v4 crystal layout recomendations

August 30, 2010, 09:06:48 am

Here are some comments about the crystal layout in the BPv4 PCB.

One big issue is that the crystal you've selected has two ground pins which you've allowed to float. That's probably because the Eagle package does not show these pins. Search for a proper package or edit your own. Then connect those two pins to ground. This will shield the case of the crystal and suppress the broadcast of MHz EMI in the general area.

As easy issue related to the above is the two bypass caps for the crystal. They should be placed as close as possible to the crystal. Once you ground the two floating pins, then it will be really easy to place those caps right where they'll connect to both ground and the related half of the crystal.

Another big issue concerns stray capacitance of the traces leading to the crystal. It's a good idea to place a grounded trace in between the two crystal traces, starting as close as possible to the PIC. You obviously can't run a ground trace between the PIC pins, but if you move around the traces that you already have then you could run a ground trace very close to the PIC. This ground trace will help prevent crosstalk between the opposite sides of the crystal which is equivalent to [s:]have[/s:] having a capacitor short out the crystal.

These are all fairly minor changes physically, but the performance of the oscillator circuit will be improved. Microchip may even have these recommendations in the PIC Data Sheet or in a related Application Note.

Re: v4 crystal layout recomendations

Reply #1 – August 30, 2010, 05:25:13 pm

Thank you, noted and will be done.