说明：  这是一个基于VHDL的等精度频率计的程序，计完数之后采用四位数码管显示，当频率上K之后，自动切换量程，很实用。
(This is a VHDL-based, such as the frequency accuracy of the procedure, including the use of four after the End of number of digital display, when the frequency K, the automatic switching range, it is practical.)