On Tue, 2008-06-17 at 14:39 -0400, Olin Lathrop wrote:
> I guess you missed the point that a ceramic resonator is *easier* than a
> canned oscillator.  You hook it up and you're done.  I've seen canned
> oscillators cause trouble in circuits, usually because someone didn't pay
> attention to bypassing and high frequency loop currents.  In other words,
> canned oscillators are more advanced and require more expertise to get
> right.  They look easy, but they're really not.  They are not for beginners.
> 
> On the other hand, I don't remember having a problem with a PIC crystal
> oscillator.  My experience is that they just work.  If you want to use a
> junkbox crystal that you can't identify to find the datasheet, just use 22pF
> caps.  The frequency might be a little off, but it will oscillate.

My experience is the exact opposite. While canned oscillators have been
100% for me (even surviving insertion backwards), crystals are another
story.

While in general the PIC XTAL driver seems to be above average, I have
had issues where everything is done supposedly correctly, yet the
crystal either doesn't start up, or runs erratically.

To be fair all problems were on prototype circuits, usually soldered
breadboard. Of course, this is EXACTLY the type of circuit construction
someone just starting out with PICs will likely use, so my experience is
relevant.

FWIW I've never had a problem with lower frequency crystals (say in the
4-8MHz range), but 16-20MHz HAVE been a problem, especially with the
older 16F parts (for example, upgrading a design from an 877 to an 877a
part resulted in no oscillation). I haven't experienced a problem with
the newer parts, but then the sample space is much smaller since either
I use the internal RC, or use a lower crystal with the 4x PLL enabled.

Outside of the PIC world I pretty much always either use an internal
oscillator if available, or I use a lower crystal and PLL my way up to
the speeds I want. Just reduces the headache.

TTYL
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