ARM - STM32 使用11.0592MHz晶振 （3）

yuyang911220

• 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
  
•   {  
• <span style="color:#ff0000;">    apbclock = RCC_ClocksStatus.PCLK1_Frequency;</span>  
•   }  
• 
  
•   /* 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 */
  
• <span style="color:#ff0000;">    integerdivider = ((25 * apbclock) / (4 * (USART_InitStruct->USART_BaudRate)));  </span><span style="color:#3366ff;">//apbclock=0xA8C000=11059200 </span>
  
•   }  
• <span style="color:#ff0000;">  tmpreg = (integerdivider / 100) << 4;                                             </span><span style="color:#3333ff;">// 得到integerdivider</span><span style="color: rgb(255, 0, 0); "> </span><span style="color:#3333ff;">
  
• </span>  
• 
  
•   /* Determine the fractional part */
  
• <span style="color:#ff0000;">  fractionaldivider = integerdivider - (100 * (tmpreg >> 4));                       </span><span style="color:#3333ff;">//得到fractionaldivider</span><span style="color: rgb(255, 0, 0); "> </span><span style="color:#ff0000;">
  
• </span>  
• 
  
•   /* 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) */
  
•   {  
• <span style="color:#ff0000;">   tmpreg |= ((((fractionaldivider * 16) + 50) / 100)) & ((uint8_t)0x0F);</span>  
•   }  
• 
  
•   /* Write to USART BRR */
  
•   USARTx->BRR = (uint16_t)tmpreg;  
• }  

小数波特率发送和接收由一共用的波特率发生器驱动，当发送器和接收器的使能位分别（TE和RE）置位时，分别为其产生时钟。接收器和发送器的波特率应设置成相同。
Tx / Rx 波特率 ＝ PCLKx / (16 * USARTDIV)PCLKx可以是APB1的时钟PCLK1（用于USART1），也可以是APB2的时钟PCLK2(用于USART2、3、4、5) 。

USARTDIV = DIV_Maintissa[11:0] + DIV_Fraction[3:0] / 16USARTDIV是一个无符号的定点数，转换成对应的二进制小数后，整数部分存放在USART_BRR寄存器中的DIV_Maintissa[11:0]，小数部分存放在USART_BRR寄存器中的DIV_Fraction[3:0]。
注：在写入USART_BRR之后，波特率计数器会被波特率寄存器的新值替换。因此，不要在通信进行中改变波特率寄存器的数值。