我用外部计数STM32 ETR计算频率，测量100K频率，用定时器2定时1S，定时器三PD2（TIM3_ETR）计数，计算公式f=n/t;测量出来的结果是9999 HZ，整整少了一位数字，这是什么原因呢，大家分析一下，谢谢，附我的程序
void Time3_Configuration()
{  
   TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
   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_ITConfig(TIM3, TIM_IT_Update, ENABLE);  
   TIM_SetCounter(TIM3, 0);   
   TIM_Cmd(TIM3, ENABLE);
}
         /*1S定时*/
void Tim2_Config(void)
{
    TIM_TimeBaseInitTypeDef   TIM2_TimeBaseStructure;
    NVIC_InitTypeDef NVIC_InitStructure;  
    TIM_DeInit(TIM2);
    TIM2_TimeBaseStructure.TIM_Period =999;
    TIM2_TimeBaseStructure.TIM_Prescaler = (7200-1);
    TIM2_TimeBaseStructure.TIM_ClockDivision = 0x0;//ÉèÖÃʱÖÓϵÊý ²»·ÖƵ
  TIM2_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;//ÏòÉϼÆÊýģʽ
    TIM_TimeBaseInit(TIM2, &TIM2_TimeBaseStructure); // Time base configuration
    TIM_ClearFlag(TIM2,TIM_FLAG_Update);//Çå³þ¸üбê־λ
  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);               
}
最后的计算部分：
void TIM2_IRQHandler(void)   
{
        if (TIM_GetITStatus(TIM2, TIM_IT_Update) != RESET)
          {
             TIM_ClearITPendingBit(TIM2, TIM_IT_Update);
             CAPTURE1=(u32)TIM_GetCounter(TIM3);        
      }
        sum+=CAPTURE1;
        count++;
        if(count==4)
        {
         Frequency1=sum/4;//求频率公式F=n/t;这里求了4次的计数平均值，结果是1S的频率
       sum=0;
         count=0;
              }
        TIM_SetCounter(TIM3,0);
}
此帖出自小平头技术问答