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