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Basic Electronics » Why is the crystal connected to GND in every schematics, but not in the NerdKit?

February 16, 2010
by Frozenlock
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I've been looking at many schematics lately and it seems the crystal for clock pulse is always connected to GND and with some capacitors. Yet, in the NerdKit it's only connected to the Xtal pins.

Could someone please explain me why the difference?

Thanks in advance!

Frozenlock

February 17, 2010
by Rick_S
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Those are called load capacitors. Most crystals have a recommended load capacitance to help get them started oscillating. Because of the natural capacitance inherent in the solderless breadboard, the Kit will typically work just fine without them. If you were to move your project to a permanent etched circuit board, you may need to add those. Load capacitors typically will range from 22 to 33 pf.

Rick

February 17, 2010
by BobaMosfet
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Frozenlock--

Rick_S is correct. To give you a larger understanding, those capacitors are there to "tune" the crystal-- to help make sure it operates within it's rated specification. There are all kinds of crystals. If you can, you want to work with 'fundamental' crystals because they are generally of higher quality. The FP1474 (14.74MHz) crystal that came with my original nerdkit was a 'fundamental' crystal, and it likes to have two 20pF capacitors tied to ground on either leg in order to help it stay right where it has to be. In breadboarding cases, it isn't always necessary-- some times it depends on how much stray noise is on the board.

If you hear of 'overtones' those are higher frequencies the same crystal can generate under the right circumstances, based on the fundamental frequency. FFT calculations and an oscilloscope can help find those overtones if you don't already know them for a given crystal.

BM

February 17, 2010
by Frozenlock
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Thank you both for your answers!

February 17, 2010
by mrobbins
(NerdKits Staff)

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Thanks to Rick_S and BobaMosfet for your help. You might also want to see BobaMosfet's post from a few months ago on this very topic!

Also, if you ever see a "three-legged crystal" where one terminal is connected to ground, it's not really a crystal -- it's a ceramic oscillator with the load capacitors built in. This may be part of the answer to your original "crystal connected to GND" question.

Mike

February 17, 2010
by Frozenlock
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My apologies for the double thread. I did a quick search for the word "crystal" in the subject, but none with oscillator.

By the way, what is the worse case scenario for a non-grounded crystal? Can they stop oscillating?

February 20, 2010
by BobaMosfet
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Any crystal can stop oscillating if it's bad, damaged, or not properly feeling the attraction of voltage to pull sufficient current through it.

Think about your question. If it has two legs, one leg is common. A third leg is another common, usually for the case (and is in fact what mrobbins above mentioned).

If you have an o-scope, it's easy to see if the crystal is working and at what rate.

BM

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