Cannot run USB and GPT at the same time

Hi all.

I'm using the C5515 eZdsp and CSL v2.10. I've set up USB using the USB-DMA CSL example successfully. Then I've set up the GPT for periodic timer interrupts using the CSL example. GPT alone works. Okay I have some small issue about the interrupt interval, see * at end of post. USB alone works as well. But if I want to run both, USB doesn't work anymore!

Here are my code snippets:

interrupt void gpt0isr (void) {
    IRQ_clear(TINT_EVENT);
    CSL_SYSCTRL_REGS->TIAFR = 0x01;
    dataReady = TRUE;
}

void init_timer (void) {
    // ...
    hGpt = GPT_open (GPT_0, &gptObj, &status);
    // ...
    status = GPT_reset(hGpt);
    // ...
    // Clear any pending interrupts
    IRQ_clearAll();
    // Disable all the interrupts
    IRQ_disableAll();
    // plug&enable Timer interrupt
    IRQ_setVecs((Uint32)(&VECSTART));
    IRQ_plug(TINT_EVENT, &gpt0isr);
    IRQ_enable(TINT_EVENT);
   
    // Configure GPT module
    hwConfig.autoLoad      = GPT_AUTO_ENABLE;
    hwConfig.ctrlTim      = GPT_TIMER_ENABLE;
    hwConfig.preScaleDiv = GPT_PRE_SC_DIV_3;    // div Sys Clk by 16 -> 160ns period
    hwConfig.prdLow      = 0x4E20;                // prd low interrupts at 3.2ms period
    hwConfig.prdHigh      = 0x0000;

    status =  GPT_config(hGpt, &hwConfig);
    // ...
    // Enable CPU Interrupts
    IRQ_globalEnable();

    // Start the Timer
    status = GPT_start(hGpt);
}

void init_all() {
    // ...
    init_usb(); // as in the USB-DMA example
    // ...
    init_timer(); // similar to GPT example, see below
    // ...
}

I think it is an interrupt issue because if I comment out the IRQ-CSL calls in init_timer() USB works properly (and the timer probably as well).
But how can I solve this issue?

* And then there's the other, "small" issue with respect to the timer period. Is my setting hwConfig.prdLow = 0x4E20 correct with hwConfig.preScaleDiv = GPT_PRE_SC_DIV_3 to have interrupts at 3.2 ms? What's strange is that I observe the interrupts (on the scope) with a PRD of 7.04 ms. But when I set the value of prdLow to 0xEA60 i suddenly have 9.6 ms periods on the scope. So maybe I did not understand something correctly about those timers... Could you help me get the settings right?

Thank you very much for your help.

Andreas