stm32基于定时器对PWM频率进行修改，以此控制步进电机，做到步进电机速度可调，只做了加速减速没有做，原理相同，实测可用(STM32 based on the timer to modify the frequency of PWM, in order to control the stepper motor, so that stepper motor speed adjustable, only to do the acceleration deceleration, did not do the same principle, measured available)