I'm using a PIC16F876 and am wondering what to use as my timing circuit. It looks like a quartz crystal would be a stable solution and appears that it is connected across pins 9 and 10, OSC1 and OSC2 respectively, obviously with some supportive circuitry (caps) as well. My question is, how do you determine what frequency of crystal to use? This particular PIC is rated at 20MHz so I'm assuming I would want to use a 20MHz crystal. What advantage is there to using a lower frequency crystal?