我为什么用输入捕获检测400hz的信号检测不出来？

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TIM_ICInitTypeDef  TIM4_ICInitStructure;
void TIM4_Cap_Init(u16 arr,u16 psc)
{  
GPIO_InitTypeDef GPIO_InitStructure;
TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
  NVIC_InitTypeDef NVIC_InitStructure;
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE); //ʹÄÜTIM4ʱÖÓ
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);  //ʹÄÜGPIOBʱÖÓ

GPIO_InitStructure.GPIO_Pin  = GPIO_Pin_6;  //PB6 Çå³ý֮ǰÉèÖà 
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPD; //PB6 ÊäÈë  
GPIO_Init(GPIOB, &GPIO_InitStructure);
GPIO_ResetBits(GPIOB,GPIO_Pin_6);       //PB6 ÏÂÀ­

//³õʼ»¯¶¨Ê±Æ÷4 TIM4
TIM_TimeBaseStructure.TIM_Period = arr; //É趨¼ÆÊýÆ÷×Ô¶¯ÖØ×°Öµ
TIM_TimeBaseStructure.TIM_Prescaler =psc;  //Ô¤·ÖƵÆ÷   
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1; //ÉèÖÃʱÖÓ·Ö¸î:TDTS = Tck_tim
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //TIMÏòÉϼÆÊýģʽ
TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure); //¸ù¾ÝTIM_TimeBaseInitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯TIMxµÄʱ¼ä»ùÊýµ¥Î»
  
//³õʼ»¯TIM2ÊäÈ벶»ñ²ÎÊý
TIM4_ICInitStructure.TIM_Channel = TIM_Channel_1; //CC1S=01  Ñ¡ÔñÊäÈë¶Ë IC1Ó³Éäµ½TI1ÉÏ
   TIM4_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Falling; //ÉÏÉýÑز¶»ñ
   TIM4_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI; //Ó³Éäµ½TI1ÉÏ
   TIM4_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;  //ÅäÖÃÊäÈë·ÖƵ,²»·ÖƵ
   TIM4_ICInitStructure.TIM_ICFilter = 0x00;//IC1F=0000 ÅäÖÃÊäÈëÂ˲¨Æ÷ ²»Â˲¨
   TIM_ICInit(TIM4, &TIM4_ICInitStructure);

//ÖжϷÖ×é³õʼ»¯
NVIC_InitStructure.NVIC_IRQChannel = TIM4_IRQn;  //TIM2ÖжÏ
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 2;  //ÏÈÕ¼ÓÅÏȼ¶2¼¶
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;  //´ÓÓÅÏȼ¶0¼¶
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //IRQͨµÀ±»Ê¹ÄÜ
NVIC_Init(&NVIC_InitStructure);  //¸ù¾ÝNVIC_InitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯ÍâÉèNVIC¼Ä´æÆ÷

TIM_ITConfig(TIM4,TIM_IT_Update|TIM_IT_CC1,ENABLE);//ÔÊÐí¸üÐÂÖÐ¶Ï ,ÔÊÐíCC1IE²¶»ñÖжÏ

  TIM_Cmd(TIM4,ENABLE );  //ʹÄܶ¨Ê±Æ÷2

}

u8  TIM4CH1_CAPTURE_STA=0; //ÊäÈ벶»ñ״̬         
u16 TIM4CH1_CAPTURE_VAL; //ÊäÈ벶»ñÖµ

//¶¨Ê±Æ÷5ÖжϷþÎñ³ÌÐò  
void TIM4_IRQHandler(void)
{
  if((TIM4CH1_CAPTURE_STA&0X80)==0)//»¹Î´³É¹¦²¶»ñ
{   
  if (TIM_GetITStatus(TIM4, TIM_IT_Update) != RESET)//·¢ÉúÖжÏ
   
  {     
   if(TIM4CH1_CAPTURE_STA&0X40)//ÒѾ­²¶»ñµ½¸ßµçƽÁË
   {
    if((TIM4CH1_CAPTURE_STA&0X3F)==0X3F)//¸ßµçƽ̫³¤ÁË
    {
     TIM4CH1_CAPTURE_STA|=0X80;//±ê¼Ç³É¹¦²¶»ñÁËÒ»´Î
     TIM4CH1_CAPTURE_VAL=0XFFFF;
    }else TIM4CH1_CAPTURE_STA++;
   }  
  }
if (TIM_GetITStatus(TIM4, TIM_IT_CC1) != RESET)//²¶»ñ1·¢Éú²¶»ñʼþ
  {
   if(TIM4CH1_CAPTURE_STA&0X40)  //²¶»ñµ½Ò»¸öϽµÑØ   
   {      
    TIM4CH1_CAPTURE_STA|=0X80;  //±ê¼Ç³É¹¦²¶»ñµ½Ò»´ÎÉÏÉýÑØ
    TIM4CH1_CAPTURE_VAL=TIM_GetCapture1(TIM4);
       TIM_OC1PolarityConfig(TIM4,TIM_ICPolarity_Rising); //CC1P=0 ÉèÖÃΪÉÏÉýÑز¶»ñ
   }
   else          //»¹Î´¿ªÊ¼,µÚÒ»´Î²¶»ñÉÏÉýÑØ
   {
    TIM4CH1_CAPTURE_STA=0;   //Çå¿Õ
    TIM4CH1_CAPTURE_VAL=0;
     TIM_SetCounter(TIM4,0);
    TIM4CH1_CAPTURE_STA|=0X40;  //±ê¼Ç²¶»ñµ½ÁËÉÏÉýÑØ
   }      
  }                     
  }

    TIM_ClearITPendingBit(TIM4, TIM_IT_CC1|TIM_IT_Update); //Çå³ýÖжϱê־λ

}