void TIM14_PWM_Init(void)//PWM配置
{         
GPIO_InitTypeDef GPIO_InitStructure;
TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
TIM_OCInitTypeDef  TIM_OCInitStructure;

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM14,ENABLE);   //
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOF, ENABLE);  //

GPIO_PinAFConfig(GPIOF,GPIO_PinSource9,GPIO_AF_TIM14); //

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9;           //GPIOF9
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;        //
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz; //
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;      //
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;        //
GPIO_Init(GPIOF,&GPIO_InitStructure);              //
   
TIM_TimeBaseStructure.TIM_Prescaler=83;  //
TIM_TimeBaseStructure.TIM_CounterMode=TIM_CounterMode_Up; //
TIM_TimeBaseStructure.TIM_Period=499;   //
TIM_TimeBaseStructure.TIM_ClockDivision=TIM_CKD_DIV1;

TIM_TimeBaseInit(TIM14,&TIM_TimeBaseStructure);//

//
TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1; //
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable; //
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_Low; //
TIM_OC1Init(TIM14, &TIM_OCInitStructure);  //
TIM_OC1PreloadConfig(TIM14, TIM_OCPreload_Enable);  //
  TIM_ARRPreloadConfig(TIM14,ENABLE);//
TIM_Cmd(TIM14, ENABLE);  //
}  
void TIM3_Int_Init(void)//定时器配置
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStructure;
NVIC_InitTypeDef NVIC_InitStructure;

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3,ENABLE);  

  TIM_TimeBaseInitStructure.TIM_Period = 99;  //
TIM_TimeBaseInitStructure.TIM_Prescaler=8399;  //
TIM_TimeBaseInitStructure.TIM_CounterMode=TIM_CounterMode_Up; //
TIM_TimeBaseInitStructure.TIM_ClockDivision=TIM_CKD_DIV1;

TIM_TimeBaseInit(TIM3,&TIM_TimeBaseInitStructure);//

TIM_ITConfig(TIM3,TIM_IT_Update,ENABLE); //
TIM_Cmd(TIM3,ENABLE); //

NVIC_InitStructure.NVIC_IRQChannel=TIM3_IRQn; //
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=0x01; //
NVIC_InitStructure.NVIC_IRQChannelSubPriority=0x03; //
NVIC_InitStructure.NVIC_IRQChannelCmd=ENABLE;
NVIC_Init(&NVIC_InitStructure);

}

u16 led0pwmval=0;   
  u8 dir=1;
void TIM3_IRQHandler(void)//定时器中断
{
if(TIM_GetITStatus(TIM3,TIM_IT_Update)==SET) //
{
    if(dir)led0pwmval++;//
  else led0pwmval--; //
   if(led0pwmval>499)dir=0;//
  if(led0pwmval==0)dir=1; //

  TIM_SetCompare1(TIM14,led0pwmval); //修改占空比的值，达到499重新从0开始计数
}
TIM_ClearITPendingBit(TIM3,TIM_IT_Update);  //
}

问题在于对应的的LED灯亮了，但是始终亮度不变，改变led0pwmval的初值，灯的亮度就会变化，这是什么原因？