Crystal Cload and ESR for microcontroller...

Mar 08, 2008 5 Replies

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Franco 18 years ago

Hi everyone:

I was wondering if the external capacitors for matching Cload are "always" neccesary when attaching a crystal to a microcontroller? If the internal capacitance of the microcontroller oscillator is the same as the specified in the datasheet of the crystal, then I don't see reasons for the external capacitors (those two ones connected between the the leads of the crystal and ground).

The other question is about the ESR indicated in the crystal datasheet. If this ESR indicated in the datasheet of the crystal is slightly higher than the one indicated as maximum for the crystal in the datasheet of the microcontroller, how could it affect the frequency?

Is there any way to correct that ESR of the crystal with an external passive component (resistor or something)?

Thanks!

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sycochkn 18 years ago

How accurate do you need the frequency to be? If you need more accuracy then an external oven controlled crystal oscillator.

Bob

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Tim Wescott 18 years ago

A microprocessor pin that has parasitic capacitance that exceeds the crystal load capacitance would be weird. What load capacitance and microprocessor are you using?

Depending on where the capacitance is things may not work -- you need capacitors to ground at each oscillator terminals to properly couple the circuit. Just putting a capacitor in parallel with the crystal will bring the resonant frequency to the correct value, but won't allow the oscillator to work.

It could bring it all the way to zero, on a bad day. The ESR of the crystal is a factor of the overall loop gain; if the crystal is too lossy then the oscillator may fail to start.

In general resistance in circuits like this bring the resonant frequency down.

No. A checkbook helps in getting better components, but it's not a passive component when used in this manner.

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

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Martin Riddle 18 years ago

Microchip has a good appnote worth reading.

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Cheers

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Franco 18 years ago

Hi Tim, thanks for the answer. The microcontroller I am using (AT91SAM7S256) happens to have integrated load capacitances in the leads of the oscillator, which ends up on a equivalent internal load capacitance of 17-23pF which matches my Cload=20pF crystal.

I managed to find a nice crystal that provide me with the recommended maximum ESR (for the frequency I selected for it).

Thanks...

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Franco 18 years ago

reading.http://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf

messagenews: snipped-for-privacy@u72g2000hsf.googlegroups.com...

Hi Martin, thanks for the doc. I think that is what I needed.

Thanks.

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