2021年10月21日 | stm32专题十一：USART（三）初始化结构体和标准库函数分析

在之前的博客中分析了stm32串口的结构，和详细的发送、接受过程。现在来分析固件库中对于USART的标准函数

typedef struct

{

  uint32_t USART_BaudRate;            // 波特率

  uint16_t USART_WordLength;          // 帧数据长度（8位还是9位）

  uint16_t USART_StopBits;            // 停止位

  uint16_t USART_Parity;              // 校验

  uint16_t USART_Mode;                // 模式：单收、单发或收发

  uint16_t USART_HardwareFlowControl; // 硬件流控

} USART_InitTypeDef;

固件库中初始化函数，就是往前一篇博客中提到的控制寄存器USART_CR1  USART_CR2中写入相应的数据，并计算波特率的值，然后写入BRR寄存器。

/**

  * @brief  Initializes the USARTx peripheral according to the specified

  *         parameters in the USART_InitStruct .

  * @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_InitStruct: pointer to a USART_InitTypeDef structure

  *         that contains the configuration information for the specified USART 

  *         peripheral.

  * @retval None

  */

void USART_Init(USART_TypeDef* USARTx, USART_InitTypeDef* USART_InitStruct)

{

  uint32_t tmpreg = 0x00, apbclock = 0x00;

  uint32_t integerdivider = 0x00;

  uint32_t fractionaldivider = 0x00;

  uint32_t usartxbase = 0;

  RCC_ClocksTypeDef RCC_ClocksStatus;

  /* The hardware flow control is available only for USART1, USART2 and USART3 */

  // 硬件流控只有在USART中才能使用，UART中不能用

  if (USART_InitStruct->USART_HardwareFlowControl != USART_HardwareFlowControl_None)

  {

    assert_param(IS_USART_123_PERIPH(USARTx));

  }

  usartxbase = (uint32_t)USARTx;

/*---------------------------- USART CR2 Configuration -----------------------*/

  // 先保存USART_CR2寄存器的状态

  tmpreg = USARTx->CR2;

  /* Clear STOP[13:12] bits */

  // 清除CR2的停止位，其他位保持不变

  tmpreg &= CR2_STOP_CLEAR_Mask;

  /* Configure the USART Stop Bits, Clock, CPOL, CPHA and LastBit ------------*/

  /* Set STOP[13:12] bits according to USART_StopBits value */

  // 根据配置的停止位，写入到相应的位置

  tmpreg |= (uint32_t)USART_InitStruct->USART_StopBits;

  /* Write to USART CR2 */

  // 八停止位配置写进CR2寄存器

  USARTx->CR2 = (uint16_t)tmpreg;

/*---------------------------- USART CR1 Configuration -----------------------*/

  tmpreg = USARTx->CR1;

  /* Clear M, PCE, PS, TE and RE bits */

  // 清除字长、校验、校验使能、发送和接收使能的位，再根据配置写入

  tmpreg &= CR1_CLEAR_Mask;

  /* Configure the USART Word Length, Parity and mode ----------------------- */

  /* Set the M bits according to USART_WordLength value */

  /* Set PCE and PS bits according to USART_Parity value */

  /* Set TE and RE bits according to USART_Mode value */

  tmpreg |= (uint32_t)USART_InitStruct->USART_WordLength | USART_InitStruct->USART_Parity |

            USART_InitStruct->USART_Mode;

  /* Write to USART CR1 */

  // 把配置的字长、校验、模式写入到CR1寄存器

  USARTx->CR1 = (uint16_t)tmpreg;

/*---------------------------- USART CR3 Configuration -----------------------*/  

  tmpreg = USARTx->CR3;

  /* Clear CTSE and RTSE bits */

  // 清除串口的发送、接收硬件流控

  tmpreg &= CR3_CLEAR_Mask;

  /* Configure the USART HFC -------------------------------------------------*/

  /* Set CTSE and RTSE bits according to USART_HardwareFlowControl value */

  tmpreg |= USART_InitStruct->USART_HardwareFlowControl;

  /* Write to USART CR3 */

  // 把配置的硬件流控制写入到CR3寄存器（通常不适用硬件流控）

  USARTx->CR3 = (uint16_t)tmpreg;

/*---------------------------- USART BRR Configuration -----------------------*/

  /* Configure the USART Baud Rate -------------------------------------------*/

  RCC_GetClocksFreq(&RCC_ClocksStatus);

  if (usartxbase == USART1_BASE)

  {

    apbclock = RCC_ClocksStatus.PCLK2_Frequency;

  }

  else

  {

    apbclock = RCC_ClocksStatus.PCLK1_Frequency;

  }

  /* Determine the integer part */

  if ((USARTx->CR1 & CR1_OVER8_Set) != 0)

  {

    /* Integer part computing in case Oversampling mode is 8 Samples */

    integerdivider = ((25 * apbclock) / (2 * (USART_InitStruct->USART_BaudRate)));    

  }

  else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */

  {

    /* Integer part computing in case Oversampling mode is 16 Samples */

    integerdivider = ((25 * apbclock) / (4 * (USART_InitStruct->USART_BaudRate)));    

  }

  tmpreg = (integerdivider / 100) << 4;

  /* Determine the fractional part */

  fractionaldivider = integerdivider - (100 * (tmpreg >> 4));

  /* Implement the fractional part in the register */

  if ((USARTx->CR1 & CR1_OVER8_Set) != 0)

  {

    tmpreg |= ((((fractionaldivider * 8) + 50) / 100)) & ((uint8_t)0x07);

  }

  else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */

  {

    tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);

  }

  /* Write to USART BRR */

  USARTx->BRR = (uint16_t)tmpreg;

}

接下来是在同步串口通信中使用的结构体，虽然这个用的很少，但学习时还是应该全面，因此也分析一下

typedef struct

{

  uint16_t USART_Clock;   // 时钟使能

  uint16_t USART_CPOL;    // 时钟极性（High或者Low）

                          // 表示串口空闲时时钟是高电平还是低电平

  uint16_t USART_CPHA;    // 时钟相位（USART_CPHA_1Edge或者USART_CPHA_2Edge）

                          // 表示在时钟的第一个边沿采集数据还是第二个边沿采集数据

  // 时钟的极性和相位一定是配合使用的

  uint16_t USART_LastBit; // 选择最后一位数据的时钟会不会发出

} USART_ClockInitTypeDef;

我们再看以一下固件库中的其他常用的配置函数

// 使能串口，配置的是CR1_UE位

void USART_Cmd(USART_TypeDef* USARTx, FunctionalState NewState)

{

  /* Check the parameters */

  assert_param(IS_USART_ALL_PERIPH(USARTx));

  assert_param(IS_FUNCTIONAL_STATE(NewState));

  if (NewState != DISABLE)

  {

    /* Enable the selected USART by setting the UE bit in the CR1 register */

    USARTx->CR1 |= CR1_UE_Set;

  }

  else

  {

    /* Disable the selected USART by clearing the UE bit in the CR1 register */

    USARTx->CR1 &= CR1_UE_Reset;

  }

}

// DMA请求

void USART_DMACmd(USART_TypeDef* USARTx, uint16_t USART_DMAReq, FunctionalState NewState)

{

  /* Check the parameters */

  assert_param(IS_USART_ALL_PERIPH(USARTx));

  assert_param(IS_USART_DMAREQ(USART_DMAReq));  

  assert_param(IS_FUNCTIONAL_STATE(NewState)); 

  if (NewState != DISABLE)

  {

    /* Enable the DMA transfer for selected requests by setting the DMAT and/or

       DMAR bits in the USART CR3 register */

    USARTx->CR3 |= USART_DMAReq;

  }

  else

  {

    /* Disable the DMA transfer for selected requests by clearing the DMAT and/or

       DMAR bits in the USART CR3 register */

    USARTx->CR3 &= (uint16_t)~USART_DMAReq;

  }

}

然后是中断配置函数，有很多个中断可以选择。这个函数写的还是有点复杂，仔细看，起始可以发现，基本就是如下过程。通过中断标志，来判断该中断由CR1（偏移地址0X0C）还是CR2（偏移0X10），或者是CR3（偏移0X14）。然后把相应寄存器的中断使能标志位置1。

/**

  * @brief  Enables or disables the specified USART interrupts.

  * @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 sources to be enabled or disabled.

  *   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:  Transmit 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_PE:   Parity Error interrupt

  *     @arg USART_IT_ERR:  Error interrupt(Frame error, noise error, overrun error)

  * @param  NewState: new state of the specified USARTx interrupts.

  *   This parameter can be: ENABLE or DISABLE.

  * @retval None

  */

void USART_ITConfig(USART_TypeDef* USARTx, uint16_t USART_IT, FunctionalState NewState)

{

  uint32_t usartreg = 0x00, itpos = 0x00, itmask = 0x00;

  uint32_t usartxbase = 0x00;

  /* Check the parameters */

  assert_param(IS_USART_ALL_PERIPH(USARTx));

  assert_param(IS_USART_CONFIG_IT(USART_IT));

  assert_param(IS_FUNCTIONAL_STATE(NewState));

  /* The CTS interrupt is not available for UART4 and UART5 */

  if (USART_IT == USART_IT_CTS)

  {

    assert_param(IS_USART_123_PERIPH(USARTx));

  }   

  usartxbase = (uint32_t)USARTx;

  /* Get the USART register index */

  usartreg = (((uint8_t)USART_IT) >> 0x05);

  /* Get the interrupt position */

  itpos = USART_IT & IT_Mask;

  itmask = (((uint32_t)0x01) << itpos);

  if (usartreg == 0x01) /* The IT is in CR1 register */

  {

    usartxbase += 0x0C;

  }

  else if (usartreg == 0x02) /* The IT is in CR2 register */