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.
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.
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