基于STM32F4的系统时钟SysTick的驱动配置-SysTick is based on the system clock drive configuration STM32F4

基于stm32f4的PWM模式输出：配置系统时钟，配置TIM2时钟输出，配置TIM2输出的时钟信号的模式（周期、极性、占空比）。-PWM mode based on stm32f4 output: configure the system clock, configuration TIM2 clock output, configure TIM2 output model of the clock signal (cycle, polarity, duty cycle).