STM32的USART

• 首先了解一下总线结构，不然的话会走很多弯路。（不说了，说多了都是泪，一定要学踏实了，不能随便拿一个代码来修改！！）
  

• 如图所示：USART1挂载APB2上面，而USART2和USART3挂载在APB1上面（就这里没看，搞了大半天才发现挂载总线错了）

• 两个AHB/APB桥在AHB和2个APB总线间提供同步连接。APB1操作速度限于36MHz，APB2操作于全速(最高72MHz)。

如图所示，在状态寄存器的有效位9-0位，第5位和第7位是发送和读取状态寄存器，由此不难理解串口的发送和接收函数的写法。

/**
  * @brief  Checks whether the specified USART flag is set or not.
  * @param  USARTx: Select the USART or the UART peripheral. 
  *   This parameter can be one of the following values:
  *   USART1, USART2, USART3, UART4 or UART5.
  * @param  USART_FLAG: specifies the flag to check.
  *   This parameter can be one of the following values:
  *     @arg USART_FLAG_CTS:  CTS Change flag (not available for UART4 and UART5)
  *     @arg USART_FLAG_LBD:  LIN Break detection flag
  *     @arg USART_FLAG_TXE:  Transmit data register empty flag
  *     @arg USART_FLAG_TC:   Transmission Complete flag
  *     @arg USART_FLAG_RXNE: Receive data register not empty flag
  *     @arg USART_FLAG_IDLE: Idle Line detection flag
  *     @arg USART_FLAG_ORE:  OverRun Error flag
  *     @arg USART_FLAG_NE:   Noise Error flag
  *     @arg USART_FLAG_FE:   Framing Error flag
  *     @arg USART_FLAG_PE:   Parity Error flag
  * @retval The new state of USART_FLAG (SET or RESET).
  */
FlagStatus USART_GetFlagStatus(USART_TypeDef* USARTx, uint16_t USART_FLAG)
{
  FlagStatus bitstatus = RESET;
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_FLAG(USART_FLAG));
  /* The CTS flag is not available for UART4 and UART5 */
  if (USART_FLAG == USART_FLAG_CTS)
  {
    assert_param(IS_USART_123_PERIPH(USARTx));
  }  
  
  if ((USARTx->SR & USART_FLAG) != (uint16_t)RESET)
  {
    bitstatus = SET;
  }
  else
  {
    bitstatus = RESET;
  }
  return bitstatus;
}

///重定向c库函数printf到USART1
int fputc(int ch, FILE *f)
{
    /* 发送一个字节数据到USART1 */
    USART_SendData(USART1, (uint8_t) ch);

    /* 等待发送完毕 */
    while (USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET);//RESET=0

    return (ch);
}

///重定向c库函数scanf到USART1
int fgetc(FILE *f)
{
    /* 等待串口1输入数据 */
    while (USART_GetFlagStatus(USART1, USART_FLAG_RXNE) == RESET);

    return (int)USART_ReceiveData(USART1);
}

•      32位数据寄存器USART_DR的有效位为[8:0]，因此只需要将带打印数据写入就可以了。

void USART_SendData(USART_TypeDef* USARTx, uint16_t Data)
{
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_DATA(Data)); 
    
  /* Transmit Data */
  USARTx->DR = (Data & (uint16_t)0x01FF);
}    

但是与上0x01ff的原因是ASCII码的范围是0-127，而寄存器只需要8-0我们只需要取后面9位就可以了。

• USART3 的初始化

void USART3_Config(void)
{
    GPIO_InitTypeDef GPIO_InitStructure;
    USART_InitTypeDef USART_InitStructure;

    /* config USART3 clock */


    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);



 /* USART3 GPIO config */
    /* Configure USART3 Tx (PB.10) as alternate function push-pull */
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOB, &GPIO_InitStructure);

    /* Configure USART3 Rx (PB.11) as input floating */
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
    GPIO_Init(GPIOB, &GPIO_InitStructure);

    /* USART3 mode config */
    USART_InitStructure.USART_BaudRate = 9600;                
    USART_InitStructure.USART_WordLength = USART_WordLength_8b;
    USART_InitStructure.USART_StopBits = USART_StopBits_1;
    USART_InitStructure.USART_Parity = USART_Parity_No ;
    USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
    USART_Init(USART3, &USART_InitStructure);

    USART_Cmd(USART3, ENABLE);
}