[time-nuts] time transfer over USB

Peter Monta pmonta at gmail.com
Mon May 13 23:36:14 EDT 2013

I wonder if the following scheme would work to improve measurement
precision over USB for use with GPS devices or similar.

Instead of USB interrupt transfers, which occur on 1 ms boundaries with
some unknown epoch, use bulk transfers.  In the good case, there are no
other USB transactions in flight, the request goes out immediately, the
device responds immediately, and the entire link has the lowest latency
possible.  If these conditions don't hold, the completion time will be
delayed.  Now use robust estimation techniques to get rid of these
exceptions and instead follow the smooth line of best-case responses.  NTP
implementations do something like this already, but at longer timescales,
and targeted at Internet links rather than a single local USB link.

If the best-case response time is calibrated out, how good can this get?


More information about the time-nuts mailing list