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ADS1263: Oscillator does not start

Steve Hendrix
Intellectual 420 points
Part Number: ADS1263

We have the ADS1263 mounted on a prototype board, orientation checked, soldering checked under a microscope. Avdd is +3.0V, Avss -2.0V, Dvdd +3.4V. There do not appear to be any register settings needed to start the oscillator, but it never runs. the output is high and the input side is low, never moving. Reading the ID register yields 0x00, so clearly something is not running correctly. Hints?

I have done many such oscillator designs, so I know all about keeping traces short, using the correct load caps, etc. Probing is via a standard x10 scope probe. This can often load down an osc when probing the input side, but not the output side. What gives?

  • 0
    TI__Mastermind 44680 points
    Hi Steve,

    I would double check that the nRESET/nPWDN pin is set high. I don't believe the crystal will start-up if the ADC is held in power down mode.

    As a sanity check, I'd also try grounding the XTAL1 pin to enable the internal oscillator. Does communication work when using the internal oscillator? If not, then that would point to a different problem.

    BTW, what is the part # for the crystal you're using?

    Thanks,
    Chris
  • 0 in reply to Christopher Hall
    Intellectual 420 points
    \RESET/\PWDN is at 3.05V.

    As described in my original post, XTAL1 is at a hard ground relative to Dvss.

    The xtal is DigiKey 887-1046-1, a TXC 9C-7.3728MAAJ-T, with 10 pF load caps. I've always had good luck going a little light on the capacitance, to allow a bit of wiggle room for stray capacitance in the layout. In the real world there's always a bit more stray capacitance than in the nice ideal EVB layout.
  • 0 in reply to Steve Hendrix
    TI__Mastermind 44680 points
    Hi Steve,

    If the external crystal is not working then I would expect the internal oscillator to turn on. To be sure, I would suggest removing the external crystal and connecting XTAL1 to DGND/DVSS, just in case the the device is still trying to drive the crystal.

    I agree about going light on the load capacitors, that can usually help with crystal start-up. One thing to note about doing so, is that decreasing the load capacitance will increase the drive level (power dissipation) across the crystal, so you'll want to make sure that the crystal can handle the increased drive strength.

    Have you seen this issue with more than one device or crystal? Crystals can be somewhat fragile and I've had to replace a few after hand soldering them to a PCB. That would be my next suggestion to try.

    Best regards,
    Chris