Suggest contacting Dan Beeker, at NXP. Heâs got some good info on crystal
placement based on the latest PC board design for low EMC.
daniel.beeker@xxxxxxx <mailto:daniel.beeker@xxxxxxx>
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On Mar 1, 2022, at 12:51 PM, Mikhail Matusov <matusov@xxxxxxxxxxxx> wrote:
Hi Peter,
First of all, is that a crystal or a crystal oscillator? If the latter I
would say that 25 MHz is very forgiving in most cases. If it is indeed a
crystal, you have to be careful with parasitic capacitances, so basically it
has to be close to the device containing the oscillator. Perhaps, an
evaluation board for the device it connects to could be used as an example.
/Mikhail
-----Original Message-----
From: contact@xxxxxxxxxxxxxxxx
Sent: Monday, February 28, 2022 12:17 PM
To: 'SI-LIST'
Subject: [SI-LIST] Reasonable Recommendations for Crystal Placement
Dear Fellows,
Could anybody please recommend any half-sensible AppNotes/Design Guides
for placing a 25MHz crystal on a PCB? I've done a quick search and a
read a few recommendations but I'm really dubious --skeptical, even--
about the validity of their content. All of them seem to repeat stuff
along these lines:
* absolutely no vias
* no right-angle bends (my favourite)
* total symmetry of everything
* XI should be closer to device pin than XOUT (so much for symmetry)
* single-point ground
* put the crystal on the same side as the device using it
My problem is that none of the recommendations take into account the
number of layers, their arrangement or their spacing. I'd have no
problem putting the crystal and the load caps on the opposite side of
the device and connect them with vias. Why? Because I have more space
and fewer noisy components down there and my parasitics can be kept way
lower than by keeping everything on the same layer and using traces for
connections.
I've reached a point in my career where I have learnt not to trust
AppNotes (except a select few) but haven't quite got far enough where I
can always follow my own lead ('my own lead' ÿhundreds of hours of
reading select literature and papers). Sometimes I need a little
reassuring push... in either the forward or backward direction :)
Many thanks for any and all, who chip in.
PS. is this is considered a noob question for the level of this list,
just let me know and I'll remove it.
Best wishes,-
Peter
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