STM32F407中PWM捕获的问题

STM32F407VGT6  
TIM3_CH3(PB0)作为PWM输出 （此路正常工作）
TIM3_CH1(PB4)作为PWM输入捕获(此路工作不正常)
配置信息如下：GPIO_InitTypeDef GPIO_InitStructure;
TIM_ICInitTypeDef  TIM_ICInitStructure;
RCC_AHB1PeriphClockCmd( RCC_AHB1Periph_GPIOB, ENABLE);
GPIO_PinAFConfig(GPIOB,GPIO_PinSource4,GPIO_AF_TIM3);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
GPIO_Init(GPIOB, &GPIO_InitStructure);
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3,ENABLE);
TIM_ICInitStructure.TIM_Channel = TIM_Channel_1;     
TIM_ICInitStructure.TIM_ICPolarity =TIM_ICPolarity_Rising;
TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_div1;
  TIM_ICInitStructure.TIM_ICFilter = 0x0;
  TIM_PWMIConfig(TIM3, &TIM_ICInitStructure);
  TIM_SelectInputTrigger(TIM3, TIM_TS_TI1FP1);
  TIM_SelectSlaveMode(TIM3, TIM_SlaveMode_Reset);
  TIM_SelectMasterSlaveMode(TIM3, TIM_MasterSlaveMode_Enable);
  TIM_Cmd(TIM3, ENABLE);
运行中，只有CCR2中有正确的脉宽值，CCR1中应有的PWM周期值一直为0，不知道是什么原因！

配置错了~~~~~~~~~~~~~~~