SLAZ320AC October   2012  – September 2021 MSP430F5635

 

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6.43 UCS11

UCS Module

Category

Functional

Function

Modifying UCSCTL4 clock control register triggers an additional erroneous clock request

Description

Changing the SELM/SELS/SELA bits in the UCSCTL4 register will correctly configure the respective clock to use the intended clock source but might also erroneously set XT1/XT2 fault flag if the crystals are not present at XT1/XT2 or not configured in the application firmware. If the NMI interrupt for the OFIFG is enabled, an unintentional NMI interrupt will be triggered and needs to be handled.

Note: The XT1/XT2 fault flag can be set regardless of which SELM/SELS/SELA bit combinations are being changed.

Workaround

Clear all the fault flags in UCSCTL7 register once after changing any of the SELM/SELS/SELA bits in the UCSCTL4 register.
If OFIFG-NMI is enabled during clock switching, disable OFIFG-NMI interrupt  during changing the SELM/SELS/SELA bits in the UCSCTL4 register to prevent unintended NMI.
Alternatively it can be handled accordingly (clear falsely set fault flags) in the Interrupt Service Routine to ensure proper OFIFG clearing.