void _delay

void _delay_us(double __us)

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

The maximal possible delay is 768μs/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 4294967.295μs/F_CPU in MHz. For values greater than the maximal possible delay, overflow results in no delay i.e., 0μs.

Conversion of __us into clock cycles may not always result in integer. By default, the clock cycles 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. Also, the backward compatible algorithm will be chosen if the code is compiled in a freestanding environment (GCC option -ffreestanding), as the math functions required for rounding are then not available to the compiler.