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TMS320C5535: External crystal oscillator for USB 12MHz oscillation circuit

user5855205
Prodigy 90 points
Part Number: TMS320C5535

I would like to use a 12MHz crystal unit.

Although the data sheet had the circuit diagram attached, we would like to add a general limiting resistor Rd in the oscillator circuit in order to lower the drive level of the oscillator circuit. Is there a problem?

Are there any precautions such as restrictions on Rd?

  • 0
    TI__Mastermind 30140 points

    Hi,

    I think there is no problem. Recommend you to stay within the crystal datasheet limits.

    You can always short across Rd so it can do no harm to the circuit

    While your at it, consider adding pads for Rbias across the crystal if not there already. Load caps are also important.

    Feel free to attach the schematic section or crystal information.

    -Mark

  • 0 in reply to Mark M
    Prodigy 90 points

    Thank you for your answer.
    As C5535, I understand that there is no particular restriction on the resistance value of Rd.
    The problem has been resolved.

  • 0 in reply to Mark M
    Prodigy 90 points

    I have additional questions.

    How should I consider the maximum value of Rd? The ESR of the C5535 data sheet is 100Ω max. Does the value obtained by adding the ESR of the crystal and the value of the limiting resistor have to be within 100Ω?

    I also didn't quite understand "Rbias across the crystal". Specifically, it would be helpful if you could show what kind of circuit in the figure.

  • 0 in reply to user5855205
    TI__Mastermind 30140 points

    Hi,

    user5855205 said:
    How should I consider the maximum value of Rd? The ESR of the C5535 data sheet is 100Ω max. Does the value obtained by adding the ESR of the crystal and the value of the limiting resistor have to be within 100Ω?

    Can you confirm the placement of your Rd is between OSC_XO and the crystal?

    The crystal has some maximum power dissipation, exceeding this can cause the crystal to age early or operate poorly.

    Find the max value of Rd experimentally by increasing Rd until crystal stops oscillating, then use 1/3 of this resistance for Rd for margin.
    Ideally this should be done across process, voltage, and temperature so try it with a few different devices.

    Rd is not related to impedance of crystal to drive OSC_XI. ESR is related to only to crystal drive strength.

    user5855205 said:
    I also didn't quite understand "Rbias across the crystal". Specifically, it would be helpful if you could show what kind of circuit in the figure.

    Rbias is a 1Megohm resistor parallel to the crystal to ensure it starts up.
    Its a weak resistor from output of an inverter to its input, and will oscillate with a period that is the propagation delay.

    Most oscillators have internal feedback resistor for this, but it does not hurt to have this additional Rbias also.

    Once the crystal begins oscillating the 1MegOhm is so weak, the crystal will dominate.

    For BOM reduction, you can experiment depopulating Rbias, but its a lot harder to add it later if it turns out to be necessary.

    Regards,
    Mark

  • 0 in reply to Mark M
    Prodigy 90 points

    Thank you very much for your explanation. Thanks to you I was able to understand the circuit.