Em 23/8/2011 03:27, Jesse Lackey escreveu:
> Hi - crystal was a SMT HC49 package (pretty large but cheap).  What I=20
> did was have the xtal so that one side of it was close to one of the PIC=
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> pins, but the other side was far away, i.e. rather than having the xtal=20
> be located parallel to the PIC (so the two traces connecting the PIC and=
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> the xtal were close to the same length), I had the xtal be 90deg rotated=
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> and so it was "perpendicular" to the PIC and it had one very short trace=
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> 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 lengths=
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> pretty much the same.  Pay more for smaller xtal if needed.  Have ground=
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> plane (on the other side of the pcb) be under xtal and traces if possible=
..


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".

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).

Best regards,

Isaac

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