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If a crystal has a rated load capacitance of 6 pF, is the right thing to do to put a 6 pF capacitor to GND on either leg of the crystal? I'm using it as the clock source (TOSC) for an XMEGA and it's got a max ESR of 50 kOhm (which is within recommendations).

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Check out http://en.wikipedia.org/wiki/Pierce_oscillator#Load_capacitance

The load capacitance is the total capacitance seen from the two pins of the crystal looking into the circuit. So if you have a 6 pF from each leg to ground, that's 3 pF total (capacitors in series halve, capacitors in parallel add).

It's really rated for 6 pF? That capacitance could be provided by the traces and PCB layout alone, I would think. Typically I've seen more like 30 pF.

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  • \$\begingroup\$ I know some USB micros use 12 pF caps. But you are correct, they say that the actual capacitance is more like 15 pF but pin/trace parasites give you the other 3 pF. \$\endgroup\$ – ajs410 Sep 1 '10 at 21:25
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Not a dumb question!

I know you need to take into account the capacitance of the pins on the micro and the capacitance of the traces to the crystal.

I bookmarked this page that walked me through the selection.

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Watch crystal oscillators in some MCUs have the feedback capacitors built-in. The MSP430 is one such family that springs to mind.

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  • \$\begingroup\$ yep I know the MSP430 has internal settings for this... I'm on an XMega though... so no such luck. \$\endgroup\$ – vicatcu Sep 2 '10 at 14:00
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Are you sure you need a capacitor at all? 6pF and 50KOhms sounds like a watch crystal application. As was mentioned usually the IC + traces provide you sufficient capacitance (3pF per connection).

The recommendations for some of the Maxim RTC clocks is a 6pF crystal. The 6pF is supplied by the IC and traces.

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  • \$\begingroup\$ I'm not sure, but the datasheet shows recommendations for explicit capacitors in schematics... \$\endgroup\$ – vicatcu Sep 2 '10 at 14:01
  • \$\begingroup\$ The schematic may be generic so that it will apply to different crystals. I happened to be looking at the ATmega328P datasheet yesterday (which I believe also shows capacitors) and the recommendation was no capacitors for a 6pF crystal since the combination of the pin capacitance and layout capacitance would be sufficient (for most applications). \$\endgroup\$ – jluciani Sep 3 '10 at 10:44

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