[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPSdecoding cards
Peter Monta
pmonta at gmail.com
Mon Feb 20 13:46:11 UTC 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
timestamp.
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.)
Cheers,
Peter
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