XTAL connection question.

[kvresto]

Hi Everyone.

Every now and again I find myself asking a question on something I would never have thought about ordinarily. This is one of those days. I’ve included 2 pics of an SMD xtal (12Mhz), each connected to my CPU in a different configuration, and my question is,.. are there any pro’s and cons on which may be the best configuration, or doesn’t it matter?

Thanks
kv

[okaresz]

In my experience it does not matter much. Of course you try to match the lengths of the XIN and XOUT traces, but I just made a mistake and forgot to do this in my recent design (see attached picture), but it works fine. At these relatively low frequencies this is not so crucial.
(R55 is just a series 47R to prevent XTAL from possible overdrive). The important is to keep the crystal close to the MCU.

[T3sl4co1l]

So what's the difference?

BTW, the vias are too close to the pads, and will absorb solder paste.  Not really any point in using via per pad anyway, these are modest impedance, limited bandwidth traces.  Length doesn't matter (as long as it's not obscenely long), grounding doesn't matter.

Tim

[okaresz]

I have full tenting on the vias, so solder mask covers them, they came back from reflow just fine. But yeah, if someone wants to be on the very safe side, leave a larger distance from the pads, like in kvresto's layout.

[voja]

Oscillator I/Os are analog signals, so you should avoid placing them close to long digital signal tracks, as they could be inductively coupled. It is also good practice to put the ground track around the Xtal and its tracks. Some ICs even have Xtal pins surrounded by Gnd pins.

[Pjotr]

As pointed out, placement orientation doesn't matter much. But you have routed signal traces in parallel to the crystal traces. Noise from that traces impairs the phase noise of the oscillator. In most applications it is not that important but if you need a "clean" oscillator it matters. Best is to surround the xtal traces with ground copper as much as possible and keep noise away from your xtal traces.

[edit]

Voja said the same

[kvresto]

Thanks everyone for your good advice, much appreciated.

cheers