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

Per Hedeland per at hedeland.org
Wed Aug 14 16:02:40 UTC 2019

In article <qj10eu$1q1r$1 at gioia.aioe.org> Miroslav Lichvar <mlichvar at redhat.com> writes:
>On 2019-08-14, Per Hedeland <per at hedeland.org> wrote:
>> Anyway, back to the FreeBSD post, it seems there are actually two
>> questions:
>> 1) Is the ~ 200 usec offset reported by ntpd really semi-constant (and
>>    thus "easy to deal with")?
>My understanding is that the error in the measurement is moving in an
>interval at a speed which depends on a hidden frequency offset. ntpd
>uses a median from multiple samples for controlling the clock. If the
>frequency offset is large enough and the polling interval long enough,
>the median should be stable and close to the middle of the interval.

Hm, I guess that could be an alternate explanation (FWIW, the ntpd
poll interval was clamped to 16 s via minpoll=maxpoll=4), but... - The
test used one USB 1.1 and one USB 2.0 adapter, which should supposedly
poll at nominally 1 kHz and 4 kHz, respectively. I.e. "middle of the
interval" ought to be 500 usec and 125 usec, respectively, plus some
"really constant" offset, while ntpd reported close to 200 usec for
both. And, I would expect such a drifting offset to be reflected in a
higher jitter.


More information about the questions mailing list