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48 lines
1.9 KiB
Markdown
48 lines
1.9 KiB
Markdown
# embassy-time
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Timekeeping, delays and timeouts.
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Timekeeping is done with elapsed time since system boot. Time is represented in
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ticks, where the tick rate is defined either by the driver (in the case of a fixed-rate
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tick) or chosen by the user with a [tick rate](#tick-rate) feature. The chosen
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tick rate applies to everything in `embassy-time` and thus determines the maximum
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timing resolution of <code>(1 / tick_rate) seconds</code>.
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Tick counts are 64 bits. The default tick rate of 1Mhz supports
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representing time spans of up to ~584558 years, which is big enough for all practical
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purposes and allows not having to worry about overflows.
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## Global time driver
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The `time` module is backed by a global "time driver" specified at build time.
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Only one driver can be active in a program.
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All methods and structs transparently call into the active driver. This makes it
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possible for libraries to use `embassy_time` in a driver-agnostic way without
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requiring generic parameters.
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For more details, check the [`embassy_time_driver`](https://crates.io/crates/embassy-time-driver) crate.
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## Instants and Durations
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[`Instant`] represents a given instant of time (relative to system boot), and [`Duration`]
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represents the duration of a span of time. They implement the math operations you'd expect,
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like addition and substraction.
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## Delays and timeouts
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[`Timer`] allows performing async delays. [`Ticker`] allows periodic delays without drifting over time.
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An implementation of the `embedded-hal` delay traits is provided by [`Delay`], for compatibility
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with libraries from the ecosystem.
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## Wall-clock time
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The `time` module deals exclusively with a monotonically increasing tick count.
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Therefore it has no direct support for wall-clock time ("real life" datetimes
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like `2021-08-24 13:33:21`).
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If persistence across reboots is not needed, support can be built on top of
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`embassy_time` by storing the offset between "seconds elapsed since boot"
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and "seconds since unix epoch".
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