2 <util/delay.h>: Convenience functions for busy-wait delay loops
Note:
The functions in this header file are wrappers around the basic busy-wait functions from <util/delay_basic.h>. They are meant as convenience functions where actual time values can be specified rather than a number of cycles to wait for. The idea behind is that compile-time constant expressions will be eliminated by compiler optimization so floating-point expressions can be used to calculate the number of delay cycles needed based on the CPU frequency passed by the macro F_CPU.As an alternative method, it is possible to pass the F_CPU macro down to the compiler from the Makefile. Obviously, in that case, no #define statement should be used.
Note:
The functions available allow the specification of microsecond, and millisecond delays directly, using the application-supplied macro F_CPU as the CPU clock frequency (in Hertz). In order for these functions to work as intended, compiler optimizations must be enabled, and the delay time must be an expression that is a known constant at compile-time. If these requirements are not met, the resulting delay will be much longer (and basically unpredictable), and applications that otherwise do not use floating-point calculations will experience severe code bloat by the floating-point library routines linked into the application.