STM32f103 TIM3输出比较pwm频率达不到500Khz

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当get_f函数输入参数f2 = 50000,d2 = 10 时不能输出500Khz PWM，小点 d2=3 时150Khz可以输出，这是怎么回事呢？

void get_f(u32 f1,u32 f2)
{
        CCR1_Val = 4000000/(2*f1*d1);
        CCR2_Val = 4000000/(2*f2*d2);
}



static void tim3_gpio_config(void)
{
        GPIO_InitTypeDef GPIO_InitStruct;
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA|RCC_APB2Periph_AFIO,ENABLE);
        GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF_PP;
        GPIO_InitStruct.GPIO_Pin = GPIO_Pin_6;
        GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
        GPIO_Init(GPIOA,&GPIO_InitStruct);
        GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF_PP;
        GPIO_InitStruct.GPIO_Pin = GPIO_Pin_7;
        GPIO_Init(GPIOA,&GPIO_InitStruct);
}

static void tim3_config(void)
{
        TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
        TIM_OCInitTypeDef  TIM_OCInitStructure;
        NVIC_InitTypeDef NVIC_InitStructure;
        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3,ENABLE);
        TIM_TimeBaseStructure.TIM_Period = 0xFFFF;
  TIM_TimeBaseStructure.TIM_Prescaler = 17;
  TIM_TimeBaseStructure.TIM_ClockDivision =TIM_CKD_DIV1;
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);
       
        TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Toggle;
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = 0;
  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;

  TIM_OC1Init(TIM3, &TIM_OCInitStructure);

  TIM_OC1PreloadConfig(TIM3, TIM_OCPreload_Disable);

  /* Output Compare Timing Mode configuration: Channel2 */
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = 0;

  TIM_OC2Init(TIM3, &TIM_OCInitStructure);

  TIM_OC2PreloadConfig(TIM3, TIM_OCPreload_Disable);
       
       
          

  /* Enable the TIM2 global Interrupt */
        NVIC_PriorityGroupConfig(NVIC_PriorityGroup_0);
  NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;

  NVIC_Init(&NVIC_InitStructure);
       
        TIM_ITConfig(TIM3, TIM_IT_CC1 | TIM_IT_CC2 , ENABLE);
        TIM_ClearITPendingBit(TIM3, TIM_IT_CC1);
        TIM_ClearITPendingBit(TIM3, TIM_IT_CC2);
        TIM_Cmd(TIM3, ENABLE);
}

void TIM3_IRQHandler(void)
{
        if (TIM_GetITStatus(TIM3, TIM_IT_CC1) != RESET)
  {
                TIM_ClearITPendingBit(TIM3, TIM_IT_CC1);
    TIM_SetCompare1(TIM3, TIM_GetCapture1(TIM3) + CCR1_Val);
  }
        if (TIM_GetITStatus(TIM3, TIM_IT_CC2) != RESET)
  {
                TIM_ClearITPendingBit(TIM3, TIM_IT_CC2);
    TIM_SetCompare2(TIM3, TIM_GetCapture2(TIM3) + CCR2_Val);
  }
}