在程序中加入定时器，在特定的地方调用TIM_Cmd(TIM3, ENABLE); 开启定时器，但是SR寄存器会置位产生中断标志，从而进入中断服务函数。单步调试发现，程序到TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);这一句的时候，SR的值会变成1，相当于产生中断！
void TIM3_Int_Init(u16 arr,u16 psc)
{
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
        NVIC_InitTypeDef NVIC_InitStructure;

        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);

        TIM_TimeBaseStructure.TIM_Period = arr;
        TIM_TimeBaseStructure.TIM_Prescaler =psc;
        TIM_TimeBaseStructure.TIM_ClockDivision = 0;
        TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  
        TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

        TIM_ITConfig(  TIM3, TIM_IT_Update ,DISABLE);
        NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;
        NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;  
        NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;  
        NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
        NVIC_Init(&NVIC_InitStructure);  

        TIM_Cmd(TIM3, DISABLE);
                                                         
}

void TIM3_IRQHandler(void)   //TIM3?D??
{
        if (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET)
                {
                TIM_ClearITPendingBit(TIM3, TIM_IT_Update  );
                 system_info = SYS_EVENT_ERR_SUCCESS;
                }
}

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