Re: [avr-gcc-list] Using delay functions with variable F_CPU clock?

[David VanHorn]

F_CPU is always evaluated upon each invocation, so if you ever change
it within one compilation unit, the next invocation of one of these
macros will reflect this.

That's compile-time, he's looking for execution-time changes in processor speed.

He didn't explicitly say so, but I think he's using xdiv to scale the CPU clock.