设计一个用等精度测频原理的频率计。 频率测量范围1~9999； 其精度为 ； 用4位带小数点数码管显示其频率； 并且具有超量程、欠量程提示功能； -Such as the design of a precision frequency measurement using the principle of the frequency meter. Frequency measuring range 1 ~ 9999 its accuracy 4 with a dec