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

Macros
#define	F_CPU   1000000UL

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

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).

Macro Definition Documentation

F_CPU

#define F_CPU   1000000UL

CPU frequency in Hz.

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.

Function Documentation

_delay_ms()

void _delay_ms ( double  __ms )

inlinestatic

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.

If the avr-gcc toolchain has __builtin_avr_delay_cycles() support, maximal possible delay is 4294967.295 ms/ F_CPU in MHz. For values greater than the maximal possible delay, overflow may result in no delay i.e., 0 ms.

Conversion of __ms into clock cycles may not always result in an integral value. By default, the clock cycles are rounded up to the next integer. This ensures that the user gets at least __ms microseconds of delay.

Alternatively, by defining the macro __DELAY_ROUND_DOWN__, or __DELAY_ROUND_CLOSEST__, before including this header file, the algorithm can be made to round down, or round to closest integer, respectively.

Note

The implementation of _delay_ms() based on __builtin_avr_delay_cycles() is not backward compatible with older implementations. In order to get functionality backward compatible with previous versions, the macro __DELAY_BACKWARD_COMPATIBLE__ must be defined before including this header file.

_delay_us()

void _delay_us ( double  __us )

inlinestatic

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 μs / 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.

If the avr-gcc toolchain has __builtin_avr_delay_cycles() support, maximal possible delay is 4294967.295 μs/ F_CPU in MHz. For values greater than the maximal possible delay, overflow may result in no delay i.e., 0 μs.

Conversion of __us into clock cycles may not always result in integer. By default, the clock cycles are rounded up to next integer. This ensures that the user gets at least __us microseconds of delay.

Alternatively, by defining the macro __DELAY_ROUND_DOWN__, or __DELAY_ROUND_CLOSEST__, before including this header file, the algorithm can be made to round down, or round to closest integer, respectively.

Note

The implementation of _delay_us() based on __builtin_avr_delay_cycles() is not backward compatible with older implementations. In order to get functionality backward compatible with previous versions, the macro __DELAY_BACKWARD_COMPATIBLE__ must be defined before including this header file.