说明：  利用8255产生波形发生器，共可实现矩形波，三角波，锯齿波，梯形波和正弦波五种波形，并且峰值可调
(Using 8255 to generate waveform generator, we can realize five kinds of waveforms: rectangular wave, triangular wave, sawtooth wave, trapezoidal wave and sine wave, and the peak value can be adjusted)