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'[PIC]: PIC crystal...'
2002\03\10@222613 by hadirian

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Hi all,

When it says on the chip 04 or 08 for the speed of the chip, is it the
crystal value or 1/4 of the crystal value.

Thanks in advance.

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2002\03\10@223238 by Rick C.

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Maximum speed of the chip.
Rick

Shirollah Ghadirian wrote:

{Quote hidden}

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2002\03\10@230701 by Fowler, Paul B.

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...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but not
a 20MHz resonator?

I have tried two different 20 MHz resonators and the chip doesn't work with
either...  4MHz is just fine.  The application is a simple LED turn on and
keep on, so speed should make no difference.

Any ideas?  - Programmed with MPLab and the properties for the project
specify 20MHz.  XT is specified as the type...

I don't need the extra speed at the moment, but I am curious what I might be
missing.  My tools to track this problem are not advanced - no oscillator,
etc...  A logic probe is about all I have.

Thank you,
Fowler

{Original Message removed}

2002\03\10@231054 by Aaron W.

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04 --> 4MHz crystal (1MHz instruction speed*)
08 --> 8MHz crystal (2MHZ instruction speed*)

*for all instructions except for the ones that take 2 clock cycles

{Original Message removed}

2002\03\10@231056 by David Duffy

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At 04:25 PM 11/03/2002 +1300, you wrote:
>Hi all,
>
>When it says on the chip 04 or 08 for the speed of the chip, is it the
>crystal value or 1/4 of the crystal value.

It's the max crystal value not the MIPS.
Regards...

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2002\03\10@231510 by Rick C.

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Try a 1 to 10 meg resistor across the osc pins. I think I saw a datasheet
somewhere with this requirement.
Rick

"Fowler, Paul B." wrote:

{Quote hidden}

> {Original Message removed}

2002\03\10@231516 by David Duffy

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At 11:05 PM 10/03/2002 -0500, you wrote:
>...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but not
>a 20MHz resonator?
>
>I have tried two different 20 MHz resonators and the chip doesn't work with
>either...  4MHz is just fine.  The application is a simple LED turn on and
>keep on, so speed should make no difference.
>
>Any ideas?  - Programmed with MPLab and the properties for the project
>specify 20MHz.  XT is specified as the type...

Change the oscillator type to HS for above 4MHz.
Regards...

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2002\03\10@231518 by Aaron W.

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are you using a decoupling cap with the pic?

----- Original Message -----
From: Fowler, Paul B. <spam_OUTpaul.fowlerTakeThisOuTspamUNISYS.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Sunday, March 10, 2002 8:05 PM
Subject: Re: [PIC]: PIC crystal...


> ...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but
not
> a 20MHz resonator?
>
> I have tried two different 20 MHz resonators and the chip doesn't work
with
> either...  4MHz is just fine.  The application is a simple LED turn on and
> keep on, so speed should make no difference.
>
> Any ideas?  - Programmed with MPLab and the properties for the project
> specify 20MHz.  XT is specified as the type...
>
> I don't need the extra speed at the moment, but I am curious what I might
be
> missing.  My tools to track this problem are not advanced - no oscillator,
> etc...  A logic probe is about all I have.
>
> Thank you,
> Fowler
>
> {Original Message removed}

2002\03\10@232133 by Fowler, Paul B.

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Changing it to HS did the trick!!!  It now runs at 20MHz.

Thank you Piclist,
Fowler

-----Original Message-----
From: David Duffy [piclistspamKILLspamAUDIOVISUALDEVICES.COM.AU]
Sent: Sunday, March 10, 2002 11:21 PM
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC crystal...


At 11:05 PM 10/03/2002 -0500, you wrote:
>...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but
not
>a 20MHz resonator?
>
>I have tried two different 20 MHz resonators and the chip doesn't work with
>either...  4MHz is just fine.  The application is a simple LED turn on and
>keep on, so speed should make no difference.
>
>Any ideas?  - Programmed with MPLab and the properties for the project
>specify 20MHz.  XT is specified as the type...

Change the oscillator type to HS for above 4MHz.
Regards...

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2002\03\11@083652 by Olin Lathrop

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> When it says on the chip 04 or 08 for the speed of the chip, is it the
> crystal value or 1/4 of the crystal value.

The oscillator frequency, or 4x the instruction speed.


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2002\03\11@083657 by Olin Lathrop

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> ...and what can cause my PIC16F84A-20P to work with a 4MHz resonator but
not
> a 20MHz resonator?

XT instead of HS oscillator mode, for one.


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2002\03\11@083700 by Olin Lathrop

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> Try a 1 to 10 meg resistor across the osc pins. I think I saw a datasheet
> somewhere with this requirement.

I haven't.  Care to provide a reference?


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2002\03\11@130040 by M. Adam Davis

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I've run into this problem with a few of my chips.  I suspect the major
reason is the extra capacitance from the breadboards I use.  One person
I talked to awhile ago measured 20pF extra capacitance from wire to wire
on their breadboard...  (I doubt you'll ever find your breadboard that
high, but they still have enough to mess with sensitive circuits)

But I've never needed the extra speed badly enough to really look into
it hard.  Use a wire wrap socket for the pic, bend the osc pins out and
up so they don't go into the breadboard, and solder the osc (or another
socket) directly to those pins with a flying lead to ground (or to the
capacitors).

-Adam

Fowler, Paul B. wrote:

{Quote hidden}

>{Original Message removed}

2002\03\11@195338 by Herbert Graf

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> When it says on the chip 04 or 08 for the speed of the chip, is it the
> crystal value or 1/4 of the crystal value.

       It is the crystal value. The actual instruction rate is 1/4 this (except
for branches). TTYL

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2002\03\11@195550 by Herbert Graf

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> ...and what can cause my PIC16F84A-20P to work with a 4MHz
> resonator but not
> a 20MHz resonator?
>
> I have tried two different 20 MHz resonators and the chip doesn't
> work with
> either...  4MHz is just fine.  The application is a simple LED turn on and
> keep on, so speed should make no difference.
>
> Any ideas?  - Programmed with MPLab and the properties for the project
> specify 20MHz.  XT is specified as the type...
>
> I don't need the extra speed at the moment, but I am curious what
> I might be
> missing.  My tools to track this problem are not advanced - no oscillator,
> etc...  A logic probe is about all I have.

       A resonator (those 4 pin beauties) or a crystal? I only ask since it a
prior message you said crystal. For a crystal you must use HS mode to get it
to work since the crystal must be driven a little "harder". For a resonator
I'm not sure, I think XT should work? TTYL

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2002\03\11@203316 by David Duffy
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Herbert wrote:
> > ...and what can cause my PIC16F84A-20P to work with a 4MHz
> > resonator but not
> > a 20MHz resonator?
<snip>
>A resonator (those 4 pin beauties) or a crystal? I only ask since it a
>prior message you said crystal. For a crystal you must use HS mode to get it
>to work since the crystal must be driven a little "harder". For a resonator
>I'm not sure, I think XT should work? TTYL

No, HS is required for a 20MHz resonator. Been there, forgot that !
Regards...

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2002\03\11@223149 by Rick C.

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You haven't looked hard enough. Just about every CMOS clock circuit I've seen
has a feedback resistor across its input/output typically 1 meg to 10 meg. This
ensures a reliable startup. The PIC16f is a CMOS device with a high input
impedance pin and a low output impedance pin (typical of CMOS clock
generators). The data sheet for the 75T202 chip recommends a 10 meg resistor.
The Basic Micro http://www.basicmicro.com/  MBasic manual page 146 recommends a
10 meg resistor across the 16f628 using a resonator. The Motorola CMOS handbook
is infested with clock circuits with this feedback resistor. I have designed
equipment with CMOS logic in commercial/broadcast circuits that use this
resistor. VDO specifies this resistor in their automotive applications in their
instrument clusters. The Piccon at http://www.qsl.net/n6bg/piccon/ shows a 1
meg across the xtal. The Simon Bus radio alert receiver uses a PIC, 4mhz xtal
with a 10 meg across its clock. I could go on and on.....
Rick

Olin Lathrop wrote:

{Quote hidden}

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2002\03\11@224522 by David Duffy

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Rick wrote:
>You haven't looked hard enough. Just about every CMOS clock circuit I've seen
>has a feedback resistor across its input/output typically 1 meg to 10 meg.
>This
>ensures a reliable startup. The PIC16f is a CMOS device with a high input
>impedance pin and a low output impedance pin (typical of CMOS clock
>generators). The data sheet for the 75T202 chip recommends a 10 meg resistor.

What Olin is saying is that Microchip don't mention it do they?
I would not say a 16F628 oscillator is the same as a standard CMOS clock
circuit.

{Quote hidden}

Are any of these examples what Microchip suggest?
I'd say some designers add the resistor out of habit with other micro's.

>Olin Lathrop wrote:
>
> > > Try a 1 to 10 meg resistor across the osc pins. I think I saw a datasheet
> > > somewhere with this requirement.
> >
> > I haven't.  Care to provide a reference?

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2002\03\11@231412 by Jinx

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> What Olin is saying is that Microchip don't mention it do they?
> I would not say a 16F628 oscillator is the same as a standard
> CMOS clock circuit.

Figure 14.2 of the 16F628 manual shows a resistor "RF" inside
the chip, across Osc1 and Osc2. Unfortunately nothing else is
said about it

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2002\03\12@090821 by Olin Lathrop

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> You haven't looked hard enough. Just about every CMOS clock circuit I've
seen
> has a feedback resistor across its input/output typically 1 meg to 10 meg.
This
> ensures a reliable startup. The PIC16f is a CMOS device with a high input
> impedance pin and a low output impedance pin (typical of CMOS clock
> generators). The data sheet for the 75T202 chip recommends a 10 meg
resistor.
> The Basic Micro http://www.basicmicro.com/  MBasic manual page 146
recommends a
> 10 meg resistor across the 16f628 using a resonator. The Motorola CMOS
handbook
> is infested with clock circuits with this feedback resistor. I have
designed
> equipment with CMOS logic in commercial/broadcast circuits that use this
> resistor. VDO specifies this resistor in their automotive applications in
their
> instrument clusters. The Piccon at http://www.qsl.net/n6bg/piccon/ shows a
1
> meg across the xtal. The Simon Bus radio alert receiver uses a PIC, 4mhz
xtal
> with a 10 meg across its clock. I could go on and on.....

But not one of these references are from Microchip specifically pertaining
to the PIC crystal driver circuit, which was topic of discussion.

Such resistors are often used as negative feedback to keep CMOS logic gates
in the "linear" region.  However, the PIC oscillator circuit was designed
specifically for that purpose and needs no such resistor.  Adding such a
resistor to the PIC crystal oscillator is merely superstition.  You might
just as well wave a dead fish over it during a full moon.


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2002\03\12@095507 by Kevin Blain

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Ahh, the technique which gets used to make the ICD work properly....


> Such resistors are often used as negative feedback to keep
> CMOS logic gates in the "linear" region.  However, the PIC
> oscillator circuit was designed specifically for that purpose
> and needs no such resistor.  Adding such a resistor to the
> PIC crystal oscillator is merely superstition.  You might
> just as well wave a dead fish over it during a full moon.
>
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, spamBeGoneolinSTOPspamspamEraseMEembedinc.com, http://www.embedinc.com
>
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2002\03\12@103509 by Dale Botkin

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On Tue, 12 Mar 2002, Olin Lathrop wrote:

> You might just as well wave a dead fish over it during a full moon.

What - am I the only one that does that??

Dale
8-)

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2002\03\12@113234 by Ted Mawson

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Actually the salts in the fish flesh cause ionic reactions which in turn
generate electrostatic fields that, while the fish is moving, interact with
the earth's magnetic field which, of course, is affected by the full moon
thus causing ionic bombardment of the silicon substrate.

Thus waving a dead fish over your PIC during a full moon CAN have an
electrical effect on it.

If you've read thus far... only joking!  ;-)

Ted Mawson

PS We engineering types do have a tendancy to reject anything that we don't
have a scientific explanation for, don't you think?


{Original Message removed}

2002\03\12@121126 by Spehro Pefhany

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At 08:14 AM 3/12/02 -0500, you wrote:

>Such resistors are often used as negative feedback to keep CMOS logic gates
>in the "linear" region.  However, the PIC oscillator circuit was designed
>specifically for that purpose and needs no such resistor.

Actually all circuits of this type (Pierce oscillator) do require a
resistor, however the  resistor is provided internally in PICs. The value is
selected from several possibilities when the oscillator type is selected.

Some micros (Motorola '05, for example) do not have the resistor internally
and it must be provided externally. This might have some slight advantage
in allowing better controlled value (5% vs. 30% tolerance) and higher
value (10M vs maybe 2M).

>  Adding such a
>resistor to the PIC crystal oscillator is merely superstition.  You might
>just as well wave a dead fish over it during a full moon.

Like waving a fish over it twice, when once has been show beyond any doubt
to be every bit as effective as twice.

Best regards,

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2002\03\12@162220 by Karl Seibert

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No, but it works better when the fish is alive.

http://whozoo.org/Intro2000/tashcorm/tempagetwo.htm

Unfortunately, they didn't include a full datasheet so I have
to guess how to use it in a circuit.

Karl

Quoting Dale Botkin <TakeThisOuTdale.....spamTakeThisOuTBOTKIN.ORG>:

{Quote hidden}

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