Crate embedded_time

Embedded Time

embedded-time provides a comprehensive library for implementing Clock abstractions over hardware to generate Instants and using Durations (Seconds, Milliseconds, etc) in embedded systems. The approach is similar to the C++ chrono library. A Duration consists of an integer value (chosen by the user from either i32 or i64) as well as a const ratio where the integer value multiplied by the ratio is the seconds represented by the Duration. Put another way, the ratio is the precision of the LSbit of the integer. This structure avoids unnecessary arithmetic. For example, if the Duration type is Milliseconds, a call to the Duration::count() method simply returns the stored integer value directly which is the number of milliseconds being represented. Conversion arithmetic is only performed when explicitly converting between time units.

Motivation

The handling of time on embedded systems is generally much different than that of OSs. For instance, on an OS, the time is measured against an arbitrary epoch. Embedded systems generally don't know (nor do they care) what the real time is, but rather how much time has passed since the system has started.

Background

Drawbacks of the core::Duration type

• The storage is u64 seconds and u32 nanoseconds.
  • This is huge overkill and adds needless complexity beyond what is required (or desired) for embedded systems.
• Any read requires arithmetic to convert to the requested units
  • This is much slower than this project's implementation of what is analogous to a tagged union of time units.

What is an Instant?

In the Rust ecosystem, it appears to be idiomatic to call a now() associated function from an Instant type. There is generally no concept of a "Clock". I believe that using the Instant in this way is a violation of the separation of concerns principle. What is an Instant? Is it a time-keeping entity from which you read the current instant in time, or is it that instant in time itself. In this case, it's both.

As an alternative, the current instant in time could be read from a Clock. The Instant read from the Clock has the same precision and width (integer type) as the Clock. Requesting the difference between two Instants gives a Duration which can have different precision and/or width.

Definitions

Clock - Any entity that periodically counts (ie a hardware timer peripheral). Generally, this needs to be monotonic. Here a wrapping timer is considered monotonic as long as it fulfills the other requirements.

Wrapping Timer - A timer that when at its maximum value, the next count is the minimum value.

Instant - A specific instant in time ("time-point") returned by calling Clock::now().

Duration - The difference of two instances (the duration of time elapsed from one instant until another).

Notes

Some parts of this crate were derived from various sources:

• RTFM
• time (Specifically the time::NumbericalDuration implementations for primitive integers)

Example Usage

struct SomeClock;
impl embedded_time::Clock for SomeClock {
    type Rep = i64;

    fn now() -> Instant<Self> {
        // ...
    }
}

impl embedded_time::Period for SomeClock {
    const PERIOD: Ratio<i32> = Ratio::<i32>::new_raw(1, 16_000_000);
}

let instant1 = SomeClock::now();
// ...
let instant2 = SomeClock::now();
assert!(instant1 < instant2);    // instant1 is *before* instant2

// duration is the difference between the instances
let duration: Option<Microseconds<i64>> = instant2.elapsed_since(&instant1);    

assert!(duration.is_some());
assert_eq!(instant1 + duration.unwrap(), instant2);

Re-exports

pub use duration::time_units;

pub use duration::Duration;

Modules

duration	Duration types/units creation and conversion.
instant
prelude	A collection of imports that are widely useful.

Structs

Ratio

Represents the ratio between two numbers.

Traits

Clock
Period
TimeRep	Create Durations from primitive and core numeric types.