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PICList Thread
'QRe: tapping xtal oscillator'
1999\08\28@114011 by Anne Ogborn

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Dave VanHorn wrote:
>
> > The obvious thing to do is to tap into the xtal output.
> > So, can somebody tell me what sort of electronic wizardry I have
> > to do to make the 4MHtz crystal drive some sort of gate so I can
> > divide by 4 and drive the ISD chip???
>
> Careful.
>
> Check the spec, there should be an output drive spec for that pin. If not,
> you're in shaky territory, no matter how many hobby projects or protos "just
> worked".  The most interesting part of a data sheet is often what is NOT in
> it.
>

Thanks - I'm with you, that it worked in the prototype means nothing. Otherwise
I'd stick a part in the breadboard and have an answer in the same time I could e
mail.

Buuuuut.......
this means I have to *understand* this circuit.

Well, found something interesting -
To quote the uChip data sheet:

OSC2 - Oscillator Crystal output. Connects to crystal or resonator in
crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4 the
frequency of OSC1, and denotes the instruction cycle time.

OK - if I can find a reasonable price can oscillator, I don't need the divide by
4
chip. That might justify some/all of the can's price.

> and the eng mgr told me that all the protos
> worked, and "don't worry.."   Guess who got to fly to kaohsiung christmas
> day to debug an assembly line full of product that didn't run... :(

Uhhh.... let me guess. It was the eng mgr? Right? No??? Did he happen to have po
inty hair?

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html

1999\08\28@141602 by Bob Drzyzgula

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On Sat, Aug 28, 1999 at 08:38:13AM -0700, Anne Ogborn wrote:
>
> OSC2 - Oscillator Crystal output. Connects to crystal or resonator in
> crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4 the
> frequency of OSC1, and denotes the instruction cycle time.

I noticed that, too, and it did seem like a way to do
it. But uChip never says anything about Fosc/4 for
anything *but* the RC mode. Does it do this in XT or
HS mode?

--Bob

--
============================================================
Bob Drzyzgula                             It's not a problem
spam_OUTbobTakeThisOuTspamdrzyzgula.org                until something bad happens
============================================================

1999\08\29@011720 by Dave VanHorn

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> Thanks - I'm with you, that it worked in the prototype means nothing.
Otherwise
> I'd stick a part in the breadboard and have an answer in the same time I
could email.
>
> Buuuuut.......
> this means I have to *understand* this circuit.

That's the fun bit.  Hanging a track on it connected to a gate isn't JUST
hanging a little extra C on it.   It couples in some noise (Remember that
"Route no tracks near the xtal tracks" directive?)  and it's a potential EMI
source. The chip you drive better have a nice low Z return to the pic.
Series R would probably help all around, as long as the edges don't get too
fuzzy.

You MIGHT be better with a discrete oscillator, feeding both circuits.  A
simple 3 gate HC14 osc can drive a ton, since you aren't needing to tap into
the xtal nodes to do it.


> OSC2 - Oscillator Crystal output. Connects to crystal or resonator in
> crystal oscillator mode. In RC mode, OSC2 pin outputs CLKOUT which has 1/4
the
> frequency of OSC1, and denotes the instruction cycle time.

But can you clock it that fast in RC mode?


> > and the eng mgr told me that all the protos
> > worked, and "don't worry.."   Guess who got to fly to kaohsiung
christmas
> > day to debug an assembly line full of product that didn't run... :(
>
> Uhhh.... let me guess. It was the eng mgr? Right? No??? Did he happen to
have pointy hair?

Nope, me.

1999\08\29@064540 by steveb

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> You MIGHT be better with a discrete oscillator, feeding both circuits.  A
> simple 3 gate HC14 osc can drive a ton, since you aren't needing to tap into
> the xtal nodes to do it.

Kill two birds with one stone and use a (74HC)4060 which has the
oscillator amp built in (equivalent to PIC HS mode). You get the
oscillator and the divider chain to get clock sources for both
chips.

Steve.


======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: .....stevebKILLspamspam@spam@tla.co.nz      fax +64 9 820-1929
======================================================

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