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(…,…)
1 /**
2 * @brief Checks whether the specified USART flag is set or not.
3 * @param USARTx: Select the USART or the UART peripheral.
4 * This parameter can be one of the following values:
5 * USART1, USART2, USART3, UART4 or UART5.
6 * @param USART_FLAG: specifies the flag to check.
7 * This parameter can be one of the following values:
8 * @arg USART_FLAG_CTS: CTS Change flag (not available for UART4 and UART5)
9 * @arg USART_FLAG_LBD: LIN Break detection flag
10 * @arg USART_FLAG_TXE: Transmit data register empty flag
11 * @arg USART_FLAG_TC: Transmission Complete flag
12 * @arg USART_FLAG_RXNE: Receive data register not empty flag
13 * @arg USART_FLAG_IDLE: Idle Line detection flag
14 * @arg USART_FLAG_ORE: OverRun Error flag
15 * @arg USART_FLAG_NE: Noise Error flag
16 * @arg USART_FLAG_FE: Framing Error flag
17 * @arg USART_FLAG_PE: Parity Error flag
18 * @retval The new state of USART_FLAG (SET or RESET).
19 */
20 FlagStatus USART_GetFlagStatus(USART_TypeDef* USARTx, uint16_t USART_FLAG)
21 {
22 FlagStatus bitstatus = RESET;
23 /* Check the parameters */
24 assert_param(IS_USART_ALL_PERIPH(USARTx));
25 assert_param(IS_USART_FLAG(USART_FLAG));
26 /* The CTS flag is not available for UART4 and UART5 */
27 if (USART_FLAG == USART_FLAG_CTS)
28 {
29 assert_param(IS_USART_123_PERIPH(USARTx));
30 }
31
32 if ((USARTx->SR & USART_FLAG) != (uint16_t)RESET)
33 {
34 bitstatus = SET;
35 }
36 else
37 {
38 bitstatus = RESET;
39 }
40 return bitstatus;
41 }
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(…,…)
1 /**
2 * @brief Checks whether the specified USART interrupt has occurred or not.
3 * @param USARTx: Select the USART or the UART peripheral.
4 * This parameter can be one of the following values:
5 * USART1, USART2, USART3, UART4 or UART5.
6 * @param USART_IT: specifies the USART interrupt source to check.
7 * This parameter can be one of the following values:
8 * @arg USART_IT_CTS: CTS change interrupt (not available for UART4 and UART5)
9 * @arg USART_IT_LBD: LIN Break detection interrupt
10 * @arg USART_IT_TXE: Tansmit Data Register empty interrupt
11 * @arg USART_IT_TC: Transmission complete interrupt
12 * @arg USART_IT_RXNE: Receive Data register not empty interrupt
13 * @arg USART_IT_IDLE: Idle line detection interrupt
14 * @arg USART_IT_ORE: OverRun Error interrupt
15 * @arg USART_IT_NE: Noise Error interrupt
16 * @arg USART_IT_FE: Framing Error interrupt
17 * @arg USART_IT_PE: Parity Error interrupt
18 * @retval The new state of USART_IT (SET or RESET).
19 */
20 ITStatus USART_GetITStatus(USART_TypeDef* USARTx, uint16_t USART_IT)
21 {
22 uint32_t bitpos = 0x00, itmask = 0x00, usartreg = 0x00;
23 ITStatus bitstatus = RESET;
24 /* Check the parameters */
25 assert_param(IS_USART_ALL_PERIPH(USARTx));
26 assert_param(IS_USART_GET_IT(USART_IT));
27 /* The CTS interrupt is not available for UART4 and UART5 */
28 if (USART_IT == USART_IT_CTS)
29 {
30 assert_param(IS_USART_123_PERIPH(USARTx));
31 }
32
33 /* Get the USART register index */
34 usartreg = (((uint8_t)USART_IT) >> 0x05);
35 /* Get the interrupt position */
36 itmask = USART_IT & IT_Mask;
37 itmask = (uint32_t)0x01 << itmask;
38
39 if (usartreg == 0x01) /* The IT is in CR1 register */
40 {
41 itmask &= USARTx->CR1;
42 }
43 else if (usartreg == 0x02) /* The IT is in CR2 register */
44 {
45 itmask &= USARTx->CR2;
46 }
47 else /* The IT is in CR3 register */
48 {
49 itmask &= USARTx->CR3;
50 }
51
52 bitpos = USART_IT >> 0x08;
53 bitpos = (uint32_t)0x01 << bitpos;
54 bitpos &= USARTx->SR;
55 if ((itmask != (uint16_t)RESET)&&(bitpos != (uint16_t)RESET))
56 {
57 bitstatus = SET;
58 }
59 else
60 {
61 bitstatus = RESET;
62 }
63
64 return bitstatus;
65 }
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(…,…)
1 /**
2 * @brief Clears the USARTx's pending flags.
3 * @param USARTx: Select the USART or the UART peripheral.
4 * This parameter can be one of the following values:
5 * USART1, USART2, USART3, UART4 or UART5.
6 * @param USART_FLAG: specifies the flag to clear.
7 * This parameter can be any combination of the following values:
8 * @arg USART_FLAG_CTS: CTS Change flag (not available for UART4 and UART5).
9 * @arg USART_FLAG_LBD: LIN Break detection flag.
10 * @arg USART_FLAG_TC: Transmission Complete flag.
11 * @arg USART_FLAG_RXNE: Receive data register not empty flag.
12 *
13 * @note
14 * - PE (Parity error), FE (Framing error), NE (Noise error), ORE (OverRun
15 * error) and IDLE (Idle line detected) flags are cleared by software
16 * sequence: a read operation to USART_SR register (USART_GetFlagStatus())
17 * followed by a read operation to USART_DR register (USART_ReceiveData()).
18 * - RXNE flag can be also cleared by a read to the USART_DR register
19 * (USART_ReceiveData()).
20 * - TC flag can be also cleared by software sequence: a read operation to
21 * USART_SR register (USART_GetFlagStatus()) followed by a write operation
22 * to USART_DR register (USART_SendData()).
23 * - TXE flag is cleared only by a write to the USART_DR register
24 * (USART_SendData()).
25 * @retval None
26 */
27 void USART_ClearFlag(USART_TypeDef* USARTx, uint16_t USART_FLAG)
28 {
29 /* Check the parameters */
30 assert_param(IS_USART_ALL_PERIPH(USARTx));
31 assert_param(IS_USART_CLEAR_FLAG(USART_FLAG));
32 /* The CTS flag is not available for UART4 and UART5 */
33 if ((USART_FLAG & USART_FLAG_CTS) == USART_FLAG_CTS)
34 {
35 assert_param(IS_USART_123_PERIPH(USARTx));
36 }
37
38 USARTx->SR = (uint16_t)~USART_FLAG;
39 }
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(…,…)
1 /**
2 * @brief Clears the USARTx's interrupt pending bits.
3 * @param USARTx: Select the USART or the UART peripheral.
4 * This parameter can be one of the following values:
5 * USART1, USART2, USART3, UART4 or UART5.
6 * @param USART_IT: specifies the interrupt pending bit to clear.
7 * This parameter can be one of the following values:
8 * @arg USART_IT_CTS: CTS change interrupt (not available for UART4 and UART5)
9 * @arg USART_IT_LBD: LIN Break detection interrupt
10 * @arg USART_IT_TC: Transmission complete interrupt.
11 * @arg USART_IT_RXNE: Receive Data register not empty interrupt.
12 *
13 * @note
14 * - PE (Parity error), FE (Framing error), NE (Noise error), ORE (OverRun
15 * error) and IDLE (Idle line detected) pending bits are cleared by
16 * software sequence: a read operation to USART_SR register
17 * (USART_GetITStatus()) followed by a read operation to USART_DR register
18 * (USART_ReceiveData()).
19 * - RXNE pending bit can be also cleared by a read to the USART_DR register
20 * (USART_ReceiveData()).
21 * - TC pending bit can be also cleared by software sequence: a read
22 * operation to USART_SR register (USART_GetITStatus()) followed by a write
23 * operation to USART_DR register (USART_SendData()).
24 * - TXE pending bit is cleared only by a write to the USART_DR register
25 * (USART_SendData()).
26 * @retval None
27 */
28 void USART_ClearITPendingBit(USART_TypeDef* USARTx, uint16_t USART_IT)
29 {
30 uint16_t bitpos = 0x00, itmask = 0x00;
31 /* Check the parameters */
32 assert_param(IS_USART_ALL_PERIPH(USARTx));
33 assert_param(IS_USART_CLEAR_IT(USART_IT));
34 /* The CTS interrupt is not available for UART4 and UART5 */
35 if (USART_IT == USART_IT_CTS)
36 {
37 assert_param(IS_USART_123_PERIPH(USARTx));
38 }
39
40 bitpos = USART_IT >> 0x08;
41 itmask = ((uint16_t)0x01 << (uint16_t)bitpos);
42 USARTx->SR = (uint16_t)~itmask;
43 }
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。
来源：https://www.cnblogs.com/renhao1024/p/12010809.html