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PICList Thread
'Tapping the Xtal Oscillator'
1996\06\11@211932 by Newfound Electronics

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face
G'day all,

Is it possible to "TAP" the crystal oscillator from a PIC device using a
high impedance schmitt trigger (74HC14?) or something the like?.

I need a buffered 4MHz signal for the rest of the system and it would be nice
if I can save $$ on a seperate "canned" oscillator.

The frequency of the crystal MUST NOT be altered as it also sets baud rates.
(Actually, some slight, (<1%) variation can be tolarated.)

Anyone out there with any ideas, pointers, or various other pieces of info
that might be construed as useful to me in my quest for cheapness?

Jim

1996\06\12@072654 by Rick Miller

picon face
Newfound Electronics wrote:
> Is it possible to "TAP" the crystal oscillator from a PIC device using a
> high impedance schmitt trigger (74HC14?) or something the like?.
>
> I need a buffered 4MHz signal for the rest of the system and it would be nice
> if I can save $$ on a seperate "canned" oscillator.
>
> The frequency of the crystal MUST NOT be altered as it also sets baud rates.
> (Actually, some slight, (<1%) variation can be tolarated.)

Hang the gate on the CLKOUT line (not CLKIN), and make sure it has a FET
input circuit.  Just about any CMOS part should be fine.  If you find that
the frequency varies (which it may, due to gate-source capacitance), you
might try reducing or eliminating the capacitor on that leg of the crystal.

(Note that I've never tried this myself.  It just seems to make sense in
an electrical sort of way to my warped [pre-coffee] brain at 6:00am.)
--
Rick Miller
<spam_OUTrdmillerTakeThisOuTspamexecpc.com>

1996\06\12@093524 by Mark K Sullivan

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>Is it possible to "TAP" the crystal oscillator from a PIC device using a
>high impedance schmitt trigger (74HC14?) or something the like?.

I often use the PIC oscillator to drive auxillary circuits.  I normally replace
one of the 20 pF load capacitors to ground with a 20 pF coupling capacitor to
the auxillary circuit.  You can use the PIC to bias the aux circuit to approx
VDD/2 by shunting this capacitor with 100K or so.

If you keep the lead capacitance down, I don't see why a 74HC14 wouldn't work.
Check the input capacitance of the 'HC14 and try to keep the crystal load
capacitance in the right range.

- Mark Sullivan -

1996\06\13@051758 by Kalle Pihlajasaari

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Hi,

> > Is it possible to "TAP" the crystal oscillator from a PIC device using a
> > high impedance schmitt trigger (74HC14?) or something the like?.
> >
> >i I need a buffered 4MHz for the rest of the system and it would be nice
> > if I can save $$ on a seperate "canned" oscillator.
> >
> > The frequency of the crystal MUST NOT be altered as it also sets baud rates.
> > (Actually, some slight, (<1%) variation can be tolarated.)
>
> Hang the gate on the CLKOUT line (not CLKIN), and make sure it has a FET
> input circuit.  Just about any CMOS part should be fine.  If you find that
> the frequency varies (which it may, due to gate-source capacitance), you
> might try reducing or eliminating the capacitor on that leg of the crystal.
>
> (Note that I've never tried this myself.  It just seems to make sense in
> an electrical sort of way to my warped [pre-coffee] brain at 6:00am.)

I have used one crystal to drive two 16C5X-HS PICs at 20 MHz in production
with no problems.

Use a standard occilator circuit (Xtal + 2 x 27pF) and feed OSC out
to the OSC in of the second PIC.  I also prototyped a design with
a dozen (12) PICs working in series at 20MHz (did not go into production)
and also tested it out at 25 and 32 MHz but the internal OSC is not
able to buffer at > 20MHz clock rates.  The first PIC (driven from an
oscillator can) worked fine though.

Cheers
--
Kalle Pihlajasaari     .....kalleKILLspamspam@spam@data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751

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