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

Tom Van Baak tvb at LeapSecond.com
Mon Feb 20 23:48:54 EST 2017


Can you convince me your USB time transfer method works?

It seems to me that if the read path and the write path are different it breaks down. Given how USB works, not to mention all the layers of software involved I can't imagine the paths are equal. There's no doubt you can get great consistency from your method. But turning that into precise time requires some kind of calibration of the actual code path delays. In other words, it sounds to me like your method is valid for frequency transfer but not time transfer.


----- Original Message ----- 
From: "Peter Monta" <pmonta at gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Monday, February 20, 2017 5:46 AM
Subject: Re: [time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs.PPSdecoding cards

> 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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