BF531的SPI的DMA问题

DMA设定为实际接收字节数时，DMA使能后，总是检测到DMA在运行，也即在数据传送完毕后，DMA还在运行，此时可以检测到当前传送计数已经变为0了，为什么DMA还不停止？另外一个问题，如果把DMA设定计数减少4个的话，DMA就可以停止，为什么？

DMA初始化代码如下：

void InitDMARead(unsigned CHAR * pDstBuf, unsigned int uiNumBytesToRead)

{

    

    // Set destination address for DMA

    *pDMA5_START_ADDR = pDstBuf;

    

    // Set the count to read in the DMA

    *pDMA5_X_COUNT = uiNumBytesToRead;

    *pDMA5_X_MODIFY = 1;

    // Set the DMA Enable bit, Increment Destination, Don’t Increment Source, 8-bit Trasnfers

    *pDMA5_CONFIG = 0x0003;  //DMA enable, 8 bit transfer, write operation

    

    // Initialize the SPI PORT: Slave, 8-bit Xfer, Clock SPI Opposite

    *pSPI_CTL = 0x4412; //spi slave, spi 8 bit, DMA reciever

    

    // DMA is now ready to receive the data...

}

DMA读程序

void FPSGetTESTImage (unsigned CHAR * pDstBuf)

{

    

    // Initialize and enable the DMA engine

    InitDMARead(pDstBuf, 1703);

    // Send "Get TEST Image" command to the sensor

    FPSWriteData(0x82);

    FPSWriteData(0x40); // Enable Operations

    FPSWriteData(0x80);

    FPSWriteData(0x04); // Enable Operations

    FPSWriteData(0x88);

    FPSWriteData(0x02); // change to 32uS column scan

    FPSWriteData(0xA2);

    FPSWriteData(0x02); // Force Finger On (simulate touching)

    FPSWriteData(0x98);

    FPSWriteData(0x2A); // Enable Histogram data

    FPSWriteData(0xAA);

    FPSWriteData(0x10); // image = 31 byte repeating pattern after E0 HEADER:

           FPSWriteData(0xAB);

    FPSWriteData(0x00);

    FPSWriteData(0x81);

    FPSWriteData(0x44); // Request 1 image slice

    //while (*pDMA5_CURR_X_COUNT != 0)

/*    while ((*pDMA5_IRQ_STATUS & DMA_RUN) != 0)

    {

        printD(".");

        printD("%4X\n",*pDMA5_IRQ_STATUS);

        printD("%4X\n",*pDMA5_CURR_X_COUNT);

        ezDelay(1);                

    }

*/

    ezDelay(3);

    printD("%4X\n",*pDMA5_IRQ_STATUS);

    printD("%4X\n",*pDMA5_CURR_X_COUNT);

         *pDMA5_CONFIG = 0x0002;  //DMA disable, 8 bit transfer, write operation

    printD("%4X\n",*pDMA5_IRQ_STATUS);

    // Now the TEST image data is available in the data buffer

}