本代码是以stm32f407本身自带的dac为硬件基础，可以产生出正弦波，矩形波，锯齿波，三角波，噪声，并且频率和幅值可以改变。(This code is stm32f407 itself comes dac-based hardware, you can produce sine, square wave, sawtooth, triangle, noise, and frequency and amplitude can be changed.)