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Can the FLL in MCG be used open-loop as a DCO, without FLL corrections?

Question asked by Steve Rhodes on Sep 25, 2017
Latest reply on Sep 27, 2017 by Hui_Ma

I'm using an MKV30F64VFM10, but this is a pretty general question about the MCG. As I understand it, the DCO in the FLL has a period that adjusts in increments of about 150 ps (the FLL RMS jitter spec). That means the total error in one period of the reference clock can be 150 ps times the multiplication factor, say 384 ns if multiplying by 2560. Averaged over many reference periods with small DCO period changes, the FLL accuracy is good. But from reference period to period, there  can be a large jump, which appears as jitter on an FTM output if it's clocked by the FLL. If the FLL would just run open-loop, and stay at a single DCO  setting, then there wouldn't be any jitter. Software could adjust the FTM period to lock to an external reference at known times, and achieve very low jitter. So... is there any way to make the FLL not try to track a reference? I thought the MCG_SC.FLTPRSRV might do it, but I can't find anything about it beyond the register description.

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