[ntp:questions] NTP server with Garmin GPS 18 LVC

unruh unruh at invalid.ca
Thu Dec 19 22:06:50 UTC 2013

On 2013-12-19, David Lord <snews at lordynet.org> wrote:
> unruh wrote:
>> On 2013-12-19, David Lord <snews at lordynet.org> wrote:
>>> Adrian P wrote:
>>> .....
>>>> Everywhere is written that FreeBSD is the best OS for time keeper,
>>>> since its kernel supports nanosecond accuracies. However, if GPS 18x
>>>> LVC PPS signal has only microsecond accuracy, then I suppose there is
>>>> no point on using FreeBSD, any linux would be okay as well. So what do
>>>> you think, looking from NTP point of view only, is this nanosecond
>>>> accuracy the only reason why someone would use FreeBSD, or there are
>>>> some other reasons as well? I see you have experience with all kind of
>>>> OSes.
>>> I think the combination of hardware used is at least as important
>>> as the OS.
>>> <http://www.febo.com/pages/soekris/> 100ns accuracy
>> AGreed. But their comparison is strongly cooked. At least on my system,
>> the serial port interrupt performace is far better than theirs--
>> something like 2 us, not the 50us they are showing. Certainly 100ns is
>> better, but more like one order of magnitude, not more than 2. 
> It might not be very clear but PPS to serial port isn't used
> other than as a comparison, ntpd has been modified to use the
> Elan Timer with PPS input to the GPIO.

I understand that they do not use pps to serial port, but they compare
to ntpd with serial input to show how much better they are. What I
object to is that that comparison is cooked. ntpd with pps to serial is
a lot better than their graph indicates. They are doing something very
wrongly to get such bad results. And it is unnecessary because they are
better than ntpd with pps to serial. 

> I have a couple of motherboards with chipsets that have higher
> resolution timestamping but the GPIO pins aren't accessible.
> David
>>> <http://www.synclab.org/radclock/> An alternative to ntpd
>>> David

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