AVR-Chat

I was looking at Jameco for crystals. Under impedance is says 18pf and 
series. What is the difference on how they are hooked up to a uC?

Brian

--- In AVR-Chat@yahoogroups.com, "Brian" <blue_eagle74@...> wrote:
> Under impedance is says 18pf and series.
The specified impedance affects the size of the external load 
capacitors that you must use.  There is a formula given in the 
crystal's datasheet that you can use to compute the size of the load 
capacitors.  When computing the load capacitor size, you must take 
into account both the stray capacitance of the crystal/capacitor 
circuit and the the impedance of the X1/X2 pins of the AVR.

With the crystal and capacitors located very close to the AVR on my 
circuit boards, I use 27pF capacitors with an 18pF crystal.  Others 
seem to prefer to use 22pF capacitors but my calculations indicate 
that the latter value may be a bit low.  For many applications, I 
suspect that either size, or anything in between, would work just as 
well.

The load capacitance does affect the actual oscillator frequency.  
If the load is too high or too low the oscillator frequency will be 
too low or too high.  That is the reason that you should calculate 
the load capacitor value carefully.

Don Kinzer
ZBasic Microcontrollers
http://www.zbasic.net

crystals w/ caps are wired exactly the same as the resonator w/  
capacitors, but the fuse settings will probably be different (ie:  
Settling time and low/high voltage).
The base documentation is pretty good about describing it.   There is  
also an app. note on the atmel website about oscillator selection if  
you want to get down into the finer details.

Selecting an 18pf crystal is a good choice.   I made a mistake of 1st  
picking one surplus that needed too much loading and I couldn't find  
caps in that picofarad range that would match up.

hope that helped...

Eric Linn Rabinowitz
N6LG







On Oct 22, 2008, at 6:45 PM, Brian wrote:

I was looking at Jameco for crystals. Under impedance is says 18pf and
series. What is the difference on how they are hooked up to a uC?

Brian

The 18pF crystals are what's known as Parallel Resonant, and the
Series ones are what's known as Series Resonant. It has to do with how
you connect a load capacitance to the crystal to get it to oscillate.

The important thing is, for microcontrollers, you want Parallel Resonant.

Note that a 18 pF crystal is NOT used with 18 pF capacitors - you need
to do a little math to figure out how to size the loading capacitors.
There's information available on this in
http://www.avrfreaks.net/index.php?module=FreaksArticles&func=downloadArticle&id=21

-Randy

On Wed, Oct 22, 2008 at 7:45 PM, Brian <blue_eagle74@yahoo.com> wrote:
> I was looking at Jameco for crystals. Under impedance is says 18pf and
> series. What is the difference on how they are hooked up to a uC?
>
> Brian
>
>

A crystal is only a part of a crystal oscillator circuit.. therefore
crystals are specified to work in circuit in either a series or parallel
mode, for a given load capacitance.

Here's a bit of info on series vs parallel resonance.
http://www.circuitcellar.com/library/print/0298/bujanos91/8.htm
http://drt.com.au/Technotes.htm

your spec for your crystal means that the particular crystal needs to be in
a series resonant circuit with 18pf of load on the crystal, for the crystal
to run at its specified frequency.

Running the crystal in a parallel resonant configuration, or with different
load capacitance might still make the crystal work - but it won't be at the
specified frequency.



Julian Higginson
Bruttour International P/L
http://www.bruttour.com.au
Ph: +612 9987 1581

-----Original Message-----
From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Brian
Sent: Thursday, 23 October 2008 10:45 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] crystals

I was looking at Jameco for crystals. Under impedance is says 18pf and 
series. What is the difference on how they are hooked up to a uC?

Brian