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'[PIC]: Serial resistor for the oscillator of a PIC'
2002\07\09@135352 by Carlos Ojea

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Hi!

I have a PIC 16F876 with the oscillator configuration:

OSC1                                      OSC2
----------CRYSTAL-------------RESISTOR-------
    |               |
   CAP             CAP
    |               |
    _               _

where CRYSTAL is a 16MHz quartz (-----> UKI brand <----)
CAP are 22pF ceramic capacitor

The point is that I need to know some suitable value for the serial
RESISTOR, since the oscillator doesn't want to start and I need it
running with that brand of crystal (UKI).

Thank you in advance!

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2002\07\09@152404 by Olin Lathrop

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> I have a PIC 16F876 with the oscillator configuration:
>
> OSC1                                      OSC2
> ----------CRYSTAL-------------RESISTOR-------
>      |               |
>     CAP             CAP
>      |               |
>      _               _
>
> where CRYSTAL is a 16MHz quartz (-----> UKI brand <----)
> CAP are 22pF ceramic capacitor
>
> The point is that I need to know some suitable value for the serial
> RESISTOR,

I always use 0Ohms.  Where did you get the idea from to put a resistor there
anyway?  I don't recall ever seeing a resistor in any Microchip
documentation.

> since the oscillator doesn't want to start

Well, yeah.  Can't blame it with a resistor in there.  You also have to
remember to use HS oscillator mode.


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2002\07\09@155655 by Drew Vassallo

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>OSC1                                      OSC2
>----------CRYSTAL-------------RESISTOR-------
>      |               |
>     CAP             CAP
>      |               |
>      _               _
>
>where CRYSTAL is a 16MHz quartz (-----> UKI brand <----)
>CAP are 22pF ceramic capacitor
>
>The point is that I need to know some suitable value for the serial
>RESISTOR,

Why would you put a resistor in there?  I've never seen it as you've
described above.  My datasheet shows an example connection and it doesn't
include a resistor anywhere.

Leave the resistor out completely and you'll be fine.

--Andrew

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2002\07\09@194303 by Mike Singer

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part 1 1371 bytes content-type:text/plain; (decoded 7bit)

Image was taken from
  DS30292B-page 123 (PIC16F87X 28/40-pin
8-Bit CMOS FLASH Microcontrollers).
  Rs may be required in HS mode, as well
as XT mode, to avoid overdriving crystals
with low drive level specification.

  Mike.

Olin Lathrop wrote:
{Quote hidden}


part 2 15485 bytes content-type:image/gif; (decode)


part 3 136 bytes
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2002\07\09@195629 by Sean Alcorn - Avion Sydney

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> Image was taken from
>    DS30292B-page 123 (PIC16F87X 28/40-pin
> 8-Bit CMOS FLASH Microcontrollers).
>    Rs may be required in HS mode, as well
> as XT mode, to avoid overdriving crystals
> with low drive level specification.

Thanks Mike,

I knew I'd seen it before! If you hadn't posted that, I would have!

Sean

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2002\07\09@202252 by Drew Vassallo
picon face
>Image was taken from
>    DS30292B-page 123 (PIC16F87X 28/40-pin
>8-Bit CMOS FLASH Microcontrollers).
>    Rs may be required in HS mode, as well
>as XT mode, to avoid overdriving crystals
>with low drive level specification.
><< resistor.gif >>


I noticed that it says "May be required for AT strip cut crystals."

Is that what you're using?  Series and parallel cuts behave differently, I
think.  I don't know any more than that.  All I know is that I use their
recommended crystals and never used a series or parallel resistor.  And
everything works just fine.

--Andrew

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2002\07\10@044031 by Michael Rigby-Jones

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{Quote hidden}

I thought it was a fairly widely known method to induce crystal oscillators
to start reliably.  The resistor should be a carbon composition type (i.e.
noisy) for best results.  Certainly I have seen this technique mentioned in
several books, although I have no idea what value would be optimal.  Having
said that I have never had a problem with crystals or resonators starting
reliably  on PIC's from 1MHz to 20MHz, even though it seems some people have
no end of trouble.

Regards

Mike

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2002\07\10@074137 by Olin Lathrop

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> I noticed that it says "May be required for AT strip cut crystals."
>
> Is that what you're using?  Series and parallel cuts behave differently, I
> think.  I don't know any more than that.

AT cut is independent of whether the crystal is intended for series or
parallel operation.  There is no series or parallel "cut", even though the
Microchip docs use that term.

> All I know is that I use their
> recommended crystals and never used a series or parallel resistor.  And
> everything works just fine.

Same here.


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2002\07\11@063746 by Alan B. Pearce

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>I noticed that it says "May be required for AT strip cut crystals."

>Is that what you're using?  Series and parallel cuts behave differently, I
>think.  I don't know any more than that.  All I know is that I use their
>recommended crystals and never used a series or parallel resistor.  And
>everything works just fine.

I suspect that the strip cut crystals may be what are used in low profile
cans like the HC49/3U to get the crystal small enough. Normally they are a
round disc that would fill the width of an HC49/U can.

This is just my surmising, I have not opened any of these to have a look.

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2002\07\11@083736 by Spehro Pefhany

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At 07:40 AM 7/10/02 -0400, you wrote:
> > I noticed that it says "May be required for AT strip cut crystals."
> >
> > Is that what you're using?  Series and parallel cuts behave differently, I
> > think.  I don't know any more than that.
>
>AT cut is independent of whether the crystal is intended for series or
>parallel operation.  There is no series or parallel "cut", even though the
>Microchip docs use that term.

There is a difference between "AT strip cut" and "AT cut", however.

> > All I know is that I use their
> > recommended crystals and never used a series or parallel resistor.  And
> > everything works just fine.
>
>Same here.

With small SMT crystals this may result in problems (premature aging and
even destruction of the crystal) due to overdrive. The maximum drive
levels are an order of magnitude or more lower than the larger
crystals.

Best regards,

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2002\07\11@094939 by Matt Pobursky

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You are correct Alan. Most any crystal that is packaged in less
than a HC49/U can is a strip cut of some kind or another.

The designer should always look a the drive power specifications
for any crystal they are considering, especially if they are
considering SMD or small package crystals. Overdriving the
crystal can kill them or degrade them over time. This is
especially true of very tiny crystals like 32KHz watch crystals.
You often need 180-330K of series resistance for a watch crystal
circuit to keep from overdriving it. I've seen *lots* of
electronic assemblies where the crystal is seriously overdriven
and could have used a series resistor.

You can tell if you are overdriving the crystal by looking at the
OSC waveform. It should be a nice clean sine wave, slightly less
than rail-to-rail. If you are getting a clipped sine wave then
you are overdriving the crystal.

I put a series resistor in almost all my oscillator circuits. You
can always use a 0 Ohm resistor if no resistance is required, but
it's very difficult to add the resistor (if necessary) "after the
fact". I should add that almost all hardware I work with
(prototypes too) are constructed on manufactured printed circuit
boards.

Matt Pobursky
Maximum Performance Systems

On Thu, 11 Jul 2002 11:37:58 +0100, Alan B. Pearce wrote:
>>I noticed that it says "May be required for AT strip cut
crystals."
>
>>Is that what you're using?  Series and parallel cuts behave
differently, I
>>think.  I don't know any more than that.  All I know is that I
use their
>>recommended crystals and never used a series or parallel
resistor.  And
>>everything works just fine.
>
>I suspect that the strip cut crystals may be what are used in
low profile
>cans like the HC49/3U to get the crystal small enough. Normally
they are a
>round disc that would fill the width of an HC49/U can.
>
>This is just my surmising, I have not opened any of these to
have a look.

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2002\07\11@101258 by John Dammeyer

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Hi Matt,

> You can tell if you are overdriving the crystal by looking at the
> OSC waveform. It should be a nice clean sine wave, slightly less
> than rail-to-rail. If you are getting a clipped sine wave then
> you are overdriving the crystal.

Which waveform?  The OSC2 or the OSC1?  Or both?



>
> I put a series resistor in almost all my oscillator circuits. You
> can always use a 0 Ohm resistor if no resistance is required, but
> it's very difficult to add the resistor (if necessary) "after the
> fact". I should add that almost all hardware I work with
> (prototypes too) are constructed on manufactured printed circuit
> boards.

For a 20MHz PIC what sort of resistor value do you find you end up
using?  1K, 10K, 100K or do you select with a particular batch of
crystals?

Cheers,

John Dammeyer

>

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2002\07\11@131839 by Matt Pobursky

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On Thu, 11 Jul 2002 07:11:30 -0700, John Dammeyer wrote:
>Hi Matt,
>
>>You can tell if you are overdriving the crystal by looking at the OSC
>>waveform. It should be a nice clean sine wave, slightly less than
>>rail-to-rail. If you are getting a clipped sine wave then you are
>>overdriving the crystal.
>
>Which waveform?  The OSC2 or the OSC1?  Or both?

I normally look at OSC2 (CLKOUT), since this is the low impedance
output drive for the oscillator circuit. Normally it will not
significantly effect the oscillator, but the 'scope probe you use has
to be considered. A good 10x probe will put about a 10MOhm/10pf load to
ground on the PIC output. A cheap 1x probe will many times kill the
oscillator.

>For a 20MHz PIC what sort of resistor value do you find you end up
>using?  1K, 10K, 100K or do you select with a particular batch of
>crystals?

It really depends on the crystal I'm going to use. If I'm using a
typical HC49/U can (1 mW drive level), then no resistor is generally
required. If I am using a HC49/US (0.1 mW drive level), then I will go
with something around 470 Ohms. For most 32KHz watch crystal circuits,
I'll start around 180K Ohms and have gone as high as 330K. 32KHz
circuits are extremely difficult to measure as they are very high
impedance all around and loading of any kind (even board moisture and
dust!) can have a significant effect on the oscillator.

>Cheers,
>
>John Dammeyer

P.S. Some interesting (I hope) crystal experiences in an OT post to
follow.

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