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PICList Thread
'crystal oscillator for 74a'
1999\03\16@115109 by Francois-Xavier Maletras

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Hi

I want to clock a 74a with a 20 MHz crystal oscillator from AEL.
The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10]
I guess I can't feed the pic with such a signal... or can I ?
Anyway, is there any chip to convert the signal to the correct
range or shall I do it by myself ?

Thanks
Francois Maletras
Kingston University
UK

1999\03\16@183403 by Mike Keitz

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On Tue, 16 Mar 1999 16:38:37 -0000 Francois-Xavier Maletras
<spam_OUTK964534TakeThisOuTspamKINGSTON.AC.UK> writes:

>I want to clock a 74a with a 20 MHz crystal oscillator from AEL.
>The oscillator gives me a 17 V peak to peak signal. [-7 ; + 10]
>I guess I can't feed the pic with such a signal... or can I ?

Reduce the signal to 5V p-p (or whatever Vdd you're using) with a
resistor divider.  Then couple it to the X1 pin of the PIC through a
"large" capacitor (1000 pF or more).  Set the PIC to HS mode.  There's no
need to connect anything to X2 or anything else to X1.  It's better to
err with a little too small an amplitude than too big.  Probably anything
from 2 V p-p and up will work.


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1999\03\17@031132 by Ravi Pailoor

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Plain resistor divder may not work. The negative swing need to be clamped.
Hence use a diode to clamp the negative swing.

Also coupling through a CAP will remove the DC contents and swing negative.
Am I right ?

Regards

Pailoor

Mike Keitz wrote:

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1999\03\17@050928 by Vincent Deno

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What about using clamping diodes.  Granted, you would have to make sure
you ensure the devices switch fast enough and are rated for that amount
ofpower.

> On Tue, 16 Mar 1999 16:38:37 -0000 Francois-Xavier Maletras
{Quote hidden}

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Vincent Deno
Design Engineer
Theta Digital Corp.
http://www.thetadigital.com
EraseMEdenovjspam_OUTspamTakeThisOuTemail.uc.edu
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1999\03\17@083305 by Amos, Jim

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I would recommend a resistive divider to remove some of the amplitude
(probably down to 2V p-p).  This would be followed by a coupling capacitor
to  remove the DC component.  You could then run it through a CMOS inverter
with the input biased to 1/2 the supply voltage.  The output of this should
be a good clean CMOS level signal.  This would work for whatever supply
voltage you are running.

The Wenzel and Associates web site at
http://www.wenzel.com/documents/library.html
has some very good waveform conversion circuits.


> {Original Message removed}

1999\03\17@090402 by Mike Keitz

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On Wed, 17 Mar 1999 12:33:19 +0530 Ravi Pailoor <chiptechspamspam_OUTVSNL.COM>
writes:
>Plain resistor divder may not work. The negative swing need to be
>clamped.
>Hence use a diode to clamp the negative swing.

It doesn't matter what DC level exists at the output of the divider since
it's coupled through a capacitor to the PIC.  But, if the oscillator's
amplitude tends to vary, it would be good to clamp with diodes in order
to  regulate the amplitude.


>Also coupling through a CAP will remove the DC contents and swing
>negative.

The PIC oscillator circuit has internal feedback that sets the DC bias at
the X1 input to about Vdd / 2.  So it will charge the capacitor enough to
prevent the input from swinging negative as long as the input amplitude
is small enough (less than Vdd/2 volts peak).


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