解决STM32F4使用高级定时器（TIM1和TIM8）输出PWM的问题

STM32F4使用高级定时器（TIM1和TIM8）输出PWM时要使用TIM_CtrlPWMOutputs使能PWM输出，否则不会输出PWM波形，这一点是和通用定时器输出PWM不一样的地方，通用定时器是不用配置TIM_CtrlPWMOutputs函数的。、

//TIM1_PWM_Init(100-1,168-1);//168M/168=1Mhz的计数频率,重装载值100，所以PWM频率为 1M/100=10Khz. 
void TIM1_PWM_Init(u32 arr,u32 psc)
{                              
    //此部分需手动修改IO口设置
    
    GPIO_InitTypeDef GPIO_InitStructure;
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
    TIM_OCInitTypeDef  TIM_OCInitStructure;
    
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_TIM1,ENABLE);      //TIM1时钟使能    
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);     //使能PORTF时钟    
    
    GPIO_PinAFConfig(GPIOA,GPIO_PinSource11,GPIO_AF_TIM1); //GPIOA11复用为定时器1
    
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;           //GPIOA11
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;        //复用功能
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;    //速度100MHz
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;      //推挽复用输出
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;        //上拉
    GPIO_Init(GPIOA,&GPIO_InitStructure);              //初始化GPIOA11
      
    TIM_TimeBaseStructure.TIM_Prescaler=psc;  //定时器分频
    TIM_TimeBaseStructure.TIM_CounterMode=TIM_CounterMode_Up; //向上计数模式
    TIM_TimeBaseStructure.TIM_Period=arr;   //自动重装载值
    TIM_TimeBaseStructure.TIM_ClockDivision=TIM_CKD_DIV1; 
    TIM_TimeBaseStructure.TIM_RepetitionCounter=0;  //20201020
    TIM_TimeBaseInit(TIM1,&TIM_TimeBaseStructure);//初始化定时器14
    
    //初始化TIM14 Channel1 PWM模式     
    TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1; //选择定时器模式:TIM脉冲宽度调制模式2
     TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable; //比较输出使能
    TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_Low; //输出极性:TIM输出比较极性低
    TIM_OC4Init(TIM1, &TIM_OCInitStructure);  //根据T指定的参数初始化外设TIM1 4OC1

    TIM_OC4PreloadConfig(TIM1, TIM_OCPreload_Enable);  //使能TIM1在CCR1上的预装载寄存器
 
    TIM_ARRPreloadConfig(TIM1,ENABLE);//ARPE使能 
    
    TIM_Cmd(TIM1, ENABLE);  //使能TIM1
    
     TIM_CtrlPWMOutputs(TIM1, ENABLE);//使能TIM1的PWM输出，TIM1与TIM8有效,如果没有这行会问题

                                          
}