13.5.6.2.9 External Clock
The output clock (GCLK_GEN) of each Generator can be sent to I/O pins (GCLK_IO).
If the Output Enable bit in the Generator Control register is set (GENCTRLn.OE = 1) and the generator is enabled (GENCTRLn.GENEN=1), the Generator requests its clock source and the GCLK_GEN clock is output to an I/O pin.
If GENCTRLn.OE is 0, the according I/O pin is set to an Output Off Value, which is selected by GENCTRLn.OOV: If GENCTRLn.OOV is '0', the output clock will be low when turned off. If this bit is '1', the output clock will be high when turned off.