Crystal/Resonator Noob question

Greetings,
  I am having a problem understanding how to setup an SMD ceramic
crystal to work with an LPC2106 micro.  Maybe its the crystal part
that I do not get.  What is the load capacitance? Is it the
capacitance that the crystal exerts on the circuit?  How do I pick the
capacitor values required, and how do I know that they are needed?
The part I am looking at is http://www.abracon.com/Resonators/abm8.pdf
which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
manufacturer's website to get some hints but was unsuccessful.

Any hints or pointers is greatly appreciated.

Thank you

-- A

amerdsp wrote:
> Greetings,
>   I am having a problem understanding how to setup an SMD ceramic
> crystal to work with an LPC2106 micro.  Maybe its the crystal part
> that I do not get.  What is the load capacitance? Is it the
> capacitance that the crystal exerts on the circuit?  How do I pick the
> capacitor values required, and how do I know that they are needed?
> The part I am looking at is http://www.abracon.com/Resonators/abm8.pdf
> which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
> manufacturer's website to get some hints but was unsuccessful.
> 
> Any hints or pointers is greatly appreciated.
> 

Answer #4 explains it:

http://www.foxonline.com/techfaqs_cry.htm#a4

-- 
Regards, Joerg

http://www.analogconsultants.com/

amerdsp wrote:
> Greetings,
>  I am having a problem understanding how to setup an SMD ceramic
> crystal to work with an LPC2106 micro.  Maybe its the crystal part
> that I do not get.  What is the load capacitance? Is it the

It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly enough 
though they don't bother stating whether the crystal is cut for parallel or 
series resonance.

> capacitance that the crystal exerts on the circuit?  How do I pick the

It is the capacitance that the crystal expects the circuit to provide so 
that it runs on its stated frequency.

> capacitor values required, and how do I know that they are needed?
> The part I am looking at is http://www.abracon.com/Resonators/abm8.pdf
> which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
> manufacturer's website to get some hints but was unsuccessful.
>
> Any hints or pointers is greatly appreciated.

Look at Section 3 of the User Manual.  You will hook it up as shown in 
figure 7b.  I suggest two 33pF caps to start. 

Anthony Fremont wrote:
> amerdsp wrote:
>> Greetings,
>>  I am having a problem understanding how to setup an SMD ceramic
>> crystal to work with an LPC2106 micro.  Maybe its the crystal part
>> that I do not get.  What is the load capacitance? Is it the
> 
> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly enough 
> though they don't bother stating whether the crystal is cut for parallel or 
> series resonance.
> 
>> capacitance that the crystal exerts on the circuit?  How do I pick the
> 
> It is the capacitance that the crystal expects the circuit to provide so 
> that it runs on its stated frequency.
> 
>> capacitor values required, and how do I know that they are needed?
>> The part I am looking at is http://www.abracon.com/Resonators/abm8.pdf
>> which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
>> manufacturer's website to get some hints but was unsuccessful.
>>
>> Any hints or pointers is greatly appreciated.
> 
> Look at Section 3 of the User Manual.  You will hook it up as shown in 
> figure 7b.  I suggest two 33pF caps to start. 
> 

It'll usually be less than that. The OP needs to take into account other 
capacitances: Traces to ground and to each other, pins to ground, etc. 
For the LPC2106 the chip capacitances might be found in a family spec, 
not necessarily in the datasheet.

-- 
Regards, Joerg

http://www.analogconsultants.com/

On Thu, 15 Nov 2007 19:51:55 -0600, Anthony Fremont wrote:

> amerdsp wrote:
>> Greetings,
>>  I am having a problem understanding how to setup an SMD ceramic
>> crystal to work with an LPC2106 micro.  Maybe its the crystal part
>> that I do not get.  What is the load capacitance? Is it the
> 
> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly enough 
> though they don't bother stating whether the crystal is cut for parallel or 
> series resonance.

The notion of load capacitance is meaningless in the context of a series
resonant crystal -- either you're operating it at series resonance, where
it's impedance is purely resistive and minimal, or you're operating it at
"parallel resonance" where the crystal is inductive and working against
the load capacitance.

-- 
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

On Nov 16, 6:31 am, amerdsp <amer...@hotmail.com> wrote:
> Greetings,
>   I am having a problem understanding how to setup an SMD ceramic
> crystal to work with an LPC2106 micro.  Maybe its the crystal part
> that I do not get.  What is the load capacitance? Is it the
> capacitance that the crystal exerts on the circuit?  How do I pick the
> capacitor values required, and how do I know that they are needed?
> The part I am looking at ishttp://www.abracon.com/Resonators/abm8.pdf
> which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
> manufacturer's website to get some hints but was unsuccessful.
>
> Any hints or pointers is greatly appreciated.
>

You can select the values of capacitors C1 and C2 to match the crystal
C_L using:
C_L = (C1 * C2)/(C1 + C2) + CS.
Here,CS is the Stray Capacitance (Typically 2-5pF)
and CL is the Circuit capacitance required by the crystal for
operating at the desired frequency.

Take care while selecting values of C1 and C2.
Large values will increase frequency stability but decrease the loop
gain,
And so, that may cause problems during oscillator startup. Take care
w.r.t C1 and C2.

Lot of tutorials are available in the internet.
And yout develpment environment manual will surely have clear
explanations for this .

Karthik Balaguru

On Nov 15, 8:51 pm, "Anthony Fremont" <nob...@noplace.net> wrote:
> amerdsp wrote:
> > Greetings,
> >  I am having a problem understanding how to setup an SMD ceramic
> > crystal to work with an LPC2106 micro.  Maybe its the crystal part
> > that I do not get.  What is the load capacitance? Is it the
>
> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly enough
> though they don't bother stating whether the crystal is cut for parallel or
> series resonance.
>
> > capacitance that the crystal exerts on the circuit?  How do I pick the
>
> It is the capacitance that the crystal expects the circuit to provide so
> that it runs on its stated frequency.
>
> > capacitor values required, and how do I know that they are needed?
> > The part I am looking at ishttp://www.abracon.com/Resonators/abm8.pdf
> > which is a 14.7456MHZ miniature SMD crystal.  I tried to look at the
> > manufacturer's website to get some hints but was unsuccessful.
>
> > Any hints or pointers is greatly appreciated.
>
> Look at Section 3 of the User Manual.  You will hook it up as shown in
> figure 7b.  I suggest two 33pF caps to start.

I am sorry, but which user manual are you referring to?  All I could
find is the datasheet and a few application notes.

Thank you,

-- A

amerdsp wrote:
> On Nov 15, 8:51 pm, "Anthony Fremont" <nob...@noplace.net> wrote:

>> Look at Section 3 of the User Manual.  You will hook it up as shown
>> in figure 7b.  I suggest two 33pF caps to start.
>
> I am sorry, but which user manual are you referring to?  All I could
> find is the datasheet and a few application notes.

http://www.nxp.com/acrobat_download/usermanuals/UM_LPC2106_2105_2104_1.pdf 

Joerg wrote:
> Anthony Fremont wrote:
>> amerdsp wrote:
>>> Greetings,
>>>  I am having a problem understanding how to setup an SMD ceramic
>>> crystal to work with an LPC2106 micro.  Maybe its the crystal part
>>> that I do not get.  What is the load capacitance? Is it the
>>
>> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly
>> enough though they don't bother stating whether the crystal is cut
>> for parallel or series resonance.
>>
>>> capacitance that the crystal exerts on the circuit?  How do I pick
>>> the
>>
>> It is the capacitance that the crystal expects the circuit to
>> provide so that it runs on its stated frequency.
>>
>>> capacitor values required, and how do I know that they are needed?
>>> The part I am looking at is
>>> http://www.abracon.com/Resonators/abm8.pdf which is a 14.7456MHZ
>>> miniature SMD crystal.  I tried to look at the manufacturer's
>>> website to get some hints but was unsuccessful. Any hints or pointers is 
>>> greatly appreciated.
>>
>> Look at Section 3 of the User Manual.  You will hook it up as shown
>> in figure 7b.  I suggest two 33pF caps to start.
>>
>
> It'll usually be less than that. The OP needs to take into account
> other capacitances: Traces to ground and to each other, pins to
> ground, etc. For the LPC2106 the chip capacitances might be found in
> a family spec, not necessarily in the datasheet.

I was thinking that Cstray wouldn't be all that high given the smd parts, 
and that a pair of 22pF might not always start reliably. 

Tim Wescott wrote:
> On Thu, 15 Nov 2007 19:51:55 -0600, Anthony Fremont wrote:
>
>> amerdsp wrote:
>>> Greetings,
>>>  I am having a problem understanding how to setup an SMD ceramic
>>> crystal to work with an LPC2106 micro.  Maybe its the crystal part
>>> that I do not get.  What is the load capacitance? Is it the
>>
>> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly
>> enough though they don't bother stating whether the crystal is cut
>> for parallel or series resonance.
>
> The notion of load capacitance is meaningless in the context of a
> series resonant crystal -- either you're operating it at series
> resonance, where it's impedance is purely resistive and minimal, or
> you're operating it at "parallel resonance" where the crystal is
> inductive and working against the load capacitance.

Thanks for the info Tim.  :-)