[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$wl.1818 at text.news.blueyonder.co.uk> "David J
> Taylor"
> <david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
> writes:
>> Harlan Stenn wrote:
>>>>>> In article <LY8Df.11489$wl.11104 at text.news.blueyonder.co.uk>,
>>>>>> "David J Taylor"
>>>>>> <david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
>>>>>> writes:
>>> John> 5) [Make sure] you have "options PPS_SYNC" in your kernel
>>> config
>>> 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
>>> UNIX.
>>> Assuming an x86 box, put the following in
>>> /usr/src/sys/i386/conf/PPS:
>> []
>> []
>> 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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