SLAZ291AC October   2012  – August 2021 MSP430F5500

 

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6.47 UCS7

UCS Module

Category

Functional

Function

DCO drifts when servicing short ISRs when in LPM0 or exiting active from ISRs for short periods of time

Description

The FLL uses two rising edges of the reference clock to compare against the DCO frequency and decide on the required modifications to the DCOx and MODx bits. If the device is in a low power mode with FLL disabled (LPM0 with DCO not sourcing ACLK/SMCLK or LPM2, LPM3, LPM4 where SCG1 bit is set) and enters a state which enables FLL (enter ISR from LPM0/LPM2 or exit active from ISRs) for a period less than 3x reference clock cycles, then the FLL will cause the DCO to drift.
This occurs because the FLL immediately begins comparing an active DCO with its reference clock and making the respective modifications to the DCOx and MODx bits. If the FLL is not given sufficient time to capture a full reference clock cycle (2 x reference clock periods) and adjust accordingly (1 x reference clock period), then the DCO will keep drifting each time the FLL is enabled.

Workaround

(1) If DCO is not sourcing ACLK or SMCLK in the application, use LPM1 instead of LPM0 to make sure FLL is disabled when interrupt service routine is serviced.
(2) When exiting active from ISRs, insert a delay of at least 3 x reference clock periods. To save on power budget, the 3 x reference clock periods could also be spent in LPM0 with TimerA or TimerB using ACLK/SMCLK sourced from DCO. This way, the FLL and DCO are still active in LPM0.