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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4 Answers
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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.
answered Aug 31, 2010 at 17:19
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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\$– ajs410Commented Sep 1, 2010 at 21:25
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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.
answered Aug 31, 2010 at 18:59
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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\$– vicatcuCommented Sep 2, 2010 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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1\$\begingroup\$ I'm not sure, but the datasheet shows recommendations for explicit capacitors in schematics... \$\endgroup\$– vicatcuCommented Sep 2, 2010 at 14:01
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\$\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\$– jlucianiCommented Sep 3, 2010 at 10:44
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Let's say you have a Crystal rated with 8pf Load Capacitance.
So how do you know which capacitors to use? Easy. Every crystal datasheet lists something called the Load Capacitance (CL). In the case of the crystal above, it’s 8 pF. C1 and C2 need to match this Load Capacitance, with the following formula being the key:
CL = (C1 * C2) / (C1 + C2) + Cstray
I got that from https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/
