基于51单片机的Keil里面的PWM当作频率计程序，需要比较器芯片，一端接被测，一端接PWM输出，比较器输出接单片机输入。-eil PWM based on 51 single-chip microcomputer as a frequency meter program and Need a comparator chip, one end being measured, end PWM output, the input comparator output