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32kHz Oscillator Question

BuffaloEngineer
Intellectual 680 points


Having trouble getting the oscillator to stabilize reliably, and I was wondering if there is some guidance in addition to what's in the SLAA322B app note for the oscillator layout.  In figure 7, it shows that there is a GND island isolated by a gap from the rest of GND.  How much of a gap between the isolated GND and the rest of GND is needed?  When making the isolated GND plane under the oscillator, how far past the oscillator footprint should this plane extend?  

  • 0
    Guru 227245 points

    The reason for an isolated GND is to prevent currents flowing below the crystal (most people don't know that the current flowing through GND is not evenly parted, but following its own paths, caused by copper thickness as well as the currents on the opposite side).

    Isolating the GND plane below the crystal keeps these currents away while still providing some EMV shielding.

    The size of the gap is not important, only that there is a gap (except for a single connection point).

    It is recommended to have a small GND wire between the XTAL wires and the adjacent processor pins (as far a s this is possible) too.

    Since the crystal is in a shielded housing, (which should be soldered to GND too), the GND pane does nto need to be much larger than the area beneath the crystal pins and the wires to the CPU.

    Keep in mind that a GND besides the wires may add parasitic capacitance and have an influence onto the capacitive load applied to the crystal. So keep the lines short and a larger gap (between XTAL lines and GND) decreases capacitance.

  • 0 in reply to Jens-Michael Gross
    Guru 58965 points

    Aside from layout, load capacitance and firmware setup, the characteristic and quality of the crystal is also important. What crystal are you using?

    I have used www.microcrystal.com MS3V-T1R 32.768kHz CL:12.5pF +/- 20ppm. They work very well for MSP430.

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