20.6.5.3 Entering Standby Mode
There may occur a delay when the device is put into Standby, until all clocks are gated off. This delay is caused by running Clock Generators: if the Run in Standby bit in the Generator Control register (GENCTRLn.RUNSTDBY) is '0', GCLK must verify that the clock is turned off properly. The duration of this verification is frequency-dependent.