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XTAL2 Layout

BuffaloEngineer
Intellectual 680 points
Other Parts Discussed in Thread: MSP-TS430PN80USB

There's an app note on the layout of a 32kHz oscillator for XTAL1, is there a layout guide for XTAL2?  

I was looking at the Launchpad gerber files which has a continuous GND plane under 32kHz XTAL1, even though the app not states that it should have a "GND Island isolated by a gap from the rest of the GND" - is this a take it or leave it recommendation?  

Is it recommended to have an isolated GND plane under XTAL2?  

  • 0
    Guru 46710 points

    BuffaloEngineer said:
    Is it recommended to have an isolated GND plane under XTAL2?  

    Yes. It will not hurt. Thou you shall connect this "island" to GND plane in place close to XTAL pins, it shall not be "galvanically isolated island".

    EMI-sensitive parts as XTAL crystals needs good shielding, but GND plane with currents flowing through it is inferior to GND island.

  • 0 in reply to Ilmars
    Intellectual 680 points

    Thanks - I'm wondering why they didn't do that with the Launchpad, it looks like they have plenty of space...

  • 0 in reply to BuffaloEngineer
    Guru 46710 points

    BuffaloEngineer said:
    Thanks - I'm wondering why they didn't do that with the Launchpad, it looks like they have plenty of space...

    Perhaps they decided that low power LaunchPad does not need any additional EMI precautions.GND plane currents are so small so they can't be source of EMI so no need for GND islands under sensitive parts

  • 0
    Guru 227245 points

    A good idea is to have a guard ring on the side with the XTAL traces (GND wires that shield the XTAL wires against crosstalk). On the other side, there should be a GND polygon that ends at the guard wires and has only one connection to system GND. If the other side is a GND plane, then the guard wires of the upper plane should be a gap on the GND plane, so that no currents will cross the GND plane below the XTAL wires and the crystal. Again, only a single-point connection to the GND plane.
    This setup should be used for all crystals, even though high-speed crystals usually are more robust than the watch crystals, as they are driven with much higher power.

  • 0 in reply to Jens-Michael Gross
    Guru 14640 points

    I have a MSP-TS430PN80USB board (Rev. 1.2) and MSP-TS430PZ100USB board (Rev 1.25). Both of these boards look like they have guard bands implemented around XT1. I tried to confirm with the MSP430 Hardware Tools User Guide (slau278q), but on the pages associated with those boards, there is no mention of guard bands.

    Can someone confirm that these boards DO in fact have guard bands on XT1?

  • 0 in reply to Todd Anderson78572
    Guru 227245 points

    The MSP430 uses a normal Pierce oscillator, so the recommendations for PCB layout are not MSP specific and apply to all situations where a crystal is used on a pierce oscillator.
    The 32KHz crystal got special attention because the MSP operates in very low power configuration here. So special precaution is required beyond the common considerations and rules.

    About your question: you have the boards. Can’t you simply look at them? And use a multimeter to check the electrical connection?

  • 0 in reply to Jens-Michael Gross
    Guru 14640 points

    Jens-Michael:

    By observation, there appears to be a separate ground plan under the LF crystal. It is a little more difficult to see if there is a guard ring around the crystal. The hardware reference guide does not go into that detail. That is why I posed the question.

  • 0 in reply to Todd Anderson78572
    Guru 227245 points

    I don’t have either of the two boards, but on my MSP-TS430PZ5x100 (non-USB), there seems to be no guard trace beside the XT2 signal traces.

    It seems that the layout was done in favor of making the crystal easier to add. Also, the two signals go to the breakout connector too. And the socket with three lines of pins rather than the lined-up pinning of the real chip, made complete guard traces impossible anyway.

    The drawback/limitation of multi-purpose boards.

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