说明：  频率测量和周期测量的基本方法是采用以固定时钟作为参考时钟，分别测量单个周期的计数为周期，单位时间的计数为频率。但是由于被测信号的频率不同，测量精度会发生变化，采用低频测量周期，高频测量频率，然后分别求倒数，便可得到对应的频率和周期
(frequency measurement and measurement cycle is the basic method used to a fixed clock as a reference clock, measured single cycle to cycle counting, the counting unit time for the frequency. However, due to the frequency of the measured signal, the measurement accuracy will be any change, the use of low-frequency measurement cycle, high-frequency measurement frequency, Then for the countdown will be corresponding to the frequency and periodicity)