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F28069U: USB with internal oscillator???

Cyril Lance
Intellectual 270 points


Hey Everyone,

I am trying to get the F28069U up and running with the embedded USB hardware on the processor.

Can anyone confirm definitively that the USB will work with the internal oscillator?  (i.e. no external crystal)

Thank you!

Cyril

  • 0
    TI__Guru* 81560 points

    Cyril,

    It will not.  The 0-pin oscillator has an out of the box 4% accuracy over temperature and a temp compensated accuracy of <1% all about 10MHz.  USB tolerances are much much tighter than even the 1% option.  External clock will be needed to guarantee USB operation timings are met.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    Intellectual 270 points

    Thanks so much for the quick answer Matthew.

    Cyril

     

  • 0 in reply to Cyril Lance
    TI__Expert 4070 points

    Hi Mathew,

    This is very confusing from what the data sheet says.  It seems to clearly indicate that if "the oscillator compensation routine" is called frequently the internal oscillator could be used.  (see below from current F28069U datasheet)

    Is there a case where this is true?  (this may be my ignorance of USB but something doesn't add up :))

    Steve S

     

     

     

     

    3.9.4 USB and HRCAP PLL Module (PLL2)

    In addition to the main system PLL, these devices also contain a second PLL (PLL2) which can be used to

    clock the USB and HRCAP peripherals. The PLL supports multipliers of 1 to 15 and has a fixed

    divide-by-two on its output.

    PLL2 may be clocked from the following three sources by modifying the PLL2CLKSRCSEL bits

    appropriately in the PLL2CTL register:

    •

     

     

    INTOSC1 (Internal Zero-pin Oscillator 1): This is the on-chip internal oscillator 1 and provides a

    10-MHz clock. If used as a clock source for USB or HRCAP, the oscillator compensation routine

    should be called frequently.

    •

     

     

    Crystal/Resonator Operation: The (crystal) oscillator enables the use of an external crystal or resonator

    attached to the device to provide the time base. The crystal or resonator is connected to the X1/X2

    pins.

    •

     

     

    External Clock Source Operation: This mode allows the reference clock to be derived from an external

    single-ended clock source connected to either GPIO19 or GPIO38. The XCLKINSEL bit in the XCLK

    register should be set appropriately to enable the selected GPIO to drive XCLKIN.

    NOTE

    For proper operation of the USB module, PLL2 should be configured to generate a 120-MHz

    clock. This will be divided by two to yield the desired 60 MHz for the USB peripheral.

    HRCAP supports a maximum clock input frequency of 120 MHz.

  • 0 in reply to Steve Sager
    TI__Guru* 81560 points

    Steve,

    This is an error in the datasheet for this device; thank you for bringing it to my attention.  We will correct it in the next revision of this DS. 

     

    The bullet should read for usage with HRCAP peripheral only for internal 0-pin oscillator on PLL2.  The oscillator in question does not fulfill the USB spec in terms of jitter/accuracy and USB should only be clocked from an external clock source.

    Best regards,

    Matthew