What value caps for Atmega328 crystal? Always 22pF?

Hi All, a question about stabilizing external Crystal for Atmega328 chip. It seems like most schematic call for use of 22pF capacitors but where did that value come from? I am asking because I am choosing few 8Mhz surface mount Crystals for Atmega328p and not quite sure if 22pF can be used with all of them because on the datasheets they have differently "Load Capacitance" values, some have 20pF some 18pF some 8pF. Are those the values I should use or its always need to be 22pF ? Thanks in advance.


Atmegs328P datasheet, 9.4:

Table 9-5. Full Swing Crystal Oscillator operating modes Frequency Range(1) (MHz) Recommended Range for CKSEL3...1 Capacitors C1 and C2 (pF) 0.4 - 20 12 - 22 011

Table 9-3. Low Power Crystal Oscillator Operating Modes(3) Frequency Range (MHz) Recommended Range for CKSEL3...1 Capacitors C1 and C2 (pF) 8.0 - 16.0 12 - 22 111

I have not had any issues with 22pF, been using that value since Fall 2010 with 328P, 1284P, and 2560 designs.

The "load capacitance" values in the crystal specification include stray capacity on the PCB or wiring, which is impossible for the manufacturer to predict. If a chosen capacitor doesn't work, your design may be at fault.

And the ATmega itself will put a lot of capacitance on the pins, perhaps 4 to 10pF or so each...

A lot of crystals want something like 15 to 18pF load, and the standard circuit uses 2 caps in series(*), so each should be 30 to 36pF (less the stray capacitance and pin capacitance), hence the generic 22pF discrete caps to get to the 30 to 36pF ballpark.

(*) with ground in the middle, but from the xtal's point of view they are simply in series.