[time-nuts] time transfer over USB
herbert at 13thfloor.at
Tue May 14 01:22:26 EDT 2013
On Mon, May 13, 2013 at 08:36:14PM -0700, Peter Monta wrote:
> 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.
IMHO the transfer mode of choice for this purpose should
be the Isochronous Transfer (in USB 2.0 and 3.0) because
it happens periodically and thus can achieve a guaranteed
maximum latency (for high speed this means 125us).
> 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? Microseconds?
With 125us latency and a fixed interval, it should be
possible to calculate the relation between the transfer
intervall (USB clock crystal) and the PPS (or whatever
information is transmitted) and thus easily get below
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