首先看看 异步通信需要定义的参数：

 再看看 此串口运行框图：

下面虚线方框的USART_BRR寄存器是设置USARTDIV分频系数的，可以看到fPCLKx(x=1,2)时钟先经过 /USARTDIV，再经过/（8x（2-OVER8）），由于OVER8一般设置为0，所以就是/8x2，即为16分频，最终发送器时钟和接收器时钟的频率（即为波特率）由下表计算：

再比如：

如果波特率是96000，那么USARTDIV=54.6875，

那么得到：

DIV_Fraction=16*0.6875=11(正好是11)=0x0B;

DIV_Mantissa=54=0x36;

下面看USART_Init函数

先上代码：

/**
  * @brief  Initializes the USARTx peripheral according to the specified
  *         parameters in the USART_InitStruct .
  * @param  USARTx: where x can be 1, 2, 3, 4, 5, 6, 7 or 8 to select the USART or 
  *         UART peripheral.
  * @param  USART_InitStruct: pointer to a USART_InitTypeDef structure that contains
  *         the configuration information for the specified USART peripheral.
  * @retval None
  */
void USART_Init(USART_TypeDef* USARTx, USART_InitTypeDef* USART_InitStruct)
{
  uint32_t tmpreg = 0x00, apbclock = 0x00;
  uint32_t integerdivider = 0x00;
  uint32_t fractionaldivider = 0x00;
  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, USART3 and USART6 */
  if (USART_InitStruct->USART_HardwareFlowControl != USART_HardwareFlowControl_None)
  {
    assert_param(IS_USART_1236_PERIPH(USARTx));
  }

/*---------------------------- USART CR2 Configuration -----------------------*/
  tmpreg = USARTx->CR2;

  /* Clear STOP[13:12] bits */
  tmpreg &= (uint32_t)~((uint32_t)USART_CR2_STOP);

  /* 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 &= (uint32_t)~((uint32_t)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 &= (uint32_t)~((uint32_t)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 ((USARTx == USART1) || (USARTx == USART6))
  {
    apbclock = RCC_ClocksStatus.PCLK2_Frequency;
  }
  else
  {
    apbclock = RCC_ClocksStatus.PCLK1_Frequency;
  }
  
  /* Determine the integer part */
  if ((USARTx->CR1 & USART_CR1_OVER8) != 0)
  {
    /* Integer part computing in case Oversampling mode is 8 Samples */
    integerdivider = ((25 * apbclock) / (2 * (USART_InitStruct->USART_BaudRate)));    
  }
  else /* if ((USARTx->CR1 & USART_CR1_OVER8) == 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 & USART_CR1_OVER8) != 0)
  {
    tmpreg |= ((((fractionaldivider * 8) + 50) / 100)) & ((uint8_t)0x07);
  }
  else /* if ((USARTx->CR1 & USART_CR1_OVER8) == 0) */
  {
    tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);
  }
  
  /* Write to USART BRR register */
  USARTx->BRR = (uint16_t)tmpreg;
}

实际上不需要完全理解这个函数是怎么写的，只需要会用，然后用熟了之后再考虑原理问题。

再分析入口参数：

其一：USART_TypeDef* USARTx

根据

 assert_param(IS_USART_ALL_PERIPH(USARTx));

#define IS_USART_ALL_PERIPH(PERIPH) (((PERIPH) == USART1) || \
                                     ((PERIPH) == USART2) || \
                                     ((PERIPH) == USART3) || \
                                     ((PERIPH) == UART4)  || \
                                     ((PERIPH) == UART5)  || \
                                     ((PERIPH) == USART6) || \
                                     ((PERIPH) == UART7)  || \
                                     ((PERIPH) == UART8))

#define USART1              ((USART_TypeDef *) USART1_BASE)
#define USART2              ((USART_TypeDef *) USART2_BASE)
#define USART3              ((USART_TypeDef *) USART3_BASE)
#define UART4               ((USART_TypeDef *) UART4_BASE)
#define UART5               ((USART_TypeDef *) UART5_BASE)
#define USART6              ((USART_TypeDef *) USART6_BASE)
#define UART7               ((USART_TypeDef *) UART7_BASE)
#define UART8               ((USART_TypeDef *) UART8_BASE)

可以看到确实是只定义了6个。：

其二：USART_InitTypeDef* USART_InitStruct

根据

typedef struct
{
  uint32_t USART_BaudRate;            /*!< This member configures the USART communication baud rate.
                                           The baud rate is computed using the following formula:
                                            - IntegerDivider = ((PCLKx) / (8 * (OVR8+1) * (USART_InitStruct->USART_BaudRate)))
                                            - FractionalDivider = ((IntegerDivider - ((u32) IntegerDivider)) * 8 * (OVR8+1)) + 0.5 
                                           Where OVR8 is the "oversampling by 8 mode" configuration bit in the CR1 register. */

  uint16_t USART_WordLength;          /*!< Specifies the number of data bits transmitted or received in a frame.
                                           This parameter can be a value of @ref USART_Word_Length */

  uint16_t USART_StopBits;            /*!< Specifies the number of stop bits transmitted.
                                           This parameter can be a value of @ref USART_Stop_Bits */

  uint16_t USART_Parity;              /*!< Specifies the parity mode.
                                           This parameter can be a value of @ref USART_Parity
                                           @note When parity is enabled, the computed parity is inserted
                                                 at the MSB position of the transmitted data (9th bit when
                                                 the word length is set to 9 data bits; 8th bit when the
                                                 word length is set to 8 data bits). */
 
  uint16_t USART_Mode;                /*!< Specifies wether the Receive or Transmit mode is enabled or disabled.
                                           This parameter can be a value of @ref USART_Mode */

  uint16_t USART_HardwareFlowControl; /*!< Specifies wether the hardware flow control mode is enabled
                                           or disabled.
                                           This parameter can be a value of @ref USART_Hardware_Flow_Control */
} USART_InitTypeDef;

逐个分析每个成员的作用：

USART_BaudRate：

BaudRate就是波特率，顾名思义就是配置波特率的。

USART_WordLength：

指定帧中传输或接收的数据位数，顾名思义就是字长。

USART_StopBits：

指定传输的停止位数。

USART_Parity：

Parity同等的，对等的，就是指定奇偶校验模式。

USART_Mode：

指定启用或禁用接收或发送模式。

USART_HardwareFlowControl：

指定硬件流控制模式是启用还是禁用。

用法下一篇再说。

Fin.