On 3/5/2020 8:19 AM, Frantisek Rysanek wrote: > during the initial settling of the PHC frequency / servo loop, > I can see offsets in low units of ns, not aligned in any way. > But: once the PHC settles to offset==0 && ppb==0, > any measured edges captured via EXTTS channel 0 or 1 > will be quantised on 8ns boundaries. > This is kind of spooky :-) > Maybe the quantisation just doesn't jump at me so bad > while the frequency (ppb) is still a little off... > and it's true that if I keep capturing on both channels, > their mutual difference is always quantised at 8 ns, > even though the difference against the internal unsettled > PHC is not aligned that way. > > I'm giving up this train of research. > Comments welcome :-) > 8 ns granularity is better than nothing at all.
This makes sense. The clock source has a period of 8 ns of I recall. We only update once every period. In order to have lower granularity, you'd have to update at a higher rate. We can change the exact amount that we update by modifying INCVAL, but that doesn't change the underlying clock source. Thus, significantly different INCVALs would not produce multiples of 8, but if you eventually sync down to a ppb of 0 it would end up being there. Thanks, Jake _______________________________________________ Linuxptp-devel mailing list Linuxptp-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linuxptp-devel
