void USART_Init(USART_TypeDef* USARTx, USART_InitTypeDef* USART_InitStruct)
{
  uint32_t tmpreg = 0x00, apbclock = 0x00;
  uint32_t integerdivider = 0x00;
  uint32_t fractionaldivider = 0x00;
  uint32_t usartxbase = 0;
  RCC_ClocksTypeDef RCC_ClocksStatus;
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_BAUDRATE(USART_InitStruct->USART_BaudRate));  
  assert_param(IS_USART_WORD_LENGTH(USART_InitStruct->USART_WordLength));
  assert_param(IS_USART_STOPBITS(USART_InitStruct->USART_StopBits));
  assert_param(IS_USART_PARITY(USART_InitStruct->USART_Parity));
  assert_param(IS_USART_MODE(USART_InitStruct->USART_Mode));
  assert_param(IS_USART_HARDWARE_FLOW_CONTROL(USART_InitStruct->USART_HardwareFlowControl));
  /* The hardware flow control is available only for USART1, USART2 and USART3 */
  if (USART_InitStruct->USART_HardwareFlowControl != USART_HardwareFlowControl_None)
  {
    assert_param(IS_USART_123_PERIPH(USARTx));
  }

  usartxbase = (uint32_t)USARTx;

/*---------------------------- USART CR2 Configuration -----------------------*/
  tmpreg = USARTx->CR2;
  /* Clear STOP[13:12] bits */
  tmpreg &= CR2_STOP_CLEAR_Mask;
  /* Configure the USART Stop Bits, Clock, CPOL, CPHA and LastBit ------------*/
  /* Set STOP[13:12] bits according to USART_StopBits value */
  tmpreg |= (uint32_t)USART_InitStruct->USART_StopBits;

  /* Write to USART CR2 */
  USARTx->CR2 = (uint16_t)tmpreg;

/*---------------------------- USART CR1 Configuration -----------------------*/
  tmpreg = USARTx->CR1;
  /* Clear M, PCE, PS, TE and RE bits */
  tmpreg &= CR1_CLEAR_Mask;
  /* Configure the USART Word Length, Parity and mode ----------------------- */
  /* Set the M bits according to USART_WordLength value */
  /* Set PCE and PS bits according to USART_Parity value */
  /* Set TE and RE bits according to USART_Mode value */
  tmpreg |= (uint32_t)USART_InitStruct->USART_WordLength | USART_InitStruct->USART_Parity |
            USART_InitStruct->USART_Mode;
  /* Write to USART CR1 */
  USARTx->CR1 = (uint16_t)tmpreg;

/*---------------------------- USART CR3 Configuration -----------------------*/  
  tmpreg = USARTx->CR3;
  /* Clear CTSE and RTSE bits */
  tmpreg &= CR3_CLEAR_Mask;
  /* Configure the USART HFC -------------------------------------------------*/
  /* Set CTSE and RTSE bits according to USART_HardwareFlowControl value */
  tmpreg |= USART_InitStruct->USART_HardwareFlowControl;
  /* Write to USART CR3 */
  USARTx->CR3 = (uint16_t)tmpreg;

/*---------------------------- USART BRR Configuration -----------------------*/
  /* Configure the USART Baud Rate -------------------------------------------*/
  RCC_GetClocksFreq(&RCC_ClocksStatus);
  if (usartxbase == USART1_BASE)
  {
    apbclock = RCC_ClocksStatus.PCLK2_Frequency;
  }
  else
  {
    apbclock = RCC_ClocksStatus.PCLK1_Frequency;
  }

  /* Determine the integer part */
if ((USARTx->CR1 & CR1_OVER8_Set) != 0)   //这一句为什么是USARTx->CR1和CR1_OVER8_Set相与？CR1寄存器不是只有最低位0~13位有效吗，而且CR1_OVER8_Set宏定义的值是(u16)0x8000，这个值的0~13位全部是0啊，不管CR1寄存器的值是多少，这个if语句是恒成立的啊，还有必要判断吗？这句要是我理解有问题，请问这句判断的是什么？还有后面的if语句好像都是这句语句，有点迷糊了，新手初入，还请高手不吝指教，谢谢！！！
  {
    /* Integer part computing in case Oversampling mode is 8 Samples */
    integerdivider = ((25 * apbclock) / (2 * (USART_InitStruct->USART_BaudRate)));   
  }
  else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */
  {
    /* Integer part computing in case Oversampling mode is 16 Samples */
    integerdivider = ((25 * apbclock) / (4 * (USART_InitStruct->USART_BaudRate)));   
  }
  tmpreg = (integerdivider / 100) << 4;

  /* Determine the fractional part */
  fractionaldivider = integerdivider - (100 * (tmpreg >> 4));

  /* Implement the fractional part in the register */
  if ((USARTx->CR1 & CR1_OVER8_Set) != 0)
  {
    tmpreg |= ((((fractionaldivider * 8) + 50) / 100)) & ((uint8_t)0x07);
  }
  else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */
  {
    tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);
  }

  /* Write to USART BRR */
  USARTx->BRR = (uint16_t)tmpreg;
}