Basic

On a microcontroller¶

exact_units is no_std, needs no allocator, and has no dependencies unless a feature asks for one.

Pick the features¶

The defaults (std, shrink_wrap) suit a host. A firmware turns them off and picks its unit groups:

[dependencies]
exact_units = { version = "0.1", default-features = false, features = ["time", "electrical", "defmt"] }

Feature	What it adds
none	`Fixed`, `Point`, `Magnitude`, the markers that need no group (`One`, `Milli`, ...)
`time`, `electrical`, ...	the unit groups, see Units and dimensions
`defmt`	`defmt::Format` for every type
`libm`	`to_db` / `from_db` without `std`, from the pure-Rust `libm`
`shrink_wrap`	the wire format, see The wire
`serde`	`serde` for the numbers, `no_std` without alloc, see The wire
`std`	every unit group but `customary`; the owned wire types

Nothing at run time¶

Everything about units is decided at compile time:

A scale is a type parameter.
A conversion factor is a constant the compiler works out.
A literal (fixed!, db!) is a constant.

So a Fixed costs what its integer costs. Cost on Cortex-M has the instruction counts.

What CI builds¶

thumbv6m-none-eabi (Cortex-M0, no FPU, no 64-bit multiply) without std.
thumbv7em-none-eabihf (Cortex-M4F) with libm, for levels in dB in hardware f32.

A firmware that builds for an M0 builds for anything bigger.

A first firmware value¶

use exact_units::{Fixed, Micros, Millis, prefix::Micro};

/// A timer compare value from a timeout in ms: exact, or refused at the edge.
fn compare_value(timeout: Millis<u16>) -> Option<Micros<u32>> {
    timeout.to::<Micro, u32>()
}

assert_eq!(compare_value(Fixed::new(5)), Some(Fixed::new(5000)));

The examples are small firmware stories in this style.