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I'm working with an LPC2387 (datasheet), and pp. 55-56 of the datasheet specifies several options for the RTC crystal with different load capacitance: 11pf, 13pf, and 15pf. For each of these crystals, different load capacitors are specified.

First, the datasheet speaks exclusively of "32 KHz" crystals. It seems to me that what they really mean is "32.768 KHz", is that correct?

Second, these load capacitances seem odd to me, since neither Digi-Key nor Mouser carry 32KHz crystals with these specifications. There is a 12.5pf crystal, which is possibly close enough, but not exact. Am I expected to calculate the capacitance of my traces and factor that into my choice, or do I simply choose the closest match?

Third, if I do end up having a choice of crystals, should I choose the highest load capacitance? Does that provide a more stable oscillation?

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  • \$\begingroup\$ electronics.stackexchange.com/questions/4225/… \$\endgroup\$ – endolith Sep 30 '11 at 17:55
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    \$\begingroup\$ @endolith: It's a related question, but not the same. In my case, the datasheet tells me exactly what load capacitance to choose for my crystal, and what capacitors to attach and where. It's just that none of the specified crystals seem to actually exist. \$\endgroup\$ – Mark Sep 30 '11 at 18:16
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You need to add a capacitor to each crystal terminal making up a total capacitance of 12.5 pF, including tracks etc. The two capacitors are effectively in series, so you need something like 20 pF.

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  • \$\begingroup\$ The datasheet (apparently) takes these into account. For example, given a crystal of load capacitance 11pf, the datasheet specifies external load capacitors of 18pf. For a 13pf crystal, the caps are 22pf. \$\endgroup\$ – Mark Sep 30 '11 at 16:54
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Yes this question did look familiar ;-p. Here's my take.

  1. The capacitance of the traces should be a non-issue, because the recommendation is typically to locate the crystal as close to the drive pins as is practical.

  2. I'm pretty confident that the mouser 12.5pF capacitors you referenced in the question will do a splendid job at satisfying the load capacitance requirements. Don't worry too terribly much about it, experience suggests that these things sort of just work.

  3. Yes, when they say 32 kHz they really mean 32.768 kHz, the standard frequency of a quartz crystal.

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I read the data sheet for the RTC chip I am using, and it seemed to indicate that the load capacitance of 12.5 pF included stray capacitance ie the PCB capacitance. So my understanding is, use a 12 pF cap.

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  • \$\begingroup\$ Welcome to EE SE. Please take the tour at electronics.stackexchange.com/Tour to get the most out of this site. Meanwhile, I see you posted a statement. What is the problem and specific question related to your project? \$\endgroup\$ – SDsolar Aug 10 '17 at 5:08

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