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<main>
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<h1 id="the-background-task-idle"><a class="header" href="#the-background-task-idle">The background task <code>#[idle]</code></a></h1>
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<p>A function marked with the <code>idle</code> attribute can optionally appear in the module. This becomes the special <em>idle task</em> and must have signature <code>fn(idle::Context) -> !</code>.</p>
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<p>When present, the runtime will execute the <code>idle</code> task after <code>init</code>. Unlike <code>init</code>, <code>idle</code> will run <em>with interrupts enabled</em> and must never return, as the <code>-> !</code> function signature indicates.
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<a href="https://doc.rust-lang.org/core/primitive.never.html">The Rust type <code>!</code> means “never”</a>.</p>
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<p>Like in <code>init</code>, locally declared resources will have <code>'static</code> lifetimes that are safe to access.</p>
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<p>The example below shows that <code>idle</code> runs after <code>init</code>.</p>
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<pre><code class="language-rust noplayground">//! examples/idle.rs
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#![no_main]
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#![no_std]
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#![deny(warnings)]
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#![deny(unsafe_code)]
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#![deny(missing_docs)]
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965)]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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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(_: init::Context) -> (Shared, Local) {
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hprintln!("init");
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(Shared {}, Local {})
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}
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#[idle(local = [x: u32 = 0])]
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fn idle(cx: idle::Context) -> ! {
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// Locals in idle have lifetime 'static
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let _x: &'static mut u32 = cx.local.x;
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hprintln!("idle");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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loop {
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cortex_m::asm::nop();
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}
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example idle
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</code></pre>
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<pre><code class="language-console">init
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idle
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</code></pre>
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<p>By default, the RTIC <code>idle</code> task does not try to optimize for any specific targets.</p>
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<p>A common useful optimization is to enable the <a href="https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit">SLEEPONEXIT</a> and allow the MCU to enter sleep when reaching <code>idle</code>.</p>
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<blockquote>
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<p><strong>Caution</strong>: some hardware unless configured disables the debug unit during sleep mode.</p>
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<p>Consult your hardware specific documentation as this is outside the scope of RTIC.</p>
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</blockquote>
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<p>The following example shows how to enable sleep by setting the
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<a href="https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit"><code>SLEEPONEXIT</code></a> and providing a custom <code>idle</code> task replacing the default <a href="https://developer.arm.com/documentation/dui0662/b/The-Cortex-M0--Instruction-Set/Miscellaneous-instructions/NOP"><code>nop()</code></a> with <a href="https://developer.arm.com/documentation/dui0662/b/The-Cortex-M0--Instruction-Set/Miscellaneous-instructions/WFI"><code>wfi()</code></a>.</p>
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<pre><code class="language-rust noplayground">//! examples/idle-wfi.rs
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#![no_main]
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#![no_std]
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#![deny(warnings)]
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#![deny(unsafe_code)]
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#![deny(missing_docs)]
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use panic_semihosting as _;
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#[rtic::app(device = lm3s6965)]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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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(mut cx: init::Context) -> (Shared, Local) {
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hprintln!("init");
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// Set the ARM SLEEPONEXIT bit to go to sleep after handling interrupts
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// See https://developer.arm.com/docs/100737/0100/power-management/sleep-mode/sleep-on-exit-bit
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cx.core.SCB.set_sleepdeep();
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(Shared {}, Local {})
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}
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#[idle(local = [x: u32 = 0])]
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fn idle(cx: idle::Context) -> ! {
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// Locals in idle have lifetime 'static
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let _x: &'static mut u32 = cx.local.x;
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hprintln!("idle");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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loop {
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// Now Wait For Interrupt is used instead of a busy-wait loop
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// to allow MCU to sleep between interrupts
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// https://developer.arm.com/documentation/ddi0406/c/Application-Level-Architecture/Instruction-Details/Alphabetical-list-of-instructions/WFI
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rtic::export::wfi()
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}
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}
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}</code></pre>
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<pre><code class="language-console">$ cargo xtask qemu --verbose --example idle-wfi
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</code></pre>
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<pre><code class="language-console">init
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idle
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</code></pre>
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<blockquote>
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<p><strong>Notice</strong>: The <code>idle</code> task cannot be used together with <em>software</em> tasks running at priority zero. The reason is that <code>idle</code> is running as a non-returning Rust function at priority zero. Thus there would be no way for an executor at priority zero to give control to <em>software</em> tasks at the same priority.</p>
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</blockquote>
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