ADAU1701 12.288MHz clock don't always start=

Hi Korey,

One of Murphy's famous laws states, "A self-starting oscillator won't." Crystal oscillators seem to be the worst of the bunch, they get downright fussy. Brett G. posted an example of a PCB layout that appears perfectly fine yet proved unreliable at https://ez.analog.com/message/33073#33073

You'll find quite a theoretical treatment of this subject (perhaps more than anyone wants to know!) in (sorry) Microchip's app note AN849, which you can view here:

http://ww1.microchip.com/downloads/en/appnotes/00849a.pdf

Bob

Thanks Bob,

I have vias at the caps so I will give this a try.

Korey

I have seen other cases where poor grounding or power supply bypassing caused excess noise on the chip supply which makes it difficult for the oscillator to maintain a constant frequency and for the PLL to lock. Solid ground and tight power supply bypassing can help a great deal in resolving these kinds of problems as well. Here's a list of a few best practices to follow: http://ez.analog.com/docs/DOC-1889

I agree with Bob that the vias could be causing reflections that are creating clock edges where they should not be, which could further "confuse" the system and prevent it from starting up reliably.

THank you both for your input. I have cut the vias and installed jumpers directly to the regulator ground and the clock seems to starting consistantly. I also changed to a 27pf and 33pf to put my load capacitance above 18pf with additive stray capacitance. This was a little confusing on the datasheet since it shows 22pf and the would give 10pf load plus the stray capacitance. I did see a post that kind of addressed and passed it to the particular xtal being used. So, in summary, I:

1) Tied the 'rock' load caps to ground directly without going through vias and ensured no sharp (90 degree) bends in any traces.

2) Changed to a 7pf shunt 'rock'

3) Changed the load capacitors to meet load capacitances of 18-25pf and also offset them per xtal tips.

Thanks for your help. We will respin and see how it goes!

Korey

Excellent news. Thanks Korey. Please let us know how the respin works.

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