The macro F_CPU specifies the CPU frequency to be considered by the delay macros. This macro is normally supplied by the environment (e.g. from within a project header, or the project's Makefile). The value 1 MHz here is only provided as a "vanilla" fallback if no such user-provided definition could be found.

In terms of the delay functions, the CPU frequency can be given as a floating-point constant (e.g. 3.6864E6 for 3.6864 MHz). However, the macros in <util/setbaud.h> require it to be an integer value.