13.9.6.4.1.5 Drift Compensation

If the Stable DFLL Frequency bit (DFLLCTRL.STABLE) in the DFLL Control register is '0', the frequency tuner will automatically compensate for drift in the CLK_DFLL48M without losing either of the locks.

Note: This means that DFLLVAL.FINE can change after every measurement of CLK_DFLL48M.

The DFLLVAL.FINE value may overflow or underflow in closed-loop mode due to large drift/instability of the clock source reference, and the DFLL Out Of Bounds bit (STATUS.DFLLOOB) in the Status register will be set. After an Out of Bounds error condition, the user must rewrite DFLLMUL.MUL to ensure correct CLK_DFLL48M frequency.

A zero-to-one transition of STATUS.DFLLOOB will generate an interrupt, if the DFLL Out Of Bounds bit in the Interrupt Enable Set register (INTENSET.DFLLOOB) is '1'. This interrupt will also be set if the tuner is not able to lock on the correct Coarse value.

To avoid this out-of-bounds error, the reference clock must be stable; an external oscillator XOSC32K is recommended.