Dave Van Horn wrote:

>>Okay, I guess that "leads" me to another question or two.  How should I tie in
>>the 2nd ground pin on an F876?  And why is it even there for that
>>matter?  I was
>>just going to connect it to the other ground pin by running a trace underneath
>>the part between them.
>
>If one of the grounds is very close to the xtal pins, that would be a clue.
>See http://www.dvanhorn.org/Micros/All/Crystals.php
>Count on this. If there are multiples, you need all of them.
>

This is one of those points with multiple opinions. As I mentioned, some
people think this gnd pin is for your analog signals. OTOH, you have Dave's
answer. In my case, I have always used a small gnd trace "island" encircling
my xtal and bypass caps, which I have always tied to this gnd, and my analog
circuitry [opamps/etc] has a separate gnd trace that goes striaght back to
the power injection point on the board.

Unfortunately, you can see that this violates the idea of having a short
noise-free gnd connection from the analog circuitry to the A/D converter
[in this case on the PIC] with a single-point gnd connection between
analog and digital under the A/D. It seems you cannot have it both ways
:).
=============

>>I believe I read one post in there about using thinner ground traces for high
>>frequency digital, so that it would cut down on the amount of frequency that
>>gets back to the central connecting point.
>
>BAD idea.
>Ground fat, VCC thin.
>

I notice I didn't fully complete my answer in my previous msg. Personally,
on 2-layer boards, I always use as wide gnd traces as possible [50-70 mils]
running on one side of the pcb directly under the digital chips, and also
use wide Vdd traces [30-50 mils]. Also, connections between the Vdd pins
and bypass caps are made as wide as possible, with the caps as close
as possible to the Vdd pins.

Dave has a slightly different take on this, but he has done a lot of
testing, and his boards may be less noisy than mine.

IIRC, one thing they do recommend when making a single-point connection
between analog and digital gnds, when made "under" the A/D, is to make
that trace thin - so DC currents will pass, but digital noise will be
somewhat blocked.

Is your head spinning yet? [it should be - this stuff is complicated,
and there are more hard questions than easy answers :)].

best regards,
- dan michaels
www.oricomtech.com
====================

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu