ADAU1701 DSP Crystal stop oscillating

We have been in production with an ADAU1701 design for 3 years but we have recently been seeing failures that we have identified as the DSP crystal oscillator stopping.
This has probably been happening for a while as we have had a few products with unexplained failures that sound like this fault in hindsight.

The product will happily run for multiple hours or days and then the crystal stops oscillating. a power cycle normally fixes the problem.

During the development we had issues with the crystal oscillator and submitted a post for help on the layout.

Dave Thib kindly helped and reviewed a revised layout which is used in production pcb so we hope this is good. The original post is here,

https://ez.analog.com/dsp/sigmadsp/f/q-a/64396/adau1701-1401-recommended-crystal-layout

Question, can you provide any suggestion as to what may be causing the crystal oscillator to extremely intermittently stop.

Are there any suggested tests we could do to identify the fault.

The crystal used is an Abracon ABLS-12.288MHZ-B4-T.

(The schematic says a ADAU1401 but we use the ADAU1701 is production)

The final PCB layout is below, It is a 4 layer design with a ground plane directly under the top layer

We have tried the following,

  • Thermally stressed the unit both hot and cold with no effect
  • Changed the drive resistor to 200R and 0R0 which appeared to do nothing
  • Performed readback every 100ms on the DSP to catch when it stops responding to SPI commands. This indicates the clock has stopped

We are struggling to think what else to test as the clock is such a self contained block in the DSP with little testing possible.

Parents
  • Hello,

    Can you measure the crystal drive level in your PCB?

    Take a look at my post:

    https://ez.analog.com/dsp/sigmadsp/f/q-a/544232/eval-adau1701miniz-high-crystal-drive-level

    The drive level should be measured right after the dumping resistor. 

    A high drive level can cause the crystal to age, and even to damage it over time.

    I've asked analog devices to explain why the crystal waveform is distorted (measured in the evaluation board), but they are ignoring my requests.

    Nir

  • I have attached both the product xtal waveform and as a comparison the analog evaluation board EVAL-ADAU1701MINIZ.

    They both look near identical. I would sincerely hope that the evaluation board does not have an incorrect crystal drive.

    Product XTAL Input

    ADAU1701 Evaluation Board XTAL input

    Product XTAL Output

    ADAU1701 Evaluation Board XTAL Output

  • Hi,

    Thanks for performing the measurement - the waveforms looks exactly like in my PCB

    The sine wave is distorted and have an amplitude of 3.5Vp-p, right?

    Thake a look in the ADAU1701 PDF, page 18:

    The 100 Ω damping resistor on OSCO gives the oscillator a voltage swing of approximately 2.2 V

    In reality, the amplitude is far above 2.2V. the crystal is indeed overloaded, and over time, it can actually "break"

    the crystal (this is an electro-mechanical component)

    Take a look at this video what can a high drive level can cause:

    https://www.youtube.com/watch?v=2Dg_GYXs4Ss

    I use a crystal oscillator in my ARM MCU, and I had never encountered any problems. the amplitude measured in my MCU crystal is 1.5 - 2Vp-p, and the sinewave is not distorted at all. 

    I've tried lowering the drive level by increasing the dumping resistor (using a resistor in the kilo-ohms range) and indeed the distortion wave gone, and the amplitude was much lower (~2Vp-p) but then, the crystal was once again not starting right.

    I strongly suggest you changing the crystal to an external CMOS oscillator - like this one:

    https://www.mouser.co.il/datasheet/2/3/ASCO-245430.pdf

    With an external XO, you will never have a startup issues ever again.

    Nir  

  • 0
    •  Super User 
    on May 16, 2021 3:33 PM in reply to Nir76

         We have made two -1701 products.  One has a packaged oscillator which always works, so that's a good idea.  The other relies upon the internal oscillator with a crystal.  We haven't thought of measuring the crystal drive.  One unit came back where its user reports it quitting when exposed to static electricity -- touching the panel after walking on carpet, for example.  We have been unable to duplicate this issue at our facility, likely because the whole place is ESD proof!  Yet it could very well be that the zap makes its oscillator stop running until power is reset.  The excessive drive problem reminds me of early amateur radio transmitters where a physically much larger crystal feeds back around a repurposed audio output or TV sweep tube.  With the circuit improperly adjusted the crystal could fracture.

         Best regards,

         Bob

  • Hi,

    Is this an official analog devices response to our crystal drive level problem?

    The crystal is operating out of spec, the drive level is way too high.

    If this is normal by the company standards, so please state that this is a given situation and there is nothing that can be done in order to solve this problem.

    Otherwise, please provide a solution to reduce the crystal drive level.

    Nir

Reply
  • Hi,

    Is this an official analog devices response to our crystal drive level problem?

    The crystal is operating out of spec, the drive level is way too high.

    If this is normal by the company standards, so please state that this is a given situation and there is nothing that can be done in order to solve this problem.

    Otherwise, please provide a solution to reduce the crystal drive level.

    Nir

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