串口DMA方式 收发9位数据问题

-心悔-

以第九位的0或1 ，区分数据或者命令，怎么实现？

-心悔-

USART_InitStructure.USART_BaudRate = 2400;
    USART_InitStructure.USART_WordLength = USART_WordLength_9b;
    USART_InitStructure.USART_StopBits = USART_StopBits_1;
    USART_InitStructure.USART_Parity = USART_Parity_Odd;
    USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
    USART_Init(USARTx, &USART_InitStructure);

-心悔-

DMA_InitStructure.DMA_BufferSize = BufferSize;/* ÅäÖÃDMA′óD¡ */
     DMA_InitStructure.DMA_FIFOMode = DMA_FIFOMode_Enable;            
     DMA_InitStructure.DMA_FIFOThreshold = DMA_FIFOThreshold_Full;      
    DMA_InitStructure.DMA_MemoryBurst = DMA_MemoryBurst_Single;        
    DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_HalfWord;        
    DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;              
    DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;                        
    DMA_InitStructure.DMA_PeripheralBaseAddr =(uint32_t) (&(USARTx->DR)) ;
  DMA_InitStructure.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;   
    DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_HalfWord;
    DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
    DMA_InitStructure.DMA_Priority = DMA_Priority_High;            
   
    /* ÅäÖÃ TX DMA */
    DMA_InitStructure.DMA_Channel = USARTx_TX_DMA_CHANNEL ;     
    DMA_InitStructure.DMA_DIR = DMA_DIR_MemoryToPeripheral ;   
    DMA_InitStructure.DMA_Memory0BaseAddr =(uint32_t)BufferSRC ;
    DMA_Init(USARTx_TX_DMA_STREAM,&DMA_InitStructure);