Merge #741

741: Docs 2 r=korken89 a=datdenkikniet

Working on the migration guide and other docs

TODO:
- [x] Migration guide
- [x] Hardcoded examples should link to example code that is tested (this was already done, AFAICT)
- [x] Address #699 
- [x] Discuss: should we remove references to non-v2, apart from the migration guide and link to the book for v1? (Off-github conclusion: yes)
- [x] RTIC {vs,and} Embassy (important: distinction between embassy runtime & HALs)
- [x] More descriptive docs on how to implement & PR implementations of `Monotonic` to `rtic-monotonics` 


Co-authored-by: datdenkikniet <jcdra1@gmail.com>

1 files changed, 64 insertions, 0 deletions

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+# Monotonic & spawn_{at/after}
+
+The understanding of time is an important concept in embedded systems, and to be able to run tasks
+based on time is essential. The framework provides the static methods
+`task::spawn_after(/* duration */)` and `task::spawn_at(/* specific time instant */)`.
+`spawn_after` is more commonly used, but in cases where it's needed to have spawns happen
+without drift or to a fixed baseline `spawn_at` is available.
+
+The `#[monotonic]` attribute, applied to a type alias definition, exists to support this.
+This type alias must point to a type which implements the [`rtic_monotonic::Monotonic`] trait.
+This is generally some timer which handles the timing of the system.
+One or more monotonics can coexist in the same system, for example a slow timer that wakes the
+system from sleep and another which purpose is for fine grained scheduling while the
+system is awake.
+
+[`rtic_monotonic::Monotonic`]: https://docs.rs/rtic-monotonic
+
+The attribute has one required parameter and two optional parameters, `binds`, `default` and
+`priority` respectively.
+The required parameter, `binds = InterruptName`, associates an interrupt vector to the timer's
+interrupt, while `default = true` enables a shorthand API when spawning and accessing
+time (`monotonics::now()` vs `monotonics::MyMono::now()`), and `priority` sets the priority
+of the interrupt vector.
+
+> The default `priority` is the **maximum priority** of the system.
+> If your system has a high priority task with tight scheduling requirements,
+> it might be desirable to demote the `monotonic` task to a lower priority
+> to reduce scheduling jitter for the high priority task.
+> This however might introduce jitter and delays into scheduling via the `monotonic`,
+> making it a trade-off.
+
+The monotonics are initialized in `#[init]` and returned within the `init::Monotonic( ... )` tuple.
+This activates the monotonics making it possible to use them.
+
+See the following example:
+
+``` rust,noplayground
+{{#include ../../../../examples/schedule.rs}}
+```
+
+``` console
+$ cargo run --target thumbv7m-none-eabi --example schedule
+{{#include ../../../../ci/expected/schedule.run}}
+```
+
+A key requirement of a Monotonic is that it must deal gracefully with
+hardware timer overruns.
+
+## Canceling or rescheduling a scheduled task
+
+Tasks spawned using `task::spawn_after` and `task::spawn_at` returns a `SpawnHandle`,
+which allows canceling or rescheduling of the task scheduled to run in the future.
+
+If `cancel` or `reschedule_at`/`reschedule_after` returns an `Err` it means that the operation was
+too late and that the task is already sent for execution. The following example shows this in action:
+
+``` rust,noplayground
+{{#include ../../../../examples/cancel-reschedule.rs}}
+```
+
+``` console
+$ cargo run --target thumbv7m-none-eabi --example cancel-reschedule
+{{#include ../../../../ci/expected/cancel-reschedule.run}}
+```