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<h1 id="the-magic-behind-monotonics"><a class="header" href="#the-magic-behind-monotonics">The magic behind Monotonics</a></h1>
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<p>Internally, all monotonics use a <a href="#the-timer-queue">Timer Queue</a>, which is a priority queue with entries describing the time at which their respective <code>Future</code>s should complete.</p>
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<h2 id="implementing-a-monotonic-timer-for-scheduling"><a class="header" href="#implementing-a-monotonic-timer-for-scheduling">Implementing a <code>Monotonic</code> timer for scheduling</a></h2>
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<p>The <a href="https://docs.rs/rtic-time/latest/rtic_time"><code>rtic-time</code></a> framework is flexible because it can use any timer which has compare-match and optionally supporting overflow interrupts for scheduling. The single requirement to make a timer usable with RTIC is implementing the <a href="https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html"><code>rtic-time::Monotonic</code></a> trait.</p>
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<p>For RTIC 2.0, we assume that the user has a time library, e.g. <a href="https://docs.rs/fugit/"><code>fugit</code></a>, as the basis for all time-based operations when implementing <a href="https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html"><code>Monotonic</code></a>. These libraries make it much easier to correctly implement the <a href="https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html"><code>Monotonic</code></a> trait, allowing the use of almost any timer in the system for scheduling.</p>
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<p>The trait documents the requirements for each method. There are reference implementations available in <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics/"><code>rtic-monotonics</code></a> that can be used for inspriation.</p>
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<ul>
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<li><a href="https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/src/systick.rs"><code>Systick based</code></a>, runs at a fixed interrupt (tick) rate - with some overhead but simple and provides support for large time spans</li>
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<li><a href="https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/src/rp2040.rs"><code>RP2040 Timer</code></a>, a "proper" implementation with support for waiting for long periods without interrupts. Clearly demonstrates how to use the <a href="https://docs.rs/rtic-time/latest/rtic_time/struct.TimerQueue.html"><code>TimerQueue</code></a> to handle scheduling.</li>
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<li><a href="https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/src/nrf.rs"><code>nRF52 timers</code></a> implements monotonic & Timer Queue for the RTC and normal timers in nRF52's</li>
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</ul>
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<h2 id="contributing"><a class="header" href="#contributing">Contributing</a></h2>
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<p>Contributing new implementations of <code>Monotonic</code> can be done in multiple ways:</p>
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<ul>
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<li>Implement the trait behind a feature flag in <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics/"><code>rtic-monotonics</code></a>, and create a PR for them to be included in the main RTIC repository. This way, the implementations of are in-tree, RTIC can guarantee their correctness, and can update them in the case of a new release.</li>
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<li>Implement the changes in an external repository. Doing so will not have them included in <a href="https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics/"><code>rtic-monotonics</code></a>, but may make it easier to do so in the future.</li>
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</ul>
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<h2 id="the-timer-queue"><a class="header" href="#the-timer-queue">The timer queue</a></h2>
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<p>The timer queue is implemented as a list based priority queue, where list-nodes are statically allocated as part of the <code>Future</code> created when <code>await</code>-ing a Future created when waiting for the monotonic. Thus, the timer queue is infallible at run-time (its size and allocation are determined at compile time).</p>
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<p>Similarly the channels implementation, the timer-queue implementation relies on a global <em>Critical Section</em> (CS) for race protection. For the examples a CS implementation is provided by adding <code>--features test-critical-section</code> to the build options.</p>
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