Hey guys,
I have a question concerning the calculation of xtal load caps. How do you do it?
Thanks,
Croc4
With the usual Pierce oscillator used in MCUs they are effectively in series, so a 20 pF crystal will need something like 33 pF capacitors, allowing for some stray capacitance.
Thanks, but I was looking for the actual nuts and bolts for coming up with the values.
Are you trying to match the xtal or the MCU or both with the load caps?
Croc4
[piclist has a nice page on crystals. In fact, the first thing on there is:
First, the crystal determines the capacitor values, not the chip.
Atmel’s application notes correctly leave off the cap values, since they can’t tell you what values to use!
There are also some good app notes on how to select set up a crystal. Microchip’s AN826 has some of the gory details. I think I’ve seen a better one out there, try searching the app notes for the other major vendors if that doesn’t answer your questions.](Processor Clock Crystals)
What do you do when the Datasheets example setup includes no caps on the xtal? Are they assuming you know to add them or are they not neccesary?
Some chips have them built-in.
Choose a 27MHz parallel resonant, fundamental mode
crystal. No external load capacitors are needed. All
capacitors required for the Pierce oscillator are included
on-chip.
Cheers Leon, found this in the datasheet 
The only problem with that approach is that, ideally, you need to find a crystal with a load capacitance that matches the internal capacitors. They sometimes have odd values, and the crystals can be hard to find. Unless you need the frequency to be spot-on, it doesn’t usually matter much, though.
Im not sure how to go about this now, its for a MAX7456 OSD IC. I suppose I can test it see how it performs before finalizing the design.
When going into production, any crystal circuit needs to be tested thoroughly under all conditions, anyway.
Thanks for all the replies
Croc4