
debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
|
||
|
|
}
|
||
|
|
}
|
||
|
|
<span class="boring">}</span></code></pre></pre>
|
||
|
|
<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example cancel-reschedule
|
||
|
|
init
|
||
|
|
foo
|
||
|
|
bar
|
||
|
|
</code></pre>
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