<util/delay.h>: Convenience functions for busy-wait delay loops(3)

NAME
<util/delay.h>: Convenience functions for busy-wait delay loops -

Detailed Description

    #define F_CPU 1000000UL  // 1 MHz
    //#define F_CPU 14.7456E6
    #include <util/delay.h>

Note:
    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.

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.

Note:
    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.

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).

  Functions
void _delay_us (double __us)
void _delay_ms (double __ms)

Function Documentation

  void _delay_ms (double __ms)

Perform a delay of __ms milliseconds, using _delay_loop_2().

The macro F_CPU is supposed to be defined to a constant defining the CPU clock frequency (in Hertz).

The maximal possible delay is 262.14 ms / F_CPU in MHz.

When the user request delay which exceed the maximum possible one, _delay_ms() provides a decreased resolution functionality. In this mode _delay_ms() will work with a resolution of 1/10 ms, providing delays up to 6.5535 seconds (independent from CPU frequency). The user will not be informed about decreased resolution.

  void _delay_us (double __us)

Perform a delay of __us microseconds, using _delay_loop_1().

The macro F_CPU is supposed to be defined to a constant defining the CPU clock frequency (in Hertz).

The maximal possible delay is 768 us / F_CPU in MHz.

If the user requests a delay greater than the maximal possible one, _delay_us() will automatically call _delay_ms() instead. The user will not be informed about this case.