void TIM3_PWM_Init(u16 arr,u16 psc)
{  
        GPIO_InitTypeDef GPIO_InitStructure;
        TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
        TIM_OCInitTypeDef  TIM_OCInitStructure;
        NVIC_InitTypeDef NVIC_InitStructure;
       

        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);        //使能定时器3时钟
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB  | RCC_APB2Periph_AFIO, ENABLE);  //使能GPIO外设和AFIO复用功能模块时钟
       
        GPIO_PinRemapConfig(GPIO_PartialRemap_TIM3, ENABLE); //Timer3部分重映射  TIM3_CH2->PB5   

   //设置该引脚为复用输出功能,输出TIM3 CH2的PWM脉冲波形        GPIOB.5
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5; //TIM_CH2
        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;  //复用推挽输出
        GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
        GPIO_Init(GPIOB, &GPIO_InitStructure);//初始化GPIO

   //初始化TIM3
        TIM_TimeBaseStructure.TIM_Period = arr; //设置在下一个更新事件装入活动的自动重装载寄存器周期的值
        TIM_TimeBaseStructure.TIM_Prescaler =psc; //设置用来作为TIMx时钟频率除数的预分频值
        TIM_TimeBaseStructure.TIM_ClockDivision = 0; //设置时钟分割:TDTS = Tck_tim
        TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //TIM向上计数模式
        TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure); //根据TIM_TimeBaseInitStruct中指定的参数初始化TIMx的时间基数单位
       
        //初始化TIM3 Channel2 PWM模式       
     TIM_OCInitStructure.TIM_Pulse = 500;       
        TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Toggle; //比较输出翻转
        TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable; //比较输出使能
        TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High; //输出极性:TIM输出比较极性高
        TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Set;
     TIM_OCInitStructure.TIM_OCNIdleState = TIM_OCIdleState_Reset;
        TIM_OC2Init(TIM3, &TIM_OCInitStructure);  //根据T指定的参数初始化外设TIM3 OC2
       
        TIM_ITConfig(TIM3,TIM_IT_CC2,ENABLE ); //使能指定的TIM3中断,允许更新中断
        TIM_OC2PreloadConfig(TIM3, TIM_OCPreload_Enable);  //使能TIM3在CCR2上的预装载寄存器

        NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;  //TIM3中断
        NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;  //先占优先级0级
        NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;  //从优先级3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(TIM3, ENABLE);  //使能TIM3
       

}

void TIM3_IRQHandler(void)   //TIM3中断
{
        if (TIM_GetITStatus(TIM3, TIM_IT_CC2) != RESET) //检查指定的TIM中断发生与否:TIM 中断源
                {
                TIM_SetCompare2(TIM3,(TIM3->CNT+5000)%0XFFFF);
                TIM_ClearITPendingBit(TIM3, TIM_IT_CC2);  //清除TIMx的中断待处理位:TIM 中断源
                }
}

程序如上，为什么翻转的时间一直是0XFFFF*0.00001S
求大神解答啊