STM32 使用TIM3定时器产生一个占空比可调的PWM波，并用该PWM波驱动一个发光二极管，使二极管由亮到暗交替变化-STM32 use TIM3 timer to generate a variable duty cycle PWM wave, and the PWM wave drive with a light-emitting diode, the diode changes from bright to dark turn