Loading Capacitors + Microcontroller

Sep 30, 2008 5 Replies

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blanko 17 years ago

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.

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Here is a link to the datasheet of the AT91SAM9261. See page 673 for the main oscillator:

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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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David L. Jones 17 years ago

No you don't, that's trivially irrelevant.

The PCB capacitance is trivially irrelevant as well.

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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John Larkin 17 years ago

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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Tim Wescott 17 years ago

What is the sound of one hand tolerancing?

Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html

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Ben Bradley 17 years ago

Just connect a few inches of wire 'antenna' to the xin pin, add a BFO and you get the sound of a Theremin. Might as well put that hand-waving to good use.

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richard 17 years ago

Use two 33pF capacitors. They are effectively in series.

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