前言

STM32固件库中提供了串口收发的标志位函数，包括USART_GetFlagStatus(…,…);和USART_GetITStatus(…,…);，两者容易混淆，重点区别就在于：前者返回值是中断标志位状态（读SR寄存器），后者返回值是中断发生与否的判断（读CR寄存器），以下主要对这两个函数进行分析。

一、USART_GETFlagStatus(…,…)

/**
  * @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;
}

1.代码解析

该函数用于检测串口中断标志位的状态。
其中，24、25、29三行用于检测所用参数是否符合该函数的范围。该函数的第一个形参只能是USART1,USART2,USART3,UART4,UART5，第二个形参只能是以下内容：

从32行开始，检测SR寄存器的状态，在SET和RESET中进行比较。

2.函数使用

函数返回值为SET或RESET，即可直接用于判断。
在没有使能相应的中断函数时，通常使用该函数来判断标志位是否置位。
但串口触发中断后，需要清除标志位，由文章后部分描述。

二、USART_GetITStatus(…,…)

/**
  * @brief  Checks whether the specified USART interrupt has occurred 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_IT: specifies the USART interrupt source to check.
  *   This parameter can be one of the following values:
  *     @arg USART_IT_CTS:  CTS change interrupt (not available for UART4 and UART5)
  *     @arg USART_IT_LBD:  LIN Break detection interrupt
  *     @arg USART_IT_TXE:  Tansmit Data Register empty interrupt
  *     @arg USART_IT_TC:   Transmission complete interrupt
  *     @arg USART_IT_RXNE: Receive Data register not empty interrupt
  *     @arg USART_IT_IDLE: Idle line detection interrupt
  *     @arg USART_IT_ORE:  OverRun Error interrupt
  *     @arg USART_IT_NE:   Noise Error interrupt
  *     @arg USART_IT_FE:   Framing Error interrupt
  *     @arg USART_IT_PE:   Parity Error interrupt
  * @retval The new state of USART_IT (SET or RESET).
  */
ITStatus USART_GetITStatus(USART_TypeDef* USARTx, uint16_t USART_IT)
{
  uint32_t bitpos = 0x00, itmask = 0x00, usartreg = 0x00;
  ITStatus bitstatus = RESET;
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_GET_IT(USART_IT));
  /* The CTS interrupt is not available for UART4 and UART5 */ 
  if (USART_IT == USART_IT_CTS)
  {
    assert_param(IS_USART_123_PERIPH(USARTx));
  }   

  /* Get the USART register index */
  usartreg = (((uint8_t)USART_IT) >> 0x05);
  /* Get the interrupt position */
  itmask = USART_IT & IT_Mask;
  itmask = (uint32_t)0x01 << itmask;

  if (usartreg == 0x01) /* The IT  is in CR1 register */
  {
    itmask &= USARTx->CR1;
  }
  else if (usartreg == 0x02) /* The IT  is in CR2 register */
  {
    itmask &= USARTx->CR2;
  }
  else /* The IT  is in CR3 register */
  {
    itmask &= USARTx->CR3;
  }

  bitpos = USART_IT >> 0x08;
  bitpos = (uint32_t)0x01 << bitpos;
  bitpos &= USARTx->SR;
  if ((itmask != (uint16_t)RESET)&&(bitpos != (uint16_t)RESET))
  {
    bitstatus = SET;
  }
  else
  {
    bitstatus = RESET;
  }

  return bitstatus;  
}

1.代码解析

代码中分别读取串口控制寄存器CR1，CR2，CR3的状态，获取中断发生的动作，返回SET或RESET。

2.函数使用

函数返回值为SET或RESET，即可直接用于判断。
除了可以判断中断标志位外，还能判断是否发生了中断。
但串口触发中断后，需要清除标志位，由文章后部分描述。

三、 USART_ClearFlag(…,…);

/**
  * @brief  Clears the USARTx's pending flags.
  * @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 clear.
  *   This parameter can be any combination 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_TC:   Transmission Complete flag.
  *     @arg USART_FLAG_RXNE: Receive data register not empty flag.
  *   
  * @note
  *   - PE (Parity error), FE (Framing error), NE (Noise error), ORE (OverRun 
  *     error) and IDLE (Idle line detected) flags are cleared by software 
  *     sequence: a read operation to USART_SR register (USART_GetFlagStatus()) 
  *     followed by a read operation to USART_DR register (USART_ReceiveData()).
  *   - RXNE flag can be also cleared by a read to the USART_DR register 
  *     (USART_ReceiveData()).
  *   - TC flag can be also cleared by software sequence: a read operation to 
  *     USART_SR register (USART_GetFlagStatus()) followed by a write operation
  *     to USART_DR register (USART_SendData()).
  *   - TXE flag is cleared only by a write to the USART_DR register 
  *     (USART_SendData()).
  * @retval None
  */
void USART_ClearFlag(USART_TypeDef* USARTx, uint16_t USART_FLAG)
{
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_CLEAR_FLAG(USART_FLAG));
  /* The CTS flag is not available for UART4 and UART5 */
  if ((USART_FLAG & USART_FLAG_CTS) == USART_FLAG_CTS)
  {
    assert_param(IS_USART_123_PERIPH(USARTx));
  } 

  USARTx->SR = (uint16_t)~USART_FLAG;
}

1.代码解析

该函数用于软件清除标志位。
例如，常用的参数为USART_FLAG_RXNE，库中定义的参数为0x0020，取反后为0xFFDF，恰好可以使SR寄存器的RXNE位置零（根据参考手册）。同时根据函数note，USART_FLAG_RXNE也可以通过读DR寄存器进行清位，即调用函数USART_ReceiveData();。

2.函数使用

可以用在中断处理函数中对标志位进行清除操作。

四、USART_ClearITPendingBit(…,…);

/**
  * @brief  Clears the USARTx's interrupt pending bits.
  * @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_IT: specifies the interrupt pending bit to clear.
  *   This parameter can be one of the following values:
  *     @arg USART_IT_CTS:  CTS change interrupt (not available for UART4 and UART5)
  *     @arg USART_IT_LBD:  LIN Break detection interrupt
  *     @arg USART_IT_TC:   Transmission complete interrupt. 
  *     @arg USART_IT_RXNE: Receive Data register not empty interrupt.
  *   
  * @note
  *   - PE (Parity error), FE (Framing error), NE (Noise error), ORE (OverRun 
  *     error) and IDLE (Idle line detected) pending bits are cleared by 
  *     software sequence: a read operation to USART_SR register 
  *     (USART_GetITStatus()) followed by a read operation to USART_DR register 
  *     (USART_ReceiveData()).
  *   - RXNE pending bit can be also cleared by a read to the USART_DR register 
  *     (USART_ReceiveData()).
  *   - TC pending bit can be also cleared by software sequence: a read 
  *     operation to USART_SR register (USART_GetITStatus()) followed by a write 
  *     operation to USART_DR register (USART_SendData()).
  *   - TXE pending bit is cleared only by a write to the USART_DR register 
  *     (USART_SendData()).
  * @retval None
  */
void USART_ClearITPendingBit(USART_TypeDef* USARTx, uint16_t USART_IT)
{
  uint16_t bitpos = 0x00, itmask = 0x00;
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_CLEAR_IT(USART_IT));
  /* The CTS interrupt is not available for UART4 and UART5 */
  if (USART_IT == USART_IT_CTS)
  {
    assert_param(IS_USART_123_PERIPH(USARTx));
  }   

  bitpos = USART_IT >> 0x08;
  itmask = ((uint16_t)0x01 << (uint16_t)bitpos);
  USARTx->SR = (uint16_t)~itmask;
}

该函数与USART_ClearFlag(…,…);功能相同，都是对SR寄存器某位进行清除操作，只是概念不一样。
例如，常用的参数为USART_IT_RXNE，库中定义的参数为0x0525，进入函数中itmask的值为0x20，取反为0xDF，恰好可以使SR寄存器的RXNE位置零（根据参考手册）。同时根据函数note，USART_FLAG_RXNE也可以通过读DR寄存器进行清位，即调用函数USART_ReceiveData();。

备注：

USART_ClearFlag(…,…);和USART_ClearITPendingBit(…,…);的函数说明里面都有各个参数的清位方式说明。