From 311291b95adbd24ecd037f5e555e4c30138f4339 Mon Sep 17 00:00:00 2001 From: datdenkikniet Date: Sat, 20 May 2023 12:02:51 +0200 Subject: Make Monotonic implementation more obvious --- book/en/src/monotonic_impl.md | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) create mode 100644 book/en/src/monotonic_impl.md (limited to 'book/en/src/monotonic_impl.md') diff --git a/book/en/src/monotonic_impl.md b/book/en/src/monotonic_impl.md new file mode 100644 index 0000000..97c0da8 --- /dev/null +++ b/book/en/src/monotonic_impl.md @@ -0,0 +1,38 @@ +# The magic behind Monotonics + +Internally, all monotonics use a [Timer Queue](#the-timer-queue), which is a priority queue with entries describing the time at which their respective `Future`s should complete. + +## Implementing a `Monotonic` timer for scheduling + +The [`rtic-time`] 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 [`rtic-time::Monotonic`] trait. + +For RTIC 2.0, we assume that the user has a time library, e.g. [`fugit`], as the basis for all time-based operations when implementing [`Monotonic`]. These libraries make it much easier to correctly implement the [`Monotonic`] trait, allowing the use of almost any timer in the system for scheduling. + +The trait documents the requirements for each method. There are reference implementations available in [`rtic-monotonics`] that can be used for inspriation. + +- [`Systick based`], runs at a fixed interrupt (tick) rate - with some overhead but simple and provides support for large time spans +- [`RP2040 Timer`], a "proper" implementation with support for waiting for long periods without interrupts. Clearly demonstrates how to use the [`TimerQueue`] to handle scheduling. +- [`nRF52 timers`] implements monotonic & Timer Queue for the RTC and normal timers in nRF52's + +## Contributing + +Contributing new implementations of `Monotonic` can be done in multiple ways: +* Implement the trait behind a feature flag in [`rtic-monotonics`], 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. +* Implement the changes in an external repository. Doing so will not have them included in [`rtic-monotonics`], but may make it easier to do so in the future. + +[`rtic-monotonics`]: https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics/ +[`fugit`]: https://docs.rs/fugit/ +[`Systick based`]: https://github.com/rtic-monotonics +[`rtic-monotonics`]: https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics +[`RP2040 Timer`]: https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/src/rp2040.rs +[`nRF52 timers`]: https://github.com/rtic-rs/rtic/blob/master/rtic-monotonics/src/nrf.rs +[`rtic-time`]: https://docs.rs/rtic-time/latest/rtic_time +[`rtic-time::Monotonic`]: https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html +[`Monotonic`]: https://docs.rs/rtic-time/latest/rtic_time/trait.Monotonic.html +[`TimerQueue`]: https://docs.rs/rtic-time/latest/rtic_time/struct.TimerQueue.html + +## The timer queue + +The timer queue is implemented as a list based priority queue, where list-nodes are statically allocated as part of the `Future` created when `await`-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). + +Similarly the channels implementation, the timer-queue implementation relies on a global *Critical Section* (CS) for race protection. For the examples a CS implementation is provided by adding `--features test-critical-section` to the build options. \ No newline at end of file -- cgit v1.2.3