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

Miroslav Lichvar mlichvar at redhat.com
Wed Aug 14 12:57:02 UTC 2019

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. If
the error is moving only very slowly, the sawtooth pattern would be
visible in the offset reported by ntpd.

Miroslav Lichvar

More information about the questions mailing list