According to a post on Reddit, it looks like this crate doesn't get rebuilt when environment variables change. For example, changing the CPU frequency doesn't cause a rebuild of the consts in this crate.

There should be a build.rs added that prints out any environment variables that should cause a rebuild. For example:

build.rs:

fn main() {
    println!("cargo:rerun-if-env-changed=AVR_CPU_FREQUENCY_HZ");
}