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Update tips on Monotonic implemenations

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book/en/src/by-example/tips_monotonic_impl.md
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# Implementing a `Monotonic` timer for scheduling
The 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.
The 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 1.0 and 2.0 we instead assume the user has a time library, e.g. [`fugit`] or [`embedded_time`], 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.
For RTIC 1.0 and 2.0 we instead assume the user has a time library, e.g. [`fugit`] or [`embedded_time`], 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, and for inspiration
there is a reference implementation based on the `SysTick` timer available on all ARM Cortex M MCUs.
The trait documents the requirements for each method. There are reference implementations available in [`rtic-monotonics`](https://github.com/rtic-rs/rtic/tree/master/rtic-monotonics/src) 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, and RTIC can guarantee their correctness, and can update them in the case of a new release.
* Implement the changes in an external repository.
<!-- TODO: remove 1.0.x examples? -->
# V1.0.x
Here is a list of `Monotonic` implementations for RTIC 1.0:
@ -27,3 +36,6 @@ If you know of more implementations feel free to add them to this list.
[`Nordic nRF52 series RTC`]: https://gist.github.com/korken89/fe94a475726414dd1bce031c76adc3dd
[`Systick based`]: https://github.com/rtic-monotonics
[`DWT and Systick based`]: https://github.com/rtic-rs/dwt-systick-monotonic
[`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