generally, the micro may have a low speed, low accuracy internal RC type osciallator.
it will have a basic means to take a crystal, and form an oscillator (see data sheet).
A crystal oscillator (with 4 pins) can also directly generate a 1MHz+ clock.
from here, a PLL on the micro can be used to synthesize a higher or lower clock freq for things like RS232 or PWM or USB, or whatnot.