不用配置引脚为输入模式,TIM_TIxExternalCLK1Source_TI2为PB7引脚
void TIM4_Init(void)
{
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;    
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE);    
    TIM_TimeBaseStructure.TIM_Period = 0xffff;
    TIM_TimeBaseStructure.TIM_Prescaler = 0;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_CounterMode =
    TIM_CounterMode_Up;
    TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);
 
    TIM_TIxExternalClockConfig(TIM4,TIM_TIxExternalCLK1Source_TI2,TIM_ICPolarity_Rising,0);
    TIM_SetCounter(TIM4, 0);    
    TIM_ClearITPendingBit(TIM4, TIM_IT_Update); 
    //打开溢出中断
    // TIM_ITConfig(TIM4, TIM_IT_Update, ENABLE);    
    TIM_Cmd(TIM4, ENABLE);  //计数器使能,开始工作   
    //NVIC_TIM4Configuration();
}