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'Crystal Oscillator'
1997\12\03@071115 by Alec Myers

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I've a PCB layout that will benefit greatly if I could (reliably) hook the
watch-crytal's capacitors to +V instead of Gnd.

If there's a sufficiently large cap. across the battery, this should be
equivalent, I think.

Has anyone tried this?

--Alec

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1997\12\03@132812 by wwl

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On Wed, 3 Dec 1997 11:58:31 +0000, you wrote:

>I've a PCB layout that will benefit greatly if I could (reliably) hook the
>watch-crytal's capacitors to +V instead of Gnd.
I've alway said they screwd up the pinout on the 18 pin devices - you
always need a gnd near the osc pins, and  MCLR is often tied to Vdd,
but they put them the wrong way round!
>If there's a sufficiently large cap. across the battery, this should be
>equivalent, I think.
As long as the cap has low impedance at the required frequency (size
isn't everything!) - shouldn't be a problem at 32KKz - the usual 100nF
ceramic ought to be OK.
My main worry would be Vcc switching noise from the PIC or elsewhere
affecting the oscillator - noise from the PIC shouldn't be too bad as
it would be synchronous, but may be an issue if there were other
clocks (e.g. using the watch xtal on the Timer 1 osc on a '74)
>Has anyone tried this?
Sorry, no - I'm always very nervous about using watch crystals - they
can be  somewhat tempramental!

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1997\12\03@145005 by Steve Smith

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This works well and is more convienent on 18pin devices to use vcc than
ground line

Cheers Steve.....

1997\12\04@062628 by paulb

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If you consider the device for a moment, you will note it is based on
a technology called CMOS where "C" stands in full for "complementary
symmetry".  This means that every part of the circuit has symmetrical
connections to both Vcc and Gnd.  Except for inputs with deliberate
asymmetry to make them compatible to TTL (which is indeed ground-
referenced) outputs, Vcc and Gnd are indistinguishable in the circuit;
their functions can be exchanged using a negative supply and suitable
alteration in logic notation!

 In other words, it doesn't have a ground reference, the threshold is
halfway between Gnd and Vcc for the function in question, so it makes
not a whit of difference to which rail you fit the load capacitors.
Lack of bypassing would be expected to cause as much mayhem with the
load caps on Gnd as on Vcc.

 Anyone fault this analysis?
       Paul B.

1997\12\04@131102 by Dwayne Reid

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>I've a PCB layout that will benefit greatly if I could (reliably) hook the
>watch-crytal's capacitors to +V instead of Gnd.
>
>If there's a sufficiently large cap. across the battery, this should be
>equivalent, I think.

This works perfectly with the 3 legged 4 MHz resonators that I use.  You
NEED a bypass cap next to the PIC anyways (I use 1uf tantalum) which
guarantees that VCC=GND at the crystal / resonator frequency.  Only caveat
is leakage resistance of the capacitors (NPO ceramic caps should be fine).

Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, Alberta, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1997\12\05@105002 by Matt Bonner

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Paul B. Webster VK2BZC wrote:
>
> If you consider the device for a moment, you will note it is based on
> a technology called CMOS where "C" stands in full for "complementary
> symmetry".  This means that every part of the circuit has symmetrical
> connections to both Vcc and Gnd.  Except for inputs with deliberate
> asymmetry to make them compatible to TTL (which is indeed ground-
> referenced) outputs, Vcc and Gnd are indistinguishable in the circuit;
> their functions can be exchanged using a negative supply and suitable
> alteration in logic notation!
If you mean exchanging Vdd and Vss, you'll most likely destroy the
device because of the parasitic diodes between inputs and Vss.  If you
mean running the HC part with a Vdd of 0V and Vss of -5V, that's
probably OK.  Don't forget that internal "resistors" are actually biased
FETs which may not like such an arrangement.
>
>   In other words, it doesn't have a ground reference, the threshold is
> halfway between Gnd and Vcc for the function in question, so it makes
> not a whit of difference to which rail you fit the load capacitors.
> Lack of bypassing would be expected to cause as much mayhem with the
> load caps on Gnd as on Vcc.
The reason the crystal capacitors can be tied to the 5V rail is because
they need to be tied to _any_ low impedance point.  A properly designed
power supply's 5V rail is a low impedance point.  Power rail noise is
another issue that has been mentioned already.

--Matt

1997\12\10@073933 by paulb

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Matt Bonner wrote:

> The reason the crystal capacitors can be tied to the 5V rail is
> because they need to be tied to _any_ low impedance point.

 My point was that in discrete/ single-ended transistor design, a
common ground point is used, with all bypassing directly to it.  Since
the power rail has the load currents impressed on it, bypassing signal
points to it rather than direct to ground is likely to introduce
feedback and instability even though the power rail is itself bypassed
to ground.

 Aside: In the earliest days, condensors{sic.} were expensive, and
bypass impedances were only *low enough* to do the job!

 CMOS on the other hand is complementary, using common-source (equiv.
to common-emitter) circuitry with one source of a pair connected to
*each* supply rail.  Its behaviour is such that the threshold varies
proportionately to the rail voltage, compensating for output
fluctutations with rail voltage.  Of course, operating mainly in a
digital mode, this should not affect any part other than the oscillator
for which it can be stated that to whatever extent that supply rail
variations do affect the oscillator, it will be an identical effect for
bypassing to either rail.

 It is of course obvious that ADCs where provided, reference to the
negative rail and bypassing must be fitted accordingly.

 Cheers,
       Paul B.


'crystal oscillator'
1999\06\27@153307 by Greg Brault
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Hi,
All the PIC projects that i've made in the past i've used the RC
oscillator mode.
I recently bought a 10 mhz crystal, but I have not been able to get it
to work.
I've followed the data sheet's instructions, the crystal is plugged into
osc1 and osc2, with a cap (47 pF) going from each pin to ground.  Can
anyone help?
Thanks in advance,
Greg
.....gjbraultKILLspamspam.....mtu.edu

1999\06\27@154805 by Joseph A. Zammit

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You must program the PIC to accept the HS oscillator mode. This is done
when seinding the Control Words before any program.

Secondly, why did you use sucha large cap. For a more stable oscillator
I usually use caps in the range < 33pF

hope this helps

1999\06\28@094152 by Francisco Armenta

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Francisco

Greg Brault wrote:

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