Datasheet

Page 22
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. Board
also contains 32.768kHz Crystal oscillator (X3) which provides external
clock for internal RTCC module.
5. Crystal Oscillator
Figure 5-1:
Crystal oscillator
module (X1)
The use of crystal in all other schematics is implied even if it is purposely left out, because of the schematics clarity.
NOTE :