minus-one

let max_byte: u8 = minus_one!(256);
assert_eq!(max_byte, 255);

let min_byte: u8 = minus_one!(1);
assert_eq!(min_byte, 0);

Often in firmware-land, you are using a register where a register value of 0 corresponds to a value of 1. This is a common pattern in hardware. However, this can be a headache when you are trying to write a value to the register, and you have to remember to subtract 1 from the value you want to write. In addition, sometimes it is not even possible to subtract 1 because the value you want to write is the maximum value.

Suppose you have a u8 and you want to write something which represents scalar 256. You cannot subtract 1 from 256 because 255 is the maximum value of an u8. The minus_one! macro allows you to write minus_one!(256) and get 255 back, and likewise minus_one!(1) will return 0.