The DPLL locks onto the nominal servo thread read time and
re-times the sampling or writing of selected values
(step generator position, encoder position, galvanometer position etc)
to remove host read or write latencies as a variable. Typically
read sample time variation can be reduced from 100s of usec
to about 250 ns. This is important because sampling jiitter
causes bogus following errors (and subsequent bogus corrections)
The DPLL is also used to start operations before the nominal read times
(like absolute encoder transfers) so the data is available at the nominal read
time without busywaiting.
