Revisions Affected

0, A

Details

The PLL has a limp mode feature to provide a slow PLLRAWCLK output even if its input OSCCLK is absent. Independently, the Missing Clock Detect (MCD) circuit will forcibly switch the system clock source to INTOSC1 when a missing OSCCLK input is detected. The MCD mux to switch between these system clock sources is not ensured to be glitch-free when both clock sources (PLLRAWCLK and INTOSC1) are still active. In rare cases, this may lead to unpredictable device behavior during a missing clock failure event.

Workarounds

When the PLL is used by the system (PLLCLKEN = 1), disable the MCD by writing MCDCR.MCLKOFF = 1.

The Dual Clock Comparator (DCC) circuit can be configured to quickly detect if the SYSCLK frequency drops outside the desired frequency to its limp mode due to a missing clock event.

When the system is operating in PLL bypass mode (PLLCLKEN = 0), the MCD circuit can still be used to detect missing clock events and switch the clock source to INTOSC1.