> Em 23/8/2011 03:27, Jesse Lackey escreveu:
> > Hi - crystal was a SMT HC49 package (pretty large but cheap).  What I
> > did was have the xtal so that one side of it was close to one of the PI=
C
> > pins, but the other side was far away, i.e. rather than having the xtal
> > be located parallel to the PIC (so the two traces connecting the PIC an=
d
> > the xtal were close to the same length), I had the xtal be 90deg rotate=
d
> > and so it was "perpendicular" to the PIC and it had one very short trac=
e
> > and one much longer trace.
> >
> > Changing cap values didn't change anything in a repeatable manner.
> >
> > So yeah.  Don't do that.  Always have xtal positioned with trace length=
s
> > pretty much the same.  Pay more for smaller xtal if needed.  Have groun=
d
> > plane (on the other side of the pcb) be under xtal and traces if possib=
le.
>=20
>=20
> Although all these recommendations are good, none of them should be
> condition "sine qua non" for the circuit to work.
> Is the OP sure that he used the correct crystal mode configuration for
> the PIC? For instance using a 20MHz crystal with mode "XT" or a 4MHz
> crystal with mode "HS".
>=20
> I already made boards without ground plane (single side), with a large
> crystal perpendicular to the PIC (one track much longer than the other)
> and of phenolic material, that is, ignoring all your recommendations and
> it worked correctly (indeed lots of boards of several different designs).

I am wondering if the long track was the one on the input side of the oscil=
lator - the high impedance side. This could have all sorts of strange picku=
p effects.

I would expect it to work fine with the long track on the output side, the =
low impedance side.


--=20
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