PCONP |= 0x80000000;        /* Turn On USB Power CLK */
    PINMODE3 |= 0x30000000; // VBUS pull-down

    OTG_CLK_CTRL = 0x1A;
    while ((OTG_CLK_STAT & 0x1A) != 0x1A);

    OTG_STAT_CTRL=3; //3-U2,0-U1

  
    /* P2.9 is USB1 SoftConnect, P1.18 is GoodLink, P0.29 is USB1_D+,
    P0.30 is USB1_D-. PINSEL1 bit 26~27 0x01, bit 28~29 0x01.
    PINSEL3 bit 4~5 0x01. PINSEL4, bit 18~19 0x01. */
    PINSEL1 &= ~0xC0000000;
    PINSEL1 |= 0x40000000;

    /* P1.30 is VBus, for testing purpose, I will try both VBus enable and
    GPIO for polling. P1.30, PINSEL3 28~29, 0x10. It should not make any difference
    on the Keil MCB2300 board. */
    PINSEL3 &= ~0x30000000;
    PINSEL3 |= 0x20000000;     // (~Overcount) USB2 off by ?

    /* SoftConnect is GPIO*/
    FIO0DIR |= (1 << 14);
    FIO0SET = 1 << 14;

static BOOL BulkDataOut = TRUE;
static BOOL BulkDataIn = FALSE;


void MSC_BulkOut()
{
    BulkDataIn = TRUE;
}


void MSC_BulkIn()
{
    BulkDataOut = TRUE;
}



void USB_EndPoint2 (DWORD event) {

  switch (event) {
    case USB_EVT_OUT:
      MSC_BulkOut();
      break;
    case USB_EVT_IN:
      MSC_BulkIn();
      break;
  }
}



.....
и далее где-то в обработчике пакетов USB


    while (1)
    {
        while (!BulkDataIn);
        
        BulkLen = USB_ReadEP(MSC_EP_OUT, BulkBufIn);
        BulkDataIn = 0;

                                .... тут что-то делаем

                                .... отсылаем данные как-то так
        while (!BulkDataOut);
        USB_WriteEP(MSC_EP_IN, (BYTE*)&flash_addr, 4);
        BulkDataOut = 0;

                 }