clock for uP

A bit more information is necessary.

But back in the old days, a crystal was directly connected to X1 & X2, with capacitors (typically 10pF ceramic) from each lead (X1 & GND / X2 & GND) to ground to help the crystal 'startup' and get to it's resonant frequency.

As for connecting directly to X1, I assume that has to do with driving the clock input of a uP directly. If I remember correctly some (if not most) can be directly driven by a TTL level signal from an external clock generator; and make sure that your driving the correct input. And then again I believe that some wanted their 'own' clock generator.

You need to be aware that some uP's require certain voltage levels, slew rates & frequency's to operate properly.

thank you