Datasheet

Page 21
6. Oscillators
Board is equipped
with 8MHz crystal
oscillator (X1) circuit
that provides external clock
waveform to the microcontroller
CLKO and CLKI pins. This base
frequency is suitable for further clock
multipliers and ideal for generation of
necessary USB clock, which ensures proper
operation of bootloader and your custom
USB-based applications.
Figure 6-1:
8MHz crystal
oscillator
module (X1)