说明：  通过单片机产生PWM，调节参数寄存器的值，产生不同占空比的PWM信号。若占空比的变化规律是正弦的，则滤波后可得到所需正弦波形。
(The PWM is generated by the microcontroller to adjust the value of the parameter register, and the PWM signals with different duty cycles are generated. If the duty cycle is sinusoidal, the sine waveform can be obtained after filtering.)