17.6.2.5 Clock Ready Flag
There is a slight delay between writing to CPUDIV until the new clock settings become effective.
During this interval, the Clock Ready flag in the Interrupt Flag Status and Clear register (INTFLAG.CKRDY) will return zero when read. If CKRDY in the INTENSET register is set to '1', the Clock Ready interrupt will be triggered when the new clock setting is effective.