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

Miroslav Lichvar mlichvar at redhat.com
Tue Aug 13 11:53:37 UTC 2019

On 2019-08-11, Per Hedeland <per at hedeland.org> wrote:
> 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.

An offset of 200 microsecond is much larger than a typical error of NTP
in a local network, so maybe that's why people don't like refclocks over

However, with a custom driver and firmware it's possible to reduce the
offset and jitter to few microseconds. See this great post from Dan


Miroslav Lichvar

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