频率计VHDL编程。设计一个4位数字显示的十进制频率计，其测量范围为1MHz，测量值通过4个数码管显示以8421BCD码形式输出，可通过开关实现量程控制，量程分10kHz、100kHz、1MHz三档(最大读数分别为9.999kHz、99.99kHz、999.9kHz)；

当输入信号的频率大于相应量程时，有溢出显示。

-Cymometer VHDL programming. Design of a 4-digit decimal display frequency, the measurement range of 1MHz, the measured value through the four LED 8421BCD code shows the form of output can be controlled through the switch range, range at 10kHz, 100kHz, 1MHz Three (maximum reading were 9.999kHz, 99.99kHz, 999.9kHz)　when the input signal is greater than the corresponding frequency range, it shows overflow.