求助定时器外部时钟源模式2

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GPIO_InitTypeDef GPIO_InitStructure;
        TIM_TimeBaseInitTypeDef TIM2_TimeBaseStructure;
        NVIC_InitTypeDef NVIC_InitStructure;
       
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);         //使能PA
        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE); //时钟使能
       
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;                               
        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;                          
        GPIO_Init(GPIOA, &GPIO_InitStructure);                                         
       
        TIM_DeInit(TIM2);
        TIM2_TimeBaseStructure.TIM_Prescaler = 0;
        TIM2_TimeBaseStructure.TIM_Period = 0xFFFF;
        TIM2_TimeBaseStructure.TIM_ClockDivision = 0;
        TIM2_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
        TIM_TimeBaseInit(TIM2, &TIM2_TimeBaseStructure);

               
        TIM_ITConfig(TIM2,TIM_IT_Update ,ENABLE);
        TIM_ETRClockMode2Config(TIM2, TIM_ExtTRGPSC_OFF, TIM_ExtTRGPolarity_NonInverted, 0);
        TIM_SetCounter(TIM2, 0);  //设置计数器为0
       
        NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;  //TIM2中断
        NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority =1;  //先占优先级2级
        NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;  //从优先级0级
        NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //IRQ通道被使能
        NVIC_Init(&NVIC_InitStructure);  //根据NVIC_InitStruct中指定的参数初始化外设NVIC寄存器
       
        TIM_Cmd(TIM2, ENABLE);



我想知道为什么要配置TIM2_TimeBaseStructure.TIM_ClockDivision = 0;和        TIM2_TimeBaseStructure.TIM_Prescaler = 0;
外部时钟的话不应该是外部来上升沿计数器加一，和定时器内部时钟的的分频滤波有关系吗？参考手册上也没配置啊

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