[ntp:questions] Re: PPS working or not? FreeBSD 5.4
David J Taylor
david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Mon Jan 30 08:38:28 UTC 2006
Per Hedeland wrote:
> In article <qObDf.11636$wl.1818 at text.news.blueyonder.co.uk> "David J
> <david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
>> Harlan Stenn wrote:
>>>>>> In article <LY8Df.11489$wl.11104 at text.news.blueyonder.co.uk>,
>>>>>> "David J Taylor"
>>>>>> <david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
>>> John> 5) [Make sure] you have "options PPS_SYNC" in your kernel
>>> David> 5 - I have not done that explicity. How could I check that,
>>> and David> change it if required? It's been 10+ years since I ran
>>> Assuming an x86 box, put the following in
>> Many thanks, Harlan. I'll look into this, although I am a little
>> disappointed to find that less than 24 hours after starting with
>> FreeBSD I'm having to recompile the kernel!
> Well, using PPS_SYNC shouldn't be *required* to use the PPS signal on
> FreeBSD - what it does is make the kernel discipline the local clock
> directly from the PPS signal, but FreeBSD also supports the PPSAPI, to
> let ntpd collect the timestamps from the PPS signal. It should be
> possible to use this via the ATOM driver. I haven't personally
> verified any of this though (hence the "should"s) - and I guess
> PPS_SYNC may give better results, but it could be interesting to try
> both variants.
> --Per Hedeland
> per at hedeland.org
Per, thanks for that. I think I understand what you are saying (although
I may not next week!):
- you need to recompile the kernel if you want the PPS signal to
discipline the local clock directly
- there is an alternative programming interface, the PPSAPI, which FreeBSD
supports, but this API is not used by the NEMA driver. (Why not?)
- you may be able to use the PPSAPI if you install the ATOM driver as well
as the NMEA driver.
I wonder which the preferred approach is, and which would give the better
I'm trying to put together a small Web page on this, so I may quote your
words (if that's OK).
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