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PCB Layout for the Crystal for the F28035

Michael Stout
Genius 16890 points

Other Parts Discussed in Thread: TDC1000, TDC1000-C2000EVM

Hello,

We have a customer that is wondering if the ground plane of the PCB should be cut out from underneath the crystal. 

Description of the problem from the customer:

 -----

We are seeing instantaneous ToF jumps in values randomly on level, temperature and concentration.  Running the TI software on the TI evaluation board (TDC1000-C2000EVM), we see no issues.  To narrow down the problem with our board, we commented out the software to not read level or concentration.  Thus eliminating 65% of the TDC1000 hardware (LNA,  PGA, TX1,TX2, RX1, RX2, and the high voltage (35v) pulse we use for level) from the circuit/test  and problem causer.

Running just temperature, we saw the Temperature ToF jump randomly.  Sometimes 25 mins apart, sometime we see calm and quiet for 36 hours.  All of this is on a bench in an office environment. 

So we got some super-fast scopes and starting triggering when the TOF value jumps and were looking at the START and STOP before and after the diodes.  (The START and STOP are the pulses from the TDC1000 to the C2000.  The C2000 measures the time between them to calculate the temperature.) They never moved.  We watched the 3.3VDC and 4.5vdc at the same time and saw no movement during the temperature jump.  That took us to the thought that the clock inside the micro was losing count in the START STOP envelope.

We disconnected the 20 MHZ crystal from the micro and shot a signal generator into the micro and the problem “seemed” to disappear.  I say seemed because we haven’t done it for 48 hours yet.

We then started looking at the TI evaluation board and we noticed that the solid ground plane and the solid power plane below the crystal was cleared on the board.  Was that done for a specific reason or just good practice?  We are trying duplicate the cut-out by lifting our crystal but we would like to discuss your thoughts on our issue.

----

Are there any general guidelines about laying out the area around and underneath the crystal that is used with the F28035?

The TI board is shown in document SNIU023, FIgure 14.  This board does not give any errors at the customer.  Their board has ground where we have the cut out.

Thanks,

Mike

  • 0
    TI__Expert 7885 points
    Hello Mike,

    I am not quite convinced that the XTAL would be causing this issue as the switching of the XTAL is constant and the failure seems to be random. You stated that the crystal has been removed and replaced by a signal generator with no failure. Have you also tried removing the signal generator and XTAL and running from the internal oscillator?

    Is the concern that the XTAL itself is acting as an antenna and coupling random noise into the device which is causing the failure?

    I understand the reasoning for creating the cutout from the GND plane beneath the XTAL, but at the same time there is not a dedicated GND for the XTAL circuit which is separate from the F28035 GND so any noise seen by the XTAL could still be coupled to the plane even with the cutout.

    Best Regards,
    Adam Dunhoft