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Register formulas¶

A datasheet writes a temperature register as

\[ T = \frac{\text{raw}}{256} + 25\ ^\circ\text{C} \]

That is a point against an origin 25 K above 0 °C, in steps of \(1/256\) K. Declare the origin once, and the raw register value is the field's type.

use exact_units::{Fixed, Offset, Point, origin, origins::Celsius, prefix::{Centi, Rational}, units::Kelvin};

origin! {
    /// The LSM6DS temperature register: raw / 256 + 25 °C.
    pub struct Lsm6dsTemp { above: Celsius, offset: Offset::int(25) }
}

type Lsm = Point<Lsm6dsTemp, Fixed<Kelvin, Rational<1, 256>, i16>>;

assert_eq!(Lsm::from_raw(128).to_string(), "25.5 °C");
assert_eq!(Lsm::from_raw(64).to::<Celsius, Centi, i16>().map(|p| p.raw()), Some(2525));

above: Celsius takes the unit, the reference and the display of °C and adds the offset.
The conversion to 0.01 °C is \(\text{raw} \cdot 100 / 256 + 2500\), by constants, checked to be whole.
On the wire it is the register's i16, nothing else.

Bus formulas¶

CAN and J1939 signals are written the same way, an offset and a step:

use exact_units::{Fixed, Offset, Point, origin, origins::Celsius, prefix::One, units::Kelvin};

origin! {
    /// J1939 engine coolant temperature (SPN 110): 1 °C per bit, −40 °C offset.
    pub struct J1939Temp { above: Celsius, offset: Offset::int(-40) }
}

type Coolant = Point<J1939Temp, Fixed<Kelvin, One, u8>>;

assert_eq!(Coolant::from_raw(130).to::<Celsius, One, i16>().map(|p| p.raw()), Some(90));

Pressure against the ambient¶

A gauge pressure is measured against the air around it, which is known only at run time. Gauge and Vacuum have different references, so they don't convert at compile time; against takes the ambient:

use exact_units::{Fixed, Point, origins::{Gauge, Vacuum}, prefix::Hecto, units::Pascal};

let tyre = Point::<Gauge, Fixed<Pascal, Hecto, i32>>::from_raw(2200);
let ambient = Point::<Vacuum, Fixed<Pascal, Hecto, i32>>::from_raw(1013);
assert_eq!(tyre.against(ambient), Some(Point::from_raw(3213)));