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();
}