STM32F407I关于定时器方面的

开发板是Open407I的。我在使用定时器2的时候居然什么都没有，定时器完全没有运行，我只是想用这个定时器做一个一毫秒的中断来测试一下。AHB1的时钟是168M，APB1时钟是42M。
下面是代码：
void STM_tim2Configuration(void)
{
        TIM_TimeBaseInitTypeDef TimeBaseInitTypeDef;
        Timx_cnt = 0;

        RCC_APB1PeriphResetCmd(RCC_APB1Periph_TIM2, ENABLE);
        TIM_DeInit(TIM2);

        TimeBaseInitTypeDef.TIM_Period = 10;
        TimeBaseInitTypeDef.TIM_Prescaler = (4200-1);
        TimeBaseInitTypeDef.TIM_ClockDivision = TIM_CKD_DIV1;
        TimeBaseInitTypeDef.TIM_CounterMode = TIM_CounterMode_Up;
        TIM_TimeBaseInit(TIM2,&TimeBaseInitTypeDef);

        TIM_ClearFlag(TIM2, TIM_FLAG_Update);
        TIM_ITConfig(TIM2, TIM_FLAG_Update, ENABLE);

        TIM_Cmd(TIM2,ENABLE);
}


void STM_Tim2NVICConfig(void)
{
        NVIC_InitTypeDef NVIC_InitStructure;

        NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;
        NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
        NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;
        NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;

        NVIC_Init(&NVIC_InitStructure);
}