低频频率计，选用24MHz的晶体，主频可达2MHz。用T1产生100us的时标，T0作信号脉冲计数器。假设晶体频率没有误差，而且稳定不变（实际上可达万分之一）；被测信号是周期性矩形波（正负脉冲宽度都不能小于0.5us），频率小于1MHz，大于1Hz。要求测量时标1S，测量精度为0.1 -Low frequency, use 24MHz crystal, frequency up to 2MHz. Generate 100us with a time scale T1, T0 for the signal pulse counter. Assuming there is no crystal frequency error, and the stability constant (in fact up to parts per million)　the measured signal is periodic square wave (both positive and negative pulse width of not less than 0.5us), the frequency is less than 1MHz, greater than 1Hz. Required measurement time scale 1S, measurement accuracy of 0.1