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[FAQ] AM623: MCU_OSC0_XI Drive Level

Kieran
Prodigy 90 points
Part Number: AM623

Hi,

I am having trouble locating the drive power of the oscillator MCU_OSC0_XI. I am driving an external quartz crystal using this pin but need to make sure the crystal is appropriately rated for the power dissipation it will experience.

Thanks,

Kieran

  • 0
    TI__Guru 63145 points

    Crystal power dissipation depends on crystal circuit variables that need to be considered.  The high frequency oscillator implemented in AM62x has an Automatic Gain Control (AGC) circuit that adjusts its drive strength to achieve resonance, which is another variable associated with crystal power dissipation.  Therefore, it is not possible for TI to define a power dissipation parameter for every crystal circuit implementation. The system designer would need to measure the voltage across their crystal circuit and use the following formula to calculate power dissipated in the crystal.

    The crystal power dissipation can be calculated by the following formula.

    Pxtal = 0.5 ESR (2 π Fxtal CL Vxtal)2, where ESR is the Effective Series Resistance of the crystal, Fxtal is the resonate frequency of the crystal, CL is the capacitive load of the crystal circuit, and Vxtal is the peak-to-peak voltage across the crystal.

    Regards,
    Paul

  • 0 in reply to peaves
    Prodigy 90 points

    Hi Paul,

    I guess what I am really looking for is the V_PP or V_RMS across the crystal. Is that information available?

  • 0 in reply to peaves
    TI__Guru 63145 points

    I forgot to mention an important point. Most crystal datasheet parameter values represent the entire family of crystals defined by the datasheet, where the range of values may be much larger than the values for a specific crystal part number. For example, the datasheet may define a maximum ESR value that applies to a range of crystal frequencies, where the actual ESR of a crystal typically decreases with frequency. So, the maximum ESR value in the datasheet could be significantly larger than the maximum ESR of your part number specific crystal if the datasheet defines a family of crystals. You should contact the crystal manufacture and request a part number specific datasheet, which should provide more accurate parameter values.

    Regards,
    Paul

  • 0 in reply to Kieran
    TI__Guru 63145 points

    The AGC will adjust automatically adjust the output voltage as appropriate to maintain oscillation.  So, the voltage across the crystal will depend on your crystal circuit implementation. I would expect it to be approximately 1V, but you need to measure it with your actual crystal circuit if you want to be sure.

    Regards,
    Paul