tm4c129encpdt ethernet (can not download)

The tm4c129encpdt cpu will be coming down  when I debug my ethernet function program on tm4c129encpdt frist time,but the cpu can not work while debug my program second time and the Keil give me a error message.And The compiler give me the same error message when downloading every time.

int
main(void)
{
    uint32_t ui32User0, ui32User1;
    uint8_t pui8MACArray[8];

    //
    // Make sure the main oscillator is enabled because this is required by
    // the PHY.  The system must have a 25MHz crystal attached to the OSC
    // pins. The SYSCTL_MOSC_HIGHFREQ parameter is used when the crystal
    // frequency is 10MHz or higher.
    //
    SysCtlMOSCConfigSet(SYSCTL_MOSC_HIGHFREQ);

    //
    // Run from the PLL at 120 MHz.
    //
    g_ui32SysClock = MAP_SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ |
                                             SYSCTL_OSC_MAIN |
                                             SYSCTL_USE_PLL |
                                             SYSCTL_CFG_VCO_480), 120000000);
    /**/
    //
    // Configure the device pins.
    //
    PinoutSet(true, false);

    //
    // Configure UART.
    //
    //UARTStdioConfig(0, 115200, g_ui32SysClock);

    //
    // Clear the terminal and print banner.
    //
    //UARTprintf("\033[2J\033[H");
    //UARTprintf("Ethernet lwIP example\n\n");

    //
    // Configure Port N1 for as an output for the animation LED.
    //
    MAP_GPIOPinTypeGPIOOutput(GPIO_PORTN_BASE, GPIO_PIN_1);

    //
    // Initialize LED to OFF (0)
    //
   MAP_GPIOPinWrite(GPIO_PORTN_BASE, GPIO_PIN_1, ~GPIO_PIN_1);

    //
    // Configure SysTick for a periodic interrupt.
    //
    MAP_SysTickPeriodSet(g_ui32SysClock / SYSTICKHZ);
    MAP_SysTickEnable();
    MAP_SysTickIntEnable();

    //
    // Configure the hardware MAC address for Ethernet Controller filtering of
    // incoming packets.  The MAC address will be stored in the non-volatile
    // USER0 and USER1 registers.
    //
  ui32User0 = 0x00123456;
  ui32User1 = 0x00123456;
    //MAP_FlashUserGet(&ui32User0, &ui32User1);
    if((ui32User0 == 0xffffffff) || (ui32User1 == 0xffffffff))
    {
        //
        // We should never get here.  This is an error if the MAC address has
        // not been programmed into the device.  Exit the program.
        // Let the user know there is no MAC address
        //
        UARTprintf("No MAC programmed!\n");
        while(1)
        {
        }
    }

    //
    // Tell the user what we are doing just now.
    //
    //UARTprintf("Waiting for IP.\n");

    //
    // Convert the 24/24 split MAC address from NV ram into a 32/16 split MAC
    // address needed to program the hardware registers, then program the MAC
    // address into the Ethernet Controller registers.
    //
  
  //ui32User0 = 0xff032001;
  //ui32User1 = 0xff032001;
  
    pui8MACArray[0] = ((ui32User0 >>  0) & 0xff);
    pui8MACArray[1] = ((ui32User0 >>  8) & 0xff);
    pui8MACArray[2] = ((ui32User0 >> 16) & 0xff);
    pui8MACArray[3] = ((ui32User1 >>  0) & 0xff);
    pui8MACArray[4] = ((ui32User1 >>  8) & 0xff);
    pui8MACArray[5] = ((ui32User1 >> 16) & 0xff);

    //
    // Initialize the lwIP library, using DHCP.
    //
    lwIPInit(g_ui32SysClock, pui8MACArray, 0, 0, 0, IPADDR_USE_DHCP);

    //
    // Setup the device locator service.
    //
    //LocatorInit();
    //LocatorMACAddrSet(pui8MACArray);
    //LocatorAppTitleSet("EK-TM4C1294XL enet_io");

    //
    // Initialize a sample httpd server.
    //
    httpd_init();

The cpu come down when debugging get to  "pui8MACArray[5] = ((ui32User1 >> 16) & 0xff);"

 Is The CPU locked?

regards

sam