Loading Capacitors + Microcontroller
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Posted by blanko on September 29, 2008, 9:35 pm
Please log in for more thread options I've reviewed all the FAQ's and documents that I could find on the web but unfortunately I still believe I am missing information on calculating the correct loading capacitance for my crystal. I am using a 18.432MHz crystal on my AT91SAM9261 and I want to be sure that the loading capacitors are exactly correct so that my system starts up every time and the crystal is oscillating at the correct frequency. When I reviewed how the pierce oscillator works and I want to calculate the resultant load capacitance. I need to know the internal load capacitance of the XOUT and XIN pins of the AT91SAM9261. Here is a link to the datasheet of the crystal I am going to be using. The crystal has a load capacitance of 16 pF. http://www.abracon.com/Resonators/ab308r-ab310r.pdf Here is a link to the datasheet of the AT91SAM9261. See page 673 for the main oscillator: http://www.atmel.com/dyn/resources/prod_documents/doc6062.pdf According to my layout guy the capacitance on the traces connected from my crystal Y1 to the AT91SAM9261 is < 1 pF. Y1.2 0.71 pF Y1.1 0.36 pF What value of capacitance should I use for the external load capacitance to ensure my boards startup every time and the frequency is accurate. What percentage of tolerance should I use on the caps? Thank you so much! -E | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by David L. Jones on September 29, 2008, 10:54 pm
Please log in for more thread options No you don't, that's trivially irrelevant. > Here is a link to the datasheet of the crystal I am going to be
> using. The crystal has a load capacitance of 16 pF. > > http://www.abracon.com/Resonators/ab308r-ab310r.pdf > > Here is a link to the datasheet of the AT91SAM9261. See page 673 for > the main oscillator: > > http://www.atmel.com/dyn/resources/prod_documents/doc6062.pdf > > According to my layout guy the capacitance on the traces connected > from my crystal Y1 to the AT91SAM9261 is < 1 pF. > > Y1.2 0.71 pF > Y1.1 0.36 pF The PCB capacitance is trivially irrelevant as well. > What value of capacitance should I use for the external load
> capacitance to ensure my boards startup every time and the frequency > is accurate. What percentage of tolerance should I use on the caps? > > Thank you so much! > > -E You are way over-analysing this. The data sheet tells you what you need for a practical oscillator. Your crystal is 16pF typical. The Atmel datasheet has recommended cap values for a 15pF crystal (near enough to your 16pF nominal) - 18pF each is the recommendation for Clext. 5% or even 10% is fine. Notice how the recommended values for Clext in the datasheet don't have a min or max value, only a typical value, that tells you it's not that critical. So use 18pF and don't worry about it, you're within the datasheet recommendations. It will start every time and it'll be more than accurate enough unless you have some exotic requirement. Dave. | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by John Larkin on September 29, 2008, 10:56 pm
Please log in for more thread options On Mon, 29 Sep 2008 18:35:25 -0700 (PDT), blanko
>
>I've reviewed all the FAQ's and documents that I could find on the web >but unfortunately I still believe I am missing information on >calculating the correct loading capacitance for my crystal. > >I am using a 18.432MHz crystal on my AT91SAM9261 and I want to be sure >that the loading capacitors are exactly correct so that my system >starts up every time and the crystal is oscillating at the correct >frequency. > >When I reviewed how the pierce oscillator works and I want to >calculate the resultant load capacitance. I need to know the >internal load capacitance of the XOUT and XIN pins of the AT91SAM9261. > > >Here is a link to the datasheet of the crystal I am going to be >using. The crystal has a load capacitance of 16 pF. > >http://www.abracon.com/Resonators/ab308r-ab310r.pdf > >Here is a link to the datasheet of the AT91SAM9261. See page 673 for >the main oscillator: > >http://www.atmel.com/dyn/resources/prod_documents/doc6062.pdf > > >According to my layout guy the capacitance on the traces connected >from my crystal Y1 to the AT91SAM9261 is < 1 pF. > >Y1.2 0.71 pF >Y1.1 0.36 pF > >What value of capacitance should I use for the external load >capacitance to ensure my boards startup every time and the frequency >is accurate. What percentage of tolerance should I use on the caps? > >Thank you so much! > >-E It's a lot easier, and generally cheaper, to just buy a packaged oscillator. Smaller, too, and a lot less risky. John | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Eeyore on September 30, 2008, 12:26 am
Please log in for more thread options
John Larkin wrote: > It's a lot easier, and generally cheaper, to just buy a packaged
> oscillator. Smaller, too, and a lot less risky. Disagree big time. Especially if you only need a resonator, not a crystal. Graham | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by John Larkin on September 30, 2008, 12:54 am
Please log in for more thread options On Tue, 30 Sep 2008 05:26:39 +0100, Eeyore
>
> >John Larkin wrote: > >> It's a lot easier, and generally cheaper, to just buy a packaged
>> oscillator. Smaller, too, and a lot less risky. >
>Disagree big time. Especially if you only need a resonator, not a crystal. > >Graham The OP specified "crystal." I've seen far too many piece-part oscillators not oscillate, or only oscillate off-frequency, or quit working if a different crystal was purchased. Packaged oscillators are cheap and tiny nowadays, always work, and usually come trimmed to a couple of PPM. If you don't need accuracy, one of the new silicon oscillators may be the way to go. SC70, 45 cents... http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4103 Everybody is doing this. http://www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1010,C1096 http://www.st.com/stonline/products/families/analog_and_mixed_signal/silicon_oscillator/silicon_oscillators.htm MEMS oscillators are becoming available, and are more accurate. http://www.sitime.com/products/sit8002.php John | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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> but unfortunately I still believe I am missing information on
> calculating the correct loading capacitance for my crystal.
>
> I am using a 18.432MHz crystal on my AT91SAM9261 and I want to be sure
> that the loading capacitors are exactly correct so that my system
> starts up every time and the crystal is oscillating at the correct
> frequency.
>
> When I reviewed how the pierce oscillator works and I want to
> calculate the resultant load capacitance. I need to know the
> internal load capacitance of the XOUT and XIN pins of the AT91SAM9261.