stm32的可编程电压检测PVD

配置PVD的顺序如：

注意一般使能PVD在系统初始化完毕开启。

/**
  * @brief  Configures EXTI Lines.
  * @param  None
  * @retval None
  */
static void EXTI_Configuration(void)
{
  EXTI_InitTypeDef EXTI_InitStructure;

  /* Configure EXTI Line16(PVD Output) to generate an interrupt on rising and
     falling edges */
  EXTI_ClearITPendingBit(EXTI_Line16);
  EXTI_InitStructure.EXTI_Line = EXTI_Line16;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising_Falling;
  EXTI_InitStructure.EXTI_LineCmd = ENABLE;
  EXTI_Init(&EXTI_InitStructure);

}

/**
  * @brief  Configures NVIC and Vector Table base location.
  * @param  None
  * @retval None
  */
static void NVIC_Configuration(void)
{
  NVIC_InitTypeDef NVIC_InitStructure;
  
  /* Configure one bit for preemption priority */
  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1);
  
  /* Enable the PVD Interrupt */
  NVIC_InitStructure.NVIC_IRQChannel = PVD_I