/*---------------------------- USART CR2 Configuration -----------------------*/
uint32_t tmpreg = 0x00  ；

  tmpreg = USARTx->CR2;
  tmpreg &= (uint32_t)~((uint32_t)USART_CR2_STOP);
  tmpreg |= (uint32_t)USART_InitStruct->USART_StopBits;
  USARTx->CR2 = (uint16_t)tmpreg;


以上代码为什么不直接写成：
uint16_t tmpreg = 0x00  ；
tmpreg = USARTx->CR2;
tmpreg &= ~(USART_CR2_STOP);
tmpreg |= USART_InitStruct->USART_StopBits;
USARTx->CR2 = tmpreg;

新手求教呀