stm32 usart的BRR寄存器的配置

版权声明：吴亚彪 https://blog.csdn.net/weixin_44288260/article/details/85793432

程序是CMSIS中的，分析它是怎么实现的？

波特率由整数和分数两部分组成。

RCC_GetClocksFreq(&RCC_ClocksStatus);
  if (usartxbase == USART1_BASE)
  {
    apbclock = RCC_ClocksStatus.PCLK2_Frequency;
  }
  else
  {
    apbclock = RCC_ClocksStatus.PCLK1_Frequency;
  }
 

整数部分的实现

USARTDIV *100
eg:USARTDIV = 25.62
则计算得 integerdivider =2562

 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)));    
 }

提取出25,并左移4位,左移是为了和低4位与操作
tmpreg = (integerdivider / 100) << 4;

提取出62
fractionaldivider = integerdivider - (100 * (tmpreg >> 4));
之后62/100*16,它可能是小数，要找到最接近的整数，此时加0.5
最后再与整数部分相与。

  /* 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;