利用stm32的TIM3产生PWM，可以调节占空比，通过寄存器配置的-Generate PWM, the duty cycle can be adjusted

STM32 的定时器能功，可以用来作为信号发生器和频率计，一方面来测频，一方面来发生频率可变，占空比可变的信号发生器(STM32 timer can work, can be used as a signal generator and frequency meter, on the one hand to measure frequency, on the one hand to variable frequency, duty cycle variable signal generator)