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document resolution and range of Instant and Duration

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Jorge Aparicio 2019-05-08 19:48:12 +02:00
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book/en/src/internals/timer-queue.md
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@ -158,6 +158,37 @@ way it will run at the right priority.
handler; basically, `enqueue_unchecked` delegates the task of setting up a new
timeout interrupt to the `SysTick` handler.
## Resolution and range of `Instant` and `Duration`
In the current implementation the `DWT`'s (Data Watchpoint and Trace) cycle
counter is used as a monotonic timer. `Instant::now` returns a snapshot of this
timer; these DWT snapshots (`Instant`s) are used to sort entries in the timer
queue. The cycle counter is a 32-bit counter clocked at the core clock
frequency. This counter wraps around every `(1 << 32)` clock cycles; there's no
interrupt associated to this counter so nothing worth noting happens when it
wraps around.
To order `Instant`s in the queue we need to compare two 32-bit integers. To
account for the wrap-around behavior we use the difference between two
`Instant`s, `a - b`, and treat the result as a 32-bit signed integer. If the
result is less than zero then `b` is a later `Instant`; if the result is greater
than zero then `b` is an earlier `Instant`. This means that scheduling a task at
an `Instant` that's `(1 << 31) - 1` cycles greater than the scheduled time
(`Instant`) of the first (earliest) entry in the queue will cause the task to be
inserted at the wrong place in the queue. There some debug assertions in place
to prevent this user error but it can't be avoided because the user can write
`(instant + duration_a) + duration_b` and overflow the `Instant`.
The system timer, `SysTick`, is a 24-bit counter also clocked at the core clock
frequency. When the next scheduled task is more than `1 << 24` clock cycles in
the future an interrupt is set to go off in `1 << 24` cycles. This process may
need to happen several times until the next scheduled task is within the range
of the `SysTick` counter.
In conclusion, both `Instant` and `Duration` have a resolution of 1 core clock
cycle and `Duration` effectively has a (half-open) range of `0..(1 << 31)` (end
not included) core clock cycles.
## Queue capacity
The capacity of the timer queue is chosen to be the sum of the capacities of all