Selecting the Clock Source for a Peripheral

When changing a peripheral clock source by writing to PCHCTRLm.GEN, the peripheral clock must be disabled before re-enabling it with the new clock source setting. This prevents glitches during the transition:

1. Disable the Peripheral Channel by writing PCHCTRLm.CHEN=0
2. Assert that PCHCTRLm.CHEN reads '0'
3. Change the source of the Peripheral Channel by writing PCHCTRLm.GEN
4. Re-enable the Peripheral Channel by writing PCHCTRLm.CHEN=1