Forums

56F803 - crystal selection??

Started by Jesse March 11, 2004
Section 3.5.1 of the DSP56F803 datasheet talks about using a crystal
oscillator. It states the no external load capacitance should be
used (the load capacitance is designed into the internal oscillator
circuitry). In their example, they have CL = 9pF.

My question is this... if they were to now choose a crystal, they'd
have to find one spec'd for a load capacitance of 9pF? Crystals with
18pF or even 12pF are common... but 9pF?

The problem is if you just go ahead and plunk in a crystal spec'd at
18pF, it will not oscillate at the correct frequency (and since it's
so far out, may have stability/startup problems).

After talking to a few crystal MFG's, it's apparent that they'll
custom make a crystal with this spec, though they don't recommend it.

Has anybody else run into this? Does Motorola recommend certain
crystals?

Any help appreciated.

Thanks
J.



Jesse -
 
It is not specifically a reccomendation to use a CL = 9 pF crystal but more of a informative statement on what the value for CL is when using our reccomended circuit.
 
A standard 18 pF crystal can be used and is very commonly used. I do not think the start up and stability are significant issues when using a standard 18 pF crystal. The main driver on start up is the ESR value. The other issue is that the oscillation frequency will be slightly incorrect. This also has not proven to be a big problem as the error frequency is quite small.
 
The objective of the data sheet showing the CL is that if you had determined that the error in using a typical standard crystal was to great then the CL value provided gives a guidleine on how to specify a crystal to get a more precise oscillating frequency. But I think it is very lilkely a standard crystal will work perfectly fine in your application. I don't know of anyone who is using a CL = 9 pF crystal in their application. Thanks.
 
- Bill

 

General Business InformationMotorola Internal Use OnlyMotorola Confidential ProprietaryPersonal Memorandum

 

-----Original Message-----
From: Jesse [mailto:e...@yahoo.ca]
Sent: Wednesday, March 10, 2004 4:15 PM
To: m...@yahoogroups.com
Subject: [motoroladsp] 56F803 - crystal selection??

Section 3.5.1 of the DSP56F803 datasheet talks about using a crystal
oscillator.  It states the no external load capacitance should be
used (the load capacitance is designed into the internal oscillator
circuitry).  In their example, they have CL = 9pF.

My question is this... if they were to now choose a crystal, they'd
have to find one spec'd for a load capacitance of 9pF?  Crystals with
18pF or even 12pF are common... but 9pF? 

The problem is if you just go ahead and plunk in a crystal spec'd at
18pF, it will not oscillate at the correct frequency (and since it's
so far out, may have stability/startup problems).

After talking to a few crystal MFG's, it's apparent that they'll
custom make a crystal with this spec, though they don't recommend it.

Has anybody else run into this?  Does Motorola recommend certain
crystals?

Any help appreciated.

Thanks
J.

 


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Thank you for your reply!

""The other issue is that the oscillation frequency will be slightly
incorrect.""

This is precisely what I am experiencing... the frequency I get is
8.002MHz (about 250ppm for 8MHz). Unfortunately, I'm keeping track
of 'seconds' and this leads to a gross error every day. If I was
confident that each unit in production would oscillate at this
frequency 'new' frequency (within tolerances of the crystal ect...),
I could offset the difference in code to get a better 1 second
interval.

Does anybody know if using a crystal with a load capacitance other
than what it was spec'd consistantly (within tolerance of the
crystal - from unit to unit) oscillate at a certain frequency (in my
case - 8.002MHz).

Thanks
Jesse

--- In , Hutchings William-p23437
<william.hutchings@m...> wrote:
> Jesse -
>
> It is not specifically a reccomendation to use a CL = 9 pF crystal
but more of a informative statement on what the value for CL is when
using our reccomended circuit.
>
> A standard 18 pF crystal can be used and is very commonly used. I
do not think the start up and stability are significant issues when
using a standard 18 pF crystal. The main driver on start up is the
ESR value. The other issue is that the oscillation frequency will be
slightly incorrect. This also has not proven to be a big problem as
the error frequency is quite small.
>
> The objective of the data sheet showing the CL is that if you had
determined that the error in using a typical standard crystal was to
great then the CL value provided gives a guidleine on how to specify
a crystal to get a more precise oscillating frequency. But I think it
is very lilkely a standard crystal will work perfectly fine in your
application. I don't know of anyone who is using a CL = 9 pF crystal
in their application. Thanks.
>
> - Bill
>
> <?xml:namespace prefix = o ns = "urn:schemas-microsoft-
com:office:office" />
>
> General Business Information Motorola Internal Use Only Motorola
Confidential Proprietary Personal Memorandum
> -----Original Message-----
> From: Jesse [mailto:electricjesster@y...]
> Sent: Wednesday, March 10, 2004 4:15 PM
> To:
> Subject: [motoroladsp] 56F803 - crystal selection?? > Section 3.5.1 of the DSP56F803 datasheet talks about using a
crystal
> oscillator. It states the no external load capacitance should be
> used (the load capacitance is designed into the internal oscillator
> circuitry). In their example, they have CL = 9pF.
>
> My question is this... if they were to now choose a crystal, they'd
> have to find one spec'd for a load capacitance of 9pF? Crystals
with
> 18pF or even 12pF are common... but 9pF?
>
> The problem is if you just go ahead and plunk in a crystal spec'd
at
> 18pF, it will not oscillate at the correct frequency (and since
it's
> so far out, may have stability/startup problems).
>
> After talking to a few crystal MFG's, it's apparent that they'll
> custom make a crystal with this spec, though they don't recommend
it.
>
> Has anybody else run into this? Does Motorola recommend certain
> crystals?
>
> Any help appreciated.
>
> Thanks
> J. >
> _____________________________________
> Note: If you do a simple "reply" with your email client, only the
author of this message will receive your answer. You need to do
a "reply all" if you want your answer to be distributed to the entire
group.
>
> _____________________________________
> About this discussion group:
>
> To Join:
>
> To Post:
>
> To Leave:
>
> Archives: http://www.yahoogroups.com/group/motoroladsp
<http://www.yahoogroups.com/group/motoroladsp>
>
> More Groups: http://www.dsprelated.com/groups.php3
<http://www.dsprelated.com/groups.php3>
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> _____
>
> > .




Jesse -
 
I am far from a Crystal expert, but the answer is yes it will be consistant. The parameters are trim sensitivity and pullability. The attached doc has a good discussion of these parameters. Thanks.
 
- Bill
 
 
 
-----Original Message-----
From: Jesse [mailto:e...@yahoo.ca]
Sent: Thursday, March 11, 2004 3:29 PM
To: m...@yahoogroups.com
Subject: [motoroladsp] Re: 56F803 - crystal selection??



Thank you for your reply!

""The other issue is that the oscillation frequency will be slightly
incorrect.""

This is precisely what I am experiencing... the frequency I get is
8.002MHz (about 250ppm for 8MHz).  Unfortunately, I'm keeping track
of 'seconds' and this leads to a gross error every day.  If I was
confident that each unit in production would oscillate at this
frequency 'new' frequency (within tolerances of the crystal ect...),
I could offset the difference in code to get a better 1 second
interval. 

Does anybody know if using a crystal with a load capacitance other
than what it was spec'd consistantly (within tolerance of the
crystal - from unit to unit) oscillate at a certain frequency (in my
case - 8.002MHz).

Thanks
Jesse

--- In m...@yahoogroups.com, Hutchings William-p23437
<william.hutchings@m...> wrote:
> Jesse -

> It is not specifically a reccomendation to use a CL = 9 pF crystal
but more of a informative statement on what the value for CL is when
using our reccomended circuit.

> A standard 18 pF crystal can be used and is very commonly used. I
do not think the start up and stability are significant issues when
using a standard 18 pF crystal. The main driver on start up is the
ESR value. The other issue is that the oscillation frequency will be
slightly incorrect. This also has not proven to be a big problem as
the error frequency is quite small.

> The objective of the data sheet showing the CL is that if you had
determined that the error in using a typical standard crystal was to
great then the CL value provided gives a guidleine on how to specify
a crystal to get a more precise oscillating frequency. But I think it
is very lilkely a standard crystal will work perfectly fine in your
application. I don't know of anyone who is using a CL = 9 pF crystal
in their application. Thanks.

> - Bill
>
>  <?xml:namespace prefix = o ns = "urn:schemas-microsoft-
com:office:office" />
>
>  General Business Information Motorola Internal Use Only Motorola
Confidential Proprietary Personal Memorandum
>

>
> -----Original Message-----
> From: Jesse [mailto:electricjesster@y...]
> Sent: Wednesday, March 10, 2004 4:15 PM
> To: m...@yahoogroups.com
> Subject: [motoroladsp] 56F803 - crystal selection??> Section 3.5.1 of the DSP56F803 datasheet talks about using a
crystal
> oscillator.  It states the no external load capacitance should be
> used (the load capacitance is designed into the internal oscillator
> circuitry).  In their example, they have CL = 9pF.
>
> My question is this... if they were to now choose a crystal, they'd
> have to find one spec'd for a load capacitance of 9pF?  Crystals
with
> 18pF or even 12pF are common... but 9pF? 
>
> The problem is if you just go ahead and plunk in a crystal spec'd
at
> 18pF, it will not oscillate at the correct frequency (and since
it's
> so far out, may have stability/startup problems).
>
> After talking to a few crystal MFG's, it's apparent that they'll
> custom make a crystal with this spec, though they don't recommend
it.
>
> Has anybody else run into this?  Does Motorola recommend certain
> crystals?
>
> Any help appreciated.
>
> Thanks
> J.
>
>  >
>
> _____________________________________
> Note: If you do a simple "reply" with your email client, only the
author of this message will receive your answer.  You need to do
a "reply all" if you want your answer to be distributed to the entire
group.
>
> _____________________________________
> About this discussion group:
>
> To Join:  m...@yahoogroups.com
>
> To Post:  m...@yahoogroups.com
>
> To Leave: m...@yahoogroups.com
>
> Archives: http://www.yahoogroups.com/group/motoroladsp
<http://www.yahoogroups.com/group/motoroladsp>
>
> More Groups: http://www.dsprelated.com/groups.php3
<http://www.dsprelated.com/groups.php3>
>
>   _____ 
>
> > .



_____________________________________
Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer.  You need to do a "reply all" if you want your answer to be distributed to the entire group.

_____________________________________
About this discussion group:

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