I'm curious if there is a discrepancy on page 528 of the Manual.
It says that IREFS value for MCG_C1 selects either the External Reference Clock or the slow internal reference clock.
But, if MCG_C2 --> IRCS is configured to use the Fast internal reference clock, then IREFS would be fast, not slow... right?
To use the Fast internal reference clock you only need to set IRCS as Fast Internal Reference Clock enabled (1), the state of IREFS is irrelevant once you set IRCS
yes, but before you select the fast clock mode, you need check the FRDIV ,and make sure the clock source for FLL is between 31.25khz-39.025khz.