AVR-Chat

I saw a lot of schematics that use 2 capacitors with 
crystal that is connected to XTAL. Is there a way
to determine the appropriate values of capacitors?

Do I need to use some equipments to do that? Or is
there a general rule somewhere?

thanks in advance

At 07:44 PM 11/20/2003 +0000, nangkon wrote:

>I saw a lot of schematics that use 2 capacitors with 
>crystal that is connected to XTAL. Is there a way
>to determine the appropriate values of capacitors?
>
>Do I need to use some equipments to do that? Or is
>there a general rule somewhere?

There used to be a page on this.. Back when I had web hosting :)

Use a crystal specified for parallel operation. It will have a loading cap spec.

Take the crystal's specified load capacitance.  (22pF typical)
Double it (44pF)
Subtract 5pF for the parasitics in the circuit (39pF)

Use two 39pF caps in this case. 
Without measuring the parasitics, or a shortwave receiver to get the crystal's exact frequency, this will get you pretty close.

Thanks for your reply.

I am planing to use a 16Mhz crystal with an
atmega128 mcu. I saw some schematic use
some values, and some do not use cap at all.

I saw some crystal spec mentioning about
load cap. Does this value come into the
formulate somehow? What if I do not use 
cap? What will happen?

I am pretty new to electronics. So when you
mention about parallel operation. What
do you mean? Outputing data to a port
for example?

thank you again

--- In AVR-Chat@yahoogroups.com, David VanHorn <dvanhorn@c...> wrote:

> 
> There used to be a page on this.. Back when I had web hosting :)
> 
> Use a crystal specified for parallel operation. It will have a
loading cap spec.
> 
> Take the crystal's specified load capacitance.  (22pF typical)
> Double it (44pF)
> Subtract 5pF for the parasitics in the circuit (39pF)
> 
> Use two 39pF caps in this case. 
> Without measuring the parasitics, or a shortwave receiver to get the
crystal's exact frequency, this will get you pretty close.

At 09:56 PM 11/20/2003 +0000, nangkon wrote:

>Thanks for your reply.
>
>I am planing to use a 16Mhz crystal with an
>atmega128 mcu. I saw some schematic use
>some values, and some do not use cap at all.

Ceramic resonators can come with the caps built in, but they have three leads. 
A crystal without caps, is just plain wrong.

>I saw some crystal spec mentioning about
>load cap. Does this value come into the
>formulate somehow? What if I do not use 
>cap? What will happen?

Good luck. A crystal with no cap spec is designed to run in series mode.
They MAY run in a parallel circuit, but you will never get them running at the right speed. 

The series and parallel resonant points can never be at the same frequency.

>I am pretty new to electronics. So when you mention about parallel operation. What
>do you mean? Outputing data to a port for example?

Nothing like that. 

I used to have some links that would explain this, but I can't find them at the moment, and I don't have time right now, perhaps someone else can step in?

On Thu, 20 Nov 2003 19:44:59 -0000, you wrote:

>I saw a lot of schematics that use 2 capacitors with 
>crystal that is connected to XTAL. Is there a way
>to determine the appropriate values of capacitors?
>
>Do I need to use some equipments to do that? Or is
>there a general rule somewhere?
>
>thanks in advance

27pf will be OK in most cases

I usually ask the vendor to find out what the crystal manufacturer suggest.
It does vary. The suggestions already posted here will probably work. If you
want to avoid the issue entirely, you can use an oscillator module that has
the necessary parts inside it. These devices usually have 4 pins on them,
power ground and output. Look online in the Digikey catalog. Make sure the
operating voltage matches your cpu supply voltage.

Al Welch

-----Original Message-----
From: nangkon [mailto:nangkon@yahoo.com]
Sent: Thursday, November 20, 2003 1:56 PM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: General Question on crystal with capacitors


Thanks for your reply.

I am planing to use a 16Mhz crystal with an
atmega128 mcu. I saw some schematic use
some values, and some do not use cap at all.

I saw some crystal spec mentioning about
load cap. Does this value come into the
formulate somehow? What if I do not use
cap? What will happen?

I am pretty new to electronics. So when you
mention about parallel operation. What
do you mean? Outputing data to a port
for example?

thank you again

--- In AVR-Chat@yahoogroups.com, David VanHorn <dvanhorn@c...> wrote:

>
> There used to be a page on this.. Back when I had web hosting :)
>
> Use a crystal specified for parallel operation. It will have a
loading cap spec.
>
> Take the crystal's specified load capacitance.  (22pF typical)
> Double it (44pF)
> Subtract 5pF for the parasitics in the circuit (39pF)
>
> Use two 39pF caps in this case.
> Without measuring the parasitics, or a shortwave receiver to get the
crystal's exact frequency, this will get you pretty close.



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Thanks for the info!

Is there an instrument (maybe Oscilloscope) to measure 
if the cap/crystal combination is ok? And what should 
I measure (frequency)?

I really want to learn this so that I can apply the
same technique for other scenario or for other chips.

Anyone can point me to tutorial or web page how to
use cap in general. I can understand resistor, but
cap usage is still like a black magic to me when
I look at different schematics.

thanks



--- In AVR-Chat@yahoogroups.com, Mike Harrison <mike@w...> wrote:

> On Thu, 20 Nov 2003 19:44:59 -0000, you wrote:
> 
> >I saw a lot of schematics that use 2 capacitors with 
> >crystal that is connected to XTAL. Is there a way
> >to determine the appropriate values of capacitors?
> >
> >Do I need to use some equipments to do that? Or is
> >there a general rule somewhere?
> >
> >thanks in advance
> 
> 27pf will be OK in most cases

I would use a frequency measurement device. We own a HP Meter that measures
frequency and voltage. Also a Fluke Frequency meter. You could only look at
general accuracy with scope. You could verify it is aprox. Correct but not
accurate enough to use for real time. If you did not own any of these I
suppose you could time and count and compare something after a reasonable
passage of time. You might use and LCD display and post the passage of time.
You could check this against a watch but this is just a close home brew way
of doing it. The frequency meter is the best way to go. It really depends on
what you need. How accurate does it need to be. If the time of day or
accurate time measurement is not important then all you care about is that
it runs reliably.

That's my two cents worth.

Al Welch 

-----Original Message-----
From: nangkon [mailto:nangkon@yahoo.com] 
Sent: Thursday, November 20, 2003 7:48 PM
To: AVR-Chat@yahoogroups.com

Thanks for the info!

Is there an instrument (maybe Oscilloscope) to measure if the cap/crystal
combination is ok? And what should I measure (frequency)?

I really want to learn this so that I can apply the same technique for other
scenario or for other chips.

Anyone can point me to tutorial or web page how to use cap in general. I can
understand resistor, but cap usage is still like a black magic to me when I
look at different schematics.

thanks



--- In AVR-Chat@yahoogroups.com, Mike Harrison <mike@w...> wrote:
> On Thu, 20 Nov 2003 19:44:59 -0000, you wrote:
> 
> >I saw a lot of schematics that use 2 capacitors with crystal that is 
> >connected to XTAL. Is there a way to determine the appropriate values 
> >of capacitors?
> >
> >Do I need to use some equipments to do that? Or is there a general 
> >rule somewhere?
> >
> >thanks in advance
> 
> 27pf will be OK in most cases


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At 03:47 AM 11/21/2003 +0000, nangkon wrote:

>Thanks for the info!
>
>Is there an instrument (maybe Oscilloscope) to measure 
>if the cap/crystal combination is ok? And what should 
>I measure (frequency)?

Probing a crystal will pull it off frequency, unless you use a special high capacitance, high impedance probe. 

What I use, is a shortwave recever, with BFO. 
This works on the radiated energy from the circuit, and does not affect the circuit at all.  If you already have a Sw receiver, then this is a very good technique, especially when you can calibrate to WWV at 10.0.. or 5.0.. MHz

I have been able to probe the crystal can, and get a recoverable signal on the scope, but it's not a very large signal, and you could be misled.

At 03:47 AM 21/11/03 -0000, you wrote:
>Anyone can point me to tutorial or web page how to
>use cap in general. I can understand resistor, but
>cap usage is still like a black magic to me when
>I look at different schematics.

How about a quick course in Electronics, any colleges
near you that offer such courses?

Regards

John Samperi

******************************************************
                        Ampertronics Pty. Ltd.
  11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
         Tel. (02) 9674-6495       Fax (02) 9674-8745
               Email: samperi@ampertronics.com.au
                 Website  http://ampertronics.com.au
* Electronic Design   * Technical Services   * Contract Assembly
******************************************************

nangkon,

I would recommend that you get a basic book on electrical networks and 
basic electronics technology.  I'm sorry but most of my books on that are 
fairly old, so I can't recommend a good one now.  You could find something 
like this almost anywhere  -- a used bookstore, even Radio Shack.  Just 
something with tutorial on electrical components and the theory behind 
electrical circuits and loops.

You will need to understand this to do anything useful, otherwise the first 
time that something doesn't work as expected, you will have no way of 
figuring out what is wrong, and as you are finding here, you won't be able 
to compute circuit parameters that you need.  The understanding of circuits 
is important because whether it is a resistor, cap, or diode, the basic 
circuit flow follows the same principals, just different behaviors with 
respect to time and thresholds..

If i get time i will look around and if i see a good tutorial, i'll send it 
to you.

cheers,
jeffrey



At 03:47 AM 11/21/2003 +0000, nangkon wrote:

>Thanks for the info!
>
>Is there an instrument (maybe Oscilloscope) to measure
>if the cap/crystal combination is ok? And what should
>I measure (frequency)?
>
>I really want to learn this so that I can apply the
>same technique for other scenario or for other chips.
>
>Anyone can point me to tutorial or web page how to
>use cap in general. I can understand resistor, but
>cap usage is still like a black magic to me when
>I look at different schematics.
>
>thanks
>
>
>
>--- In AVR-Chat@yahoogroups.com, Mike Harrison <mike@w...> wrote:
> > On Thu, 20 Nov 2003 19:44:59 -0000, you wrote:
> >
> > >I saw a lot of schematics that use 2 capacitors with
> > >crystal that is connected to XTAL. Is there a way
> > >to determine the appropriate values of capacitors?
> > >
> > >Do I need to use some equipments to do that? Or is
> > >there a general rule somewhere?
> > >
> > >thanks in advance
> >
> > 27pf will be OK in most cases
>
>
>
>To unsubscribe from this group, send an email to:
>AVR-Chat-unsubscribe@yahoogroups.com
>
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

Hi,

I am interested in processing black-and-white images from a CCD.  problem 
is, I don't know a good CCD to start with.  can anyone recommend one?  i 
have done some searches, but the ones i find (e.g. in Digikey) are overkill 
-- color and too high resolution.  thanks in advance for any suggestions ...

jeffrey

Do you Yahoo!?
Free Pop-Up Blocker - Get it now

CMOS sensors are cheaper and easier to work with. Omnivision is one brand.
National Semi as well. There are cheap kits and evals for the Omnivision
brand out there. They usually have two busses to deal with... Serial for
access to registers in the camera and parallel for data. Otherwise you can
use a composite signal one and get a decoder chip which is not my favorite.
Be careful as some brands are only OEM in large volume and they will not
support the little guy. HP and Kodak also make them.

Al Welch 

-----Original Message-----
From: Close, Jeffrey [mailto:close@cafenoir.com] 
Sent: Thursday, November 20, 2003 10:29 PM
To: AVR-Chat@yahoogroups.com
Cc: j. jeffrey close


Hi,

I am interested in processing black-and-white images from a CCD.  problem
is, I don't know a good CCD to start with.  can anyone recommend one?  i
have done some searches, but the ones i find (e.g. in Digikey) are overkill
-- color and too high resolution.  thanks in advance for any suggestions ...

jeffrey


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On Thu, Nov 20, 2003 at 11:42:55PM -0500, David VanHorn wrote:
> At 03:47 AM 11/21/2003 +0000, nangkon wrote:
> 
> >Thanks for the info!
> >
> >Is there an instrument (maybe Oscilloscope) to measure 
> >if the cap/crystal combination is ok? And what should 
> >I measure (frequency)?
> 
> Probing a crystal will pull it off frequency, unless you use a special high capacitance, high impedance probe. 
> 
> What I use, is a shortwave recever, with BFO. 
> This works on the radiated energy from the circuit, and does not affect the circuit at all.  If you already have a Sw receiver, then this is a very good technique, especially when you can calibrate to WWV at 10.0.. or 5.0.. MHz
> 
> I have been able to probe the crystal can, and get a recoverable signal on the scope, but it's not a very large signal, and you could be misled.

My favorite technique is to use one of the dual MMIC (MAR-6) preamp boards
to give about 40 dB gain. Then make a probe by removing about a 1/2" of the
outer insulation and braid from a piece of RG-58. When connected to a decent
frequency counter and put in the general vicinity of the oscillator, you 
should be able to measure the frequency easily.

Mike