This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Setting XTAL field different from actual crystal frequency on LM4F232

Hi,

what are the drawbacks on setting XTAL field to 0x15 (as if I had a 16MHz crystal), but the actual crystal is 12.288MHz to have a system clock of 61.44MHz (I need a clock multiple of 12.288MHz)

best regards

  • mastupristi,

         12.288MHz crystal is supported by LM4F232. Set the XTAL field to 0x12.

    -kel

  • Perhaps I have not explained.

    If I set XTAL to 0x12, I get a sysclock of 80MHz.

    I need to generate 192KHz, 384KHz and 768KHz as exact as possible. I need 0% error. timers cannot generate those frequency with requested precision starting from 80MHz sysclock.

    That's why I want to generate 61.44MHz sysclock. 61.44 = 5 x 12.288.

    Another way is to use 24.576MHz crystal and bypass PLL.

    best regards

  • mastupristi said:
    what are the drawbacks

    How about instant loss of JTAG - resulting from mismatch between "actual xtal freq" and your xtal field SW entry.

    It is inventive/clever idea - but don't believe tolerated by the Stellaris function.  Your idea of 24.xxx MHz xtal - and entry of that value w/in xtal field - should work nicely...

  • Thank you - appreciated.   However - not all Stellaris will accept > 20MHz xtal - be sure to check electrical specs.  (your MCU datasheet)

    We use M4F w/25 MHz xtal - works fine - but seem to recall earlier/lesser MCUs may not accept that high a Fxtal...