我在使用ETR外部时钟信号时，想实现类似于外部中断的，信号来一次进入一次中断的操作。但是发现当TIM_Period参数给1-1，也就是0时，无法正常进入中断。但是将参数调整为2-1时，来两次信号又可以进入中断，请问这是为什么？uint16_t Count;
void TIM_QuicklyInit(void);
       
int main(void)
{
        TIM_QuicklyInit();
        OLED_Init();
        OLED_ShowString(1,1,"Counts:");
        while(1)
        {
                OLED_ShowNum(2,1,Count,4);
                OLED_ShowNum(3,1,TIM_GetCounter(TIM2),1);
        }

}

void TIM_QuicklyInit(void)
{
        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2,ENABLE);
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA,ENABLE);


       
        GPIO_InitTypeDef GPIOInitstructure;
        GPIOInitstructure.GPIO_Mode = GPIO_Mode_IPD;
        GPIOInitstructure.GPIO_Pin = GPIO_Pin_0;
        GPIOInitstructure.GPIO_Speed = GPIO_Speed_50MHz;
        GPIO_Init(GPIOA,&GPIOInitstructure);


       
        TIM_ETRClockMode2Config(TIM2,TIM_ExtTRGPSC_OFF,TIM_ExtTRGPolarity_Inverted,0x0F);


       
        TIM_TimeBaseInitTypeDef TimeInitstructure;
        TimeInitstructure.TIM_CounterMode = TIM_CounterMode_Up;
        TimeInitstructure.TIM_ClockDivision = TIM_CKD_DIV4;
        TimeInitstructure.TIM_Period = 1-1;
        TimeInitstructure.TIM_Prescaler = 1-1;
        TimeInitstructure.TIM_RepetitionCounter = 0;
        TIM_TimeBaseInit(TIM2,&TimeInitstructure);


       
        TIM_ClearFlag(TIM2,TIM_FLAG_Update);
       
        TIM_Cmd(TIM2,ENABLE);
       
        TIM_ITConfig(TIM2,TIM_IT_Update,ENABLE);
       
        NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);


        NVIC_InitTypeDef NVICInitstructure;
        NVICInitstructure.NVIC_IRQChannel = TIM2_IRQn ;
        NVICInitstructure.NVIC_IRQChannelCmd = ENABLE;
        NVICInitstructure.NVIC_IRQChannelPreemptionPriority = 2;
        NVICInitstructure.NVIC_IRQChannelSubPriority = 2;
        NVIC_Init(&NVICInitstructure);
       
}

void TIM2_IRQHandler(void)
{
        if(TIM_GetFlagStatus(TIM2,TIM_IT_Update)==SET)
                Count++;
        TIM_ClearFlag(TIM2,TIM_IT_Update);
}