求助：下面的这条指令，单步执行可以清除TC标志，连续执行就不行，        USART_ClearFlag(USART3, USART_FLAG_TC);       
请求指点

void USART_ClearFlag(USART_TypeDef* USARTx, uint16_t USART_FLAG)
{
  /* Check the parameters */
  assert_param(IS_USART_ALL_PERIPH(USARTx));
  assert_param(IS_USART_CLEAR_FLAG(USART_FLAG));

  /* The CTS flag is not available for UART4 and UART5 */
  if ((USART_FLAG & USART_FLAG_CTS) == USART_FLAG_CTS)
  {
    assert_param(IS_USART_1236_PERIPH(USARTx));
  }

  USARTx->SR &= (uint16_t)~USART_FLAG;
}