_delay((unsigned long)((x)*(_XTAL_FREQ/4000000.0)))

回答: _delay((unsigned long)((x)*(_XTAL_FREQ/4000000.0)))是一个用于延时的宏定义。它的作用是根据给定的延时时间x和系统时钟频率_XTAL_FREQ来计算延时的周期数，并进行延时操作。具体的计算公式是将延时时间x乘以系统时钟频率_XTAL_FREQ除以4000000.0。这个宏定义通常用于嵌入式系统中，用于控制延时的精度和时间。\[1\]
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