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TMS570LC4357-SEP: TMS570LC4357-SEP

Jerome TUAL
Prodigy 40 points

Part Number: TMS570LC4357-SEP
Other Parts Discussed in Thread: TMS570LC4357

Hi,

I’m looking for technical guidance regarding the grounding of a 4‑pin crystal connected to the TMS570LC4357. The TMS570LC4357 provides a dedicated Kelvin GND reference for the oscillator.

But our crystal has two GND pins (pin 2&4) : image.png

My understanding is that the ground pin of the crystal should be connected to the MCU’s Kelvin GND, just like the load capacitors C40 and C42. In this configuration, the ground guard ring around the crystal would also be tied to Kelvin GND. If yes I should to update my schematic as following : 

image.png  

OR Should the crystal’s two GND pins instead be connected to the main PCB GND image.png, with the guard ring referenced to the board ground rather than Kelvin GND?

Thanks in advance for your support.

Jerome.

  • 0
    Prodigy 40 points

    HI,

    I haven’t received any reply to my question, and I’m hoping someone can help.

    Thanks, Jerome

  • 0 in reply to Jerome TUAL
    TI__Guru**** 200366 points

    Hi Jerome,

    The crystal’s local load capacitors and ground connections must be routed to the KELVIN_GND pin.

    To protect the highly sensitive oscillator circuit from digital switching noise:
    • The ground pads of the two external crystal load capacitors (C1 and C2) must be tied together and connected directly to the KELVIN_GND pin
    • KELVIN_GND must remain an isolated island on your PCB. Do not connect this trace to your main system ground plane, digital ground, or analog ground.
    • If you are using a 4-pin surface-mount crystal package where the extra two pins are tied to the metal case for shielding, those casing ground pins must also be tied to KELVIN_GND, keeping them entirely separate from the main PCB ground plane.