关于STM32的USART_GetFlagStatus和USART_GetITStatus解析（异步通信）

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。

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。

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。