在调用相关库函数对USART模块进行配置前，我们需要先设定好相应的参数。首先，定义一个结构体USART_InitStructure，其类型为USART_InitTypeDef，它的成员即为配置USART需要的相关参数。第二就是开启USART时钟，其挂载于高速APB线（APB2）上。第三是进行参数设定，这里设定波特率为115200，字长为8位，1个停止位，无奇偶校验，无硬件流控制，接收与发送均开启模式。限于文章篇幅，所以此处没有提到USART相关IO口的配置，在编码时请注意添加。参数设定代码如下：
USART_InitTypeDef USART_InitStructure; /* config USART1 clock */ RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE); /* USART1 mode config */ USART_InitStructure.USART_BaudRate = 115200; USART_InitStructure.USART_WordLength = USART_WordLength_8b; USART_InitStructure.USART_StopBits = USART_StopBits_1; USART_InitStructure.USART_Parity = USART_Parity_No ; USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None; USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx; USART_Init(USART1, &USART_InitStructure); /* 使能串口1接收中断*/ USART_ITConfig(USART1, USART_IT_RXNE, ENABLE); USART_Cmd(USART1, ENABLE);
设置好了参数，就需要调用库函数来完成对USART的配置，上面有3个库函数，我们按代码顺序来。
1、USART_Init()
函数功能：初始化USART模块，完成对它的配置。
/**
* @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;
/* Check the parameters */
assert_param(IS_USART_ALL_PERIPH(USARTx));
assert_param(IS_USART_BAUDRATE(USART_InitStruct->USART_BaudRate));
assert_param(IS_USART_WORD_LENGTH(USART_InitStruct->USART_WordLength));
assert_param(IS_USART_STOPBITS(USART_InitStruct->USART_StopBits));
assert_param(IS_USART_PARITY(USART_InitStruct->USART_Parity));
assert_param(IS_USART_MODE(USART_InitStruct->USART_Mode));
assert_param(IS_USART_HARDWARE_FLOW_CONTROL(USART_InitStruct->USART_HardwareFlowControl));
/* The hardware flow control is available only for USART1, USART2 and USART3 */
if (USART_InitStruct->USART_HardwareFlowControl != USART_HardwareFlowControl_None)
{
assert_param(IS_USART_123_PERIPH(USARTx));
}
usartxbase = (uint32_t)USARTx;
/*---------------------------- USART CR2 Configuration -----------------------*/
tmpreg = USARTx->CR2;
/* Clear STOP[13:12] bits */
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 */
tmpreg |= (uint32_t)USART_InitStruct->USART_StopBits;
/* Write to USART CR2 */
USARTx->CR2 = (uint16_t)tmpreg;
/*---------------------------- USART CR1 Configuration -----------------------*/
tmpreg = USARTx->CR1;
/* Clear M, PCE, PS, TE and RE bits */
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 */
USARTx->CR1 = (uint16_t)tmpreg;
/*---------------------------- USART CR3 Configuration -----------------------*/
tmpreg = USARTx->CR3;
/* Clear CTSE and RTSE bits */
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 */
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;
}上述代码即为USART_Init()的具体实现，现在我们以前面设定的参数为例，来分析这个库函数。
if (USART_InitStruct->USART_HardwareFlowControl != USART_HardwareFlowControl_None)
括号内条件不满足，跳过相应代码。
usartxbase = (uint32_t)USARTx;
根据USART1相关宏定义
#define USART1 ((USART_TypeDef *) USART1_BASE) #define USART1_BASE (APB2PERIPH_BASE + 0x3800) #define APB2PERIPH_BASE (PERIPH_BASE + 0x10000) #define PERIPH_BASE ((uint32_t)0x40000000) /*!< Peripheral base address in the alias region */
可以得到 usartxbase = 0x4001 3800;此即为USART1存储器映像首地址。
tmpreg = USARTx->CR2;
因为此时USART1->CR2暂未赋值，所以tmpreg = 0;
tmpreg &= CR2_STOP_CLEAR_Mask;
CR2_STOP_CLEAR_Mask定义如下
#define CR2_STOP_CLEAR_Mask ((uint16_t)0xCFFF) /*!< USART CR2 STOP Bits Mask */
因为tmpreg = 0，所以相与后temreg还是0。
tmpreg |= (uint32_t)USART_InitStruct->USART_StopBits;
因为USART_StopBits_1定义如下
#define USART_StopBits_1 ((uint16_t)0x0000)
所以相或后temreg仍然为0。
USARTx->CR2 = (uint16_t)tmpreg;
即USART1->CR2 = 0，由CR2寄存器描述
可知，USART被配置为1个停止位。
tmpreg = USARTx->CR1;
同理，此时tmpreg = 0;
tmpreg &= CR1_CLEAR_Mask;
CR1_CLEAR_Mask定义如下
#define CR1_CLEAR_Mask ((uint16_t)0xE9F3) /*!< USART CR1 Mask */
然而不管它定义如何，此时tmpreg总是为0。
tmpreg |= (uint32_t)USART_InitStruct->USART_WordLength | USART_InitStruct->USART_Parity | USART_InitStruct->USART_Mode;
先找到3个参数的定义
#define USART_WordLength_8b ((uint16_t)0x0000) #define USART_Parity_No ((uint16_t)0x0000) #define USART_Mode_Rx ((uint16_t)0x0004) #define USART_Mode_Tx ((uint16_t)0x0008)
所以tmpreg |= 0x0000 | 0x0000 | 0x0004 | 0x0008 = 0x000C;
USARTx->CR1 = (uint16_t)tmpreg;
即USART1->CR1 = 0x000C，由CR1寄存器描述
可知，USART1被配置为8位数据字长、校验控制被禁止、发送与接收均使能。
tmpreg = USARTx->CR3;
同理，tmpreg = 0;
tmpreg &= CR3_CLEAR_Mask;
CR3_CLEAR_Mask定义为
#define CR3_CLEAR_Mask ((uint16_t)0xFCFF) /*!< USART CR3 Mask */
相与后temreg = 0;
tmpreg |= USART_InitStruct->USART_HardwareFlowControl; #define USART_HardwareFlowControl_None ((uint16_t)0x0000)
所以tmpreg = 0;
USARTx->CR3 = (uint16_t)tmpreg;
即USART1->CR3 = 0，由CR3寄存器描述
可知，USART1被配置为禁止CTS硬件流控制、禁止RTS硬件流控制，即无硬件流控制。
RCC_GetClocksFreq(&RCC_ClocksStatus);
这是个相关时钟配置的函数，通过Debug我们可以看到各个时钟的配置情况
PCLK2为0x044A A200，转换为十进制即为72 000 000，即PCLK2 = 72MHz。
if (usartxbase == USART1_BASE)
{
apbclock = RCC_ClocksStatus.PCLK2_Frequency;
}满足判断条件，所以apbclock = 72 000 000;
这里说明一下：参考手册上面规定了只有USART1才能使用PCLK2时钟（最高72MHz），其他USART模块用PCLK1时钟（最高36MHz）。
if ((USARTx->CR1 & CR1_OVER8_Set) != 0)
CR1 & CR1_OVER8_Set定义如下
#define CR1_OVER8_Set ((u16)0x8000) /* USART OVER8 mode Enable Mask */
而USART1->CR1 = 0x000C，两者相与结果为0，不满足条件。
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)));
}integerdivider = ( (25 * 72 000 000) / (4 * 115200) ) = 3906.25，舍去小数部分，即为3906。
tmpreg = (integerdivider / 100) << 4;
tmpreg = ( 3906 / 100 ) << 4 = 39 << 4 = 0x27 << 4 = 0x270 = 624;
fractionaldivider = integerdivider - (100 * (tmpreg >> 4));
fractionaldivider = 3906 - ( 100 * (0x270 >> 4) ) = 3906 - ( 100 * 0x27 ) = 3906 - 100 * 39 = 3906 - 3900 = 6;
if ((USARTx->CR1 & CR1_OVER8_Set) != 0)
前面已经判断过了，不满足条件，跳过语句。
else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */
{
tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);
}tmpreg |= ( ( (6 * 16) + 50 ) / 100 ) & 0x0F = tmpreg | ( ( 96 + 50 ) / 100 ) & 0x0F = tmpreg | (146 / 100) & 0x0F = tmpreg | 1 & 0x0F = tmpreg | 0x01 = 0x270 | 0x01 = 0x271;
USARTx->BRR = (uint16_t)tmpreg;
即USART1->BRR = 0x271，由BRR寄存器描述
Mantissa：基数
Fraction：分数
所以基数部分为0x27 = 39，分数部分为0x01 = 1，根据参考手册描述
Mantissa(USARTDIV) = 39d
Fraction(USARTDIV) = 1 / 16 = 0.0625d
所以USARTDIV = 39.0625d。
再由波特率计算表
可知，USART1波特率被配置为115.2Kbps，即波特率 = 115.2 * 1000 = 115200。
至此，对USART1的配置总结为：
USART1波特率 = 115200、8位数据字长、校验控制被禁止、发送与接收均使能、1个停止位、无硬件流控制。
再回过头来看一下之前的配置代码。
USART_InitStructure.USART_BaudRate = 115200; USART_InitStructure.USART_WordLength = USART_WordLength_8b; USART_InitStructure.USART_StopBits = USART_StopBits_1; USART_InitStructure.USART_Parity = USART_Parity_No ; USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None; USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
对比上面的分析，就能知道库函数根据我们的需要帮我们准确的完成了对USART1的配置，是的，它干的就是这么件事，而我们只需要利用库函数，就能够直观且轻而易举的完成对串口模块的配置，库的存在，能够节约开发人员许多的时间，而且在很大程度上降低了开发的难度，的确是进行STM32开发非常好的一个选择。
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