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TMS320F280049: Crystal Oscillation Amplitude does not meet VIH

Wilson Pang
Intellectual 590 points
Part Number: TMS320F280049

hi Sir/Miss,

Does anyone know what to do if my crystal amplitude does not meet C2000 min VIH (VDDIO x 0.7)?

If Worst Case VDDIO = 3.37V, Minimum VIH to be recognized by C2000 is 3.37V x 0.7 = 2.359V.

But the max peak 20MHz crystal oscillation X1 (input to C2000) below has typical 2.29V. So it failed to meet VIH.

May I reduce the crystal capacitance load, CL ? Currently it is 18pF for both X1 and X2 end as per C2000 datasheet recommendation.

Shall I reduce to about 12pF to check if the VIH can be met?

Thanks for your advice in advance.

  • 0
    TI__Mastermind 28275 points

    Hi Wilson,

    The electrical oscillator has a comparator that toggles depending on the level of X1. If X1 drops too low, it will stop toggling and there will be no clock out. If the voltage swing on X1 and X2 is low, it means the overall gain of the crystal circuit is reduced. Using a low CL can help but keep in mind CL to gain relationship is a parabolic one which means there is an optimal range of CLs to use to give the best circuit gain. This is the range of CLs we have specified in our datasheet.

    Your best bet will be to use a crystal with a lower ESR or switch to a 10MHz as the gain is higher the lower the frequency of the crystal.

  • 0 in reply to Frank Ankapong
    Intellectual 590 points

    Hi Frank,

    Thanks for the advice.

    I tried replacing the crystal capacitance load 18pF with 10pF. Not much improvement from 2.166V to 2.172V. it still cannot meet worst case VIH 2.359V of C2000.

    Green plot = CL 18pF

    Pink plot = CL 10pF.

    So based on C2000 datasheet, minimum CL required is 12pF, Max is 24pF.

    Therefore, I will need to select another crystal.

    Please let me know if there is any comment.

  • 0 in reply to Wilson Pang
    Intellectual 590 points

    hi Frank and team,

    Another question is, will the internal oscillator INTOSC2 10MHz kicks in when the C2000 crystal lost oscillation because X1 (input to MicroP) level is too low (not hitting VIH)?

    Will there be other diagnostic or indication in any of the register if internal oscillator INTOSC2 10MHz kicks in?

    Will there be any indication in any of the register if we lost external crystal oscillation?

  • 0 in reply to Wilson Pang
    TI__Mastermind 28275 points

    Hi Wilson,

    Yes, if the crystal loses oscillation, the MCD will kick in and switch to INTOSC1. Please look at the "Missing Clock Detection" chapter in the TRM.

  • 0 in reply to Frank Ankapong
    TI__Genius 15385 points

    Hi Wilson,

    Do you need any more information on this topic? If the issue is resolved can we please close this thread?

    Thanks,

    Nirav

  • 0 in reply to Nirav Ginwala
    Intellectual 590 points

    Thanks Nirav and Frank. I think it is closed for now.

    I will check with my System and SW engineers on this.