[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPSdecoding cards

Peter Monta pmonta at gmail.com
Mon Feb 20 08:46:11 EST 2017

Time transfer over USB can be improved by timestamping on both ends, then
using a robust estimator for the clock offset.  For example, imagine the
USB is a small microprocessor peripheral.  It has a local timer, freely
incrementing, based on its local clock.  When it gets a USB interrupt from
the host, the timer state is read, and the USB message contains the host

This is enough information for a single-shot clock comparison.  It may be
contaminated with operating-system latency or any number of other host
latencies (bus, cache, etc.).  But generally, with a lightly loaded host,
the USB transaction goes as fast as it possibly can.  A plot of the
clock-pair points will show a heavy line with the best-case transfers and a
smattering of latency events.  A robust estimator will ignore the chaff.

So I don't see a problem with submicrosecond time transfer over USB.  (I
tried this some time back as a quick hack, with a Teensy board.)


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