STM32 外部计数ETR计算频率，测量100K频率，用定时器2定时250ms，定时器3PD2（TIM3_ETR）计数，计算公式f=n*t;

void Tim3_Configuration(void)

{  

   TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;

   TIM_TimeBaseStructure.TIM_Prescaler = 0x00; 

   TIM_TimeBaseStructure.TIM_Period = 0xFFFF; 

   TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1; 

   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_ITConfig(TIM3, TIM_IT_Update, ENABLE);  

   TIM_SetCounter(TIM3, 0);    

   TIM_Cmd(TIM3, ENABLE); 

} 

void Tim2_Config(void)

{

    TIM_TimeBaseInitTypeDef   TIM2_TimeBaseStructure;

    NVIC_InitTypeDef NVIC_InitStructure;  

    TIM_DeInit(TIM2);

    TIM2_TimeBaseStructure.TIM_Period =（2500-1）;

    TIM2_TimeBaseStructure.TIM_Prescaler = (7200-1); 

    TIM2_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;

    TIM2_TimeBaseStructure.TIM_CounterMode =TIM_CounterMode_Up;

    TIM_TimeBaseInit(TIM2, &TIM2_TimeBaseStructure); // Time base configuration

    TIM_ClearFlag(TIM2,TIM_FLAG_Update);

    TIM_ITConfig(TIM2,TIM_IT_Update,ENABLE ); 

    TIM_Cmd(TIM2, ENABLE); 

    NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;

    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;

    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;

    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

    NVIC_Init(&NVIC_InitStructure);                

} 

void TIM2_IRQHandler(void)   

{

        if (TIM_GetITStatus(TIM2, TIM_IT_Update) != RESET) 

          {

             TIM_ClearITPendingBit(TIM2, TIM_IT_Update);

             CAPTURE=(u32)TIM_GetCounter(TIM3);        

          }

        sum+=CAPTURE;

        count++;

        if(count==4)

        {

         Frequency=sum*4;//求频率公式F=n*t;这里求了4次的计数值，合起来是1S的频率

       sum=0;

        count=0;

          }

        TIM_SetCounter(TIM3,0); 

}