说明：  应用AD7528设计实现波形发生器。能够应用按键控制输出方波、正弦波、三角波，并在液晶显示器上显示当前输出波形种类。
(Application Design and Implementation of AD7528 waveform generator. Button can be used to control the output square wave, sine wave, triangle wave, and liquid crystal display shows the current output waveform types.)