A bit of a follow up...   The unit had the right fuses set (HS).  The design
for this circuit isn't new but the part (18F2520) and the layout are both
new.  The oscillator circuit is straight from the data sheet with 100 ohm
series resistor (I've tried several values here).  Before they had a similar
design using a 16F which hasn't had these problems.

I was under the impression that using series instead of parallel cut
crystals by mistake would only change the frequency that it oscillated at,
not whether it oscillated at all.  Is that a bad assumption?  However, I'm
sure we have the right cut crystal because...

  I removed the crystal and ran a generator into the osc input, this worked
and the code ran.  Also osc2 was showing the signal coming back out like I
would expect if the config was right.    I put a 1M resistor across the
crystal and the oscillator started working at the right frequency.  If it
works at all it is reliable, but ... this isn't a fix-all because a) I'm not
sure *why* this helped except a guess that the resistor in the feedback path
is helping the gate act like an amp (maybe someone here will know) and b) it
only works on 90% of the units, but for my purposes it let me get on with
the firmware.

Anyway, I guess there are no specific questions but if anyone has anything
to add or has any good ideas here I'd love to hear about it.   Thanks!




On 8/1/06, alan smith <micro_eng2@yahoo.com> wrote:
>
> The other question to ask is....have they worked in the past? Or is this a
> new design.  Could be the crystal might be series cut instead of parallel
> cut? Wrong caps?
>
> But I'd go with Jan-Erik....do you have the chip configured for the right
> type of osc.
>
> Jan-Erik Soderholm <jan-erik.soderholm@telia.com> wrote:
> Marty Robertson wrote :
>
> > My company just got in a
> > batch of boards with 18F2520's on them.
> > They seem to work fine with the internal
> > oscillator, but with the external oscillator there seems to
> > be no output. I checked with a scope and it is not oscillating.
>
> What did you do to change the *config* of the PIC from
> INTOSC to LF or HS crystal osccilator ?
>
> > The crystal is connected exactly as suggested in the
> > datasheet. Where else can I look to find this problem?
>
> Again, what was changed in the *config* ?
>
> Regards,
> Jan-Erik.
>
>
>
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