12.6.2.9 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 GCLK_IO is selected as a generator source in the GENCTRLn.SRC bit field, the external clock will be used as a source for GCLKn. When using an external GCLK_IO as a source for Generic Clock Generator 0 (GCLKGEN0), ensure the external source has stabilized before assigning to the GCLKGEN0 and disabling the previous clock source. The GCLK_IO does not have a status ready signal for an external input source. This can be achieved in software by counting clock pulses for a known time period (eg: using RTC or FREQM).

Note: The I/O pin (GCLK/IO[n]) must first be configured as a GCLK output by writing the corresponding 27 I/O Pin Controller (PORT) registers.

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. If this bit is '1', the output clock will be high.

In Standby mode, if the clock is output (GENCTRLn.OE=1), the clock on the I/O pin is frozen to the OOV value if the Run In Standby bit of the Generic Control register (GENCTRLn.RUNSTDBY) is zero. If GENCTRLn.RUNSTDBY is '1', the GCLKGEN clock is kept running and output to the I/O pin.