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

谁都会走 提交于 2019-12-09 14:45:49

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。

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