Adam stambler wrote:
> Anyway, thanks again for the suggestions.  I am just in high school and
> trying to learn electronics.  Often just reading through this weblist
> has been more educational than any of my classes and your responses
> have been really informative.

There is no substitute for actually building your own circuits, figuring out
why they don't work, and eventually getting them to do what you intended.
Of course this all needs to be grounded in theory, but tinkering is so
important.  When you get to college, you'll be surprised how many EE
students never kludged up a few transistors to make a amplifier, oscillator,
or whatever.  Whether they get good grades on tests or not, they never
amount to much.  And if you tinker and get a intuitive feel for circuits,
you'll be a natural at picking up the theory and will do fine on those tests
too.  The equations are important and you need to learn them, but to ever be
any good at designing circuits you have to be able to see the currents flow
and the voltages move in your mind.

I've interviewed EE graduates that proudly showed me how they got mostly As
and a few Bs.  That's a good thing, but I want to see evidence of real
passion for EE, not just going thru the motions because someone told them it
is a good gig.  I always ask a candidate what project he's done on the side.
Too often I get a dumbfounded look and a answer like "what do you mean,
there wasn't anything like that required?".  Those are real short
interviews.

> For example, I haven't come across this "powered clock" before.

You got some bad or at least misleading advice here.  These things are
called "oscillator modules" or the more slang term "canned oscillators".
They contain a crystal with driver circuit.  You hook up power and ground
and out comes the frequency written on the can.

These things have some drawbacks, and I think it's bad advice to use them as
a first choice, particularly in your situation.  First, they are expensive.
Second, the PIC already has the driver circuit built in.  Third they take a
great deal of power compared to the PIC crystal driver circuit.  Fourth,
they are notorious for glitching the supply and need careful bypass,
although of course you should be bypassing every separately powered device
anyway.  You're better off learning why the PIC oscillator won't run and
fixing it, as you did.  It sounds like you just got a bad resonator.

I don't use resonators much and have also heard people having problems with
the PIC driver at high frequencies with resonators.  Resonators have a bit
more attenuation than crystals, which matters when you are at the limits of
the driver circuit.

My first choice is a crystal because they are accurate and work very
reliably.  I don't recall a single case where a PIC failed to work with a
crystal when the correct oscillator mode was selected, and this includes up
to 20MHz on protoboards.  If I need more accuracy or the PIC internal
oscillator is unsuitable for other reasons, I use a crystal unless there is
a good reason not to.  I can't think of a good reason to ever use a canned
oscillator to run a PIC.

There are two primary advantages to resonators.  They are a bit cheaper and
more rugged than crystals.  For tinkering, crystals are cheap enough and
probably more available.  However since your circuit is intended for a model
rocket, rugged is a valid consideration in this case.  I think a resonator
is the right choice in this case.


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