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