Hi Javier,

I searchd first, but have not found the proper info.

Did you try entering the word "Crystal" in the field titled Search ? I did and I was amazed.

Anyway there is an authentic PDF from Intel ( AP-155)that is Titled "Oscillators for Microcontrollers". It may have the answers for you.

And finally,

On board Oscillators are relatively inaccurate as compared to what you can achive externally ( generally)

Crystals are different from Resonators if thats what you mean. And I suppose Crystals are more accurate?

Raghu