[ntp:questions] PPS via USB-to-serial adapter

Per Hedeland per at hedeland.org
Tue Aug 13 11:24:22 UTC 2019

In article <P42dnZkcsZtZH8zAnZ2dnUU78RHNnZ2d at giganews.com> Jakob Bohm <jb-usenet at wisemo.com.invalid> writes:
>On 11/08/2019 15:44, Per Hedeland wrote:
>> Hi,
>> Since the idea of using a USB-to-serial adapter for PPS is often
>> dismissed here as more or less pointless/useless, (due to the inherent
>> delays in the USB communication AFAIU), I found this recent post to a
>> couple of FreeBSD mailing lists quite interesting:
>> https://lists.freebsd.org/pipermail/freebsd-usb/2019-August/016078.html
>> TL;DR^2 The author carried out a pretty sophisticated (IMHO) test with
>> two different USB-to-serial adapters feeding PPS to ntpd, and found an
>> offset of some 200 usec with 20-30 usec jitter. You can of course tell
>> ntpd to correct for the offset (once you know how large it is...), and
>> the jitter doesn't seem too bad to me, although it is of course higher
>> than for more "direct" connections.
>> In subsequent discussion he pointed out that it is significant that
>> the test was done on FreeBSD - while he would expect similar results
>> on Linux, performance on Windows would be "all bets off" due to
>> varying driver quality.
>> --Per Hedeland
>One of the hard parts is that the USB protocol is essentially polling at
>either 1kHz (USB 1.1 speeds) or 4kHz (USB 2.0 ~280Mbps speed).  This may
>be suitable for syncing around ms range.
>Thus if the USB controller clock in the computer is running close to
>its nominal speed, the delta between actual PPS pulse and USB event
>reaching the computer will be almost constant, varying only by the
>jitter and speed error of the USB controller clock, wrapping at 1000 or
>250 usec.

Hm, I certainly don't know much about these details, but I have a hard
time seeing how this "almost constant" delta could explain the results
in the linked message. I'm not sure I'm getting the math right here,
but as far as I can see, assuming a clock that is only 1 ppm off,
(e.g.) the 1kHz clock would accumulate a full period of "offness" in
1000 seconds, and during those 1000 seconds the delta would vary
across the whole 1000 usec interval.

If this is really the case, wouldn't this show up as a very high
jitter from ntpd's point of view? And/or an offset that was at least
half the 1000 usec interval, or possibly varying across that interval?

>I haven't checked the iMX.6 datasheet for its USB controller internals
>(for example, does it take the frame clock from the 24MHz-derived bit
>lock or the from another clock that is closer to the PPS-synced clock in
>his experiments).

A subsequent message in the FreeBSD list thread
states that the USB clock in the test case is not *derived* from the
10 MHz clock that was used to generate the PPS signal, which could
otherwise explain the low jitter I guess - per above, I can't see how
the clock being "close" could do it.

>I also haven't grabbed a copy of the serial port adapter USB subclass
>specification to check if there is something not happening at frame
>boundaries somehow.

This sounds very interesting, but again my knowledge is limited...


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