实现正弦、三角、锯齿、方波等4种的频率和幅度可控输出波形，还可完成各种波形的线形叠加输出，注解详细，值得学习-Achieve sinusoidal, triangular, serrated, Fang Bo and other 4 kinds of frequency and amplitude controllable output waveform, but also to complete the linear superposition of various waveforms output, notes in detail, it is worth learning