[ntp:questions] how to have offset < 1ms

Dave Hart davehart at gmail.com
Thu Apr 15 19:12:58 UTC 2010

On Apr 15, 5:36 pm, unruh wrote:
> For ms accuracy you would just need usb for power and serial port, but
> work by Lichvar suggests that you cannot just use the serial port if you
> want usec accuracy.

It appears your bias against using DCD for PPS clouded your
comprehension of what Miroslav said.  He said he measured a 20-40us
offset between a timestamp taken in the serial driver interrupt
routine and one taken in userland by gpsd.  Incidentally, I see about
40us offset on a PII 400MHz box between serialpps.sys timestamps and
ntpd user-mode timestamps.  Of course, if it's consistent it can be
calibrated away.  I also see about twice the jitter (~4us vs ~2us)
from the userland timestamps, taken by ntpd running the realtime
priority class, as with interrupt timestamps.  The extra jitter can't
be calibrated away.  The more loaded the machine, the more the
userland route will suffer compared to interrupt-time.

Dave Hart

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