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