void TIM3_ETR_GetDropCounts_Configuration(void) 
{ 
GPIO_InitTypeDef GPIO_InitStructure; 
TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure; 

//Config PD2 为浮空输入 
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2; 
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz; 
GPIO_Init(GPIOD, &GPIO_InitStructure); 

//GPIO_PinAFConfig(GPIOD,GPIO_PinSource2,GPIO_AF_TIM3);


TIM_TimeBaseStructure.TIM_Prescaler = 0x00; 
TIM_TimeBaseStructure.TIM_Period = 0xFFFF; 
TIM_TimeBaseStructure.TIM_ClockDivision = 0x0; 
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up; 
TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);  // Time base configuration 
   
TIM_ETRClockMode2Config(TIM3, TIM_ExtTRGPSC_OFF, TIM_ExtTRGPolarity_NonInverted, 0);   
TIM_SetCounter(TIM3, 0);    
TIM_Cmd(TIM3, ENABLE); 
} 

主函数中，使用到
TIM_GetCounter(TIM3)来捕频率。没有反应。。。