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TMS320C6745: Connecting OSCVSS to GND on PCB

Naoki Kawada
Guru 19890 points
Part Number: TMS320C6745

Hello,

My customer has a question about OSCVSS connection. They are connecting xtal under data sheet recommendation. Please take a look at Figure 6-6. On-Chip 1.2V Oscillator in the data sheet:

They examined several noise tests and in some case they found DSP sometimes hang up. They investigated into the issue and found a workaround; connecting OSCVSS  to PCB ground. 
Actually, this is not allowed in the data sheet. It says:

When an external crystal is used, oscillator (OSC_VSS, RTC_VSS) ground must be kept separate from other grounds and connected directly to the crystal load capacitor ground. These pins are shorted to VSS on the device itself and should not be connected to VSS on the circuit board. If a crystal is not used and the clock input is driven directly, then the oscillator VSS may be connected to board ground.

They have confirmed the workaround works in their use case, but because of data sheet statements, they has not decided yet to go with it...
I believe you must have some reasons not to connect OSCVSS to GND for proper device operation. Do you have any comments on this ?

Best Regards,
Naoki

  • 0
    TI__Guru**** 161600 points
    Hi Naoki,

    We're looking into this and will update the thread.

    Best Regards,
    Yordan
  • 0
    TI__Guru**** 161600 points
    Hi Naoki,

    They examined several noise tests and in some case they found DSP sometimes hang up.

    Have they followed the recommendation in the Schematic Checklist (processors.wiki.ti.com/.../_AM1x_Schematic_Review_Checklist They must place the oscillator components close to the SoC package, also they should minimize parasitic capacitance, in order to reduce the noise. Perhaps this is what they are seeing.

    I believe you must have some reasons not to connect OSCVSS to GND for proper device operation. Do you have any comments on this ?

    OSCVSS is a local ground (analog ground) and it should be separated from the board ground. This is somewhat a general rule.. Can they show the corresponding schematic part?

    Best Regards,
    Yordan