引入了D 触发器的长帧同步时钟的产生,其是一个时钟分频的例子，特别提醒了如何在程序中引入触发器，适合初学者引用。-The introduction of the D flip-flop of long frame synchronization clock generation, it is an example of a clock divider, remind how the introduction of the program Trigger reference for begin