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AM335x: What are the implications of connecting VSS_OSC to VSS in a system that uses an external crystal

rperezti
Genius 12670 points

We accidentally connected VSS_OSC (Kelvin Ground) to VSS in a system that uses an extenal crystal on XTALIN/XTALOUT. From reading other posts I conclude that this will potentially cause undesired noise on the oscillator circuit, but are there any other side effects of this connection?  For example, can we expect an elevated current on one of the supplies?

More generally, what is the definition of a Kelvin ground reference in the context of AM335x?

 

Thanks,

Ruben

  • 0
    TI__Guru 63115 points

    High frequency digital switching current flowing through inductive substrate traces and bond wires that connect the silicon die to the PCB ground produces a few milivolts of ground noise on the die relative to the PCB ground.

    When the crystal circuit is referenced to the PCB ground and the oscillator input is referenced to the die ground, the oscillator input will see this high frequency ground noise summed with the crystal output voltage.  This can potentially increase clock jitter since the crystal output voltage is a relative slow changing sinusoidal signal and the high frequency ground noise can cause the oscillator input to see slightly shorter or longer clock cycles. 

    The Kelvin ground allows the crystal circuit ground to be at the same potential as the die ground and reduces the oscillator input noise.

    There should not be any change in power supply consumption.

    Regards,
    Paul