stm32 PWM 输出PWM的时候会丢一下方波

//配置成PWM输出

void TIM3_Mode_Config(u16 f,u16 p)

{

   TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;//初始化TIM3的时间基数单位

   TIM_OCInitTypeDef  TIM_OCInitStructure;//初始化TIM3的外设

   NVIC_InitTypeDef NVIC_InitStructure;



  //打开TIM3的时钟

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE); 



  //TIM3的时间基数单位设置(如计数终止值：999,从0开始 ；计数方式：向上计数）         

  TIM_TimeBaseStructure.TIM_Prescaler= (48 - 1); //时钟预分频数,时钟频率=72MHZ/(时钟预分频+1) 之后是12M    

  TIM_TimeBaseStructure.TIM_Period=STARTSPEED-1; //自动重装载寄存器的值/1ms 12KHZ         

  TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1 ;        

  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up; 

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);



  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1); //优先级分组（0~4）, 抢占式为0，没嵌套

  NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn; //制定IRQ通道

  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;        //先占优先级

  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 2;//从优先级 

  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

  NVIC_Init(&NVIC_InitStructure);

  

  //TIM3的外设的设置

  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;        //TIM脉冲宽度调制模式1    

  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;//这个暂时不知道，stm32固件库里没有搜到。应该是定时器输出声明使能的意思        

  TIM_OCInitStructure.TIM_Pulse = STARTSPEED/2;//设置了待装入捕获比较寄存器的脉冲值          

  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_Low; //TIM输出比较极性高

  TIM_OC4Init(TIM3, &TIM_OCInitStructure);

         

  TIM_OC4PreloadConfig(TIM3, TIM_OCPreload_Enable);//使能或者失能TIMx在CCR1上的预装载寄存器

  TIM_ARRPreloadConfig(TIM3, ENABLE);        //使能TIM3重载寄存器ARR  



  TIM_ClearFlag(TIM3, TIM_FLAG_Update); //清除溢出中断标志

  //TIM_ITConfig(TIM3,TIM_IT_Update,ENABLE);

  

  TIM_Cmd(TIM3, ENABLE);//使能TIM3     

  

}