Re: [chrony-users] Newbie requesting help

[Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>]

Thanks for the info Bill. I am not sure I understand all of it, so I'll 
annotate...

On 03/21/2016 02:39 PM, Bill Unruh wrote:
> Not sure why you want to do it this way. You say that you can use an
> interrupt. Have that interrupt read the system clock and output that 
> time to
> the device (eg /dev/clock). Then you have to make sure that the system 
> clock
> is, initally, within .25 sec of the UTC.

I was saying that /dev/clock would be the reference clock's vote for the 
current time.  By saying that /dev/clock can be used for the pps result, 
are you implying that I don't need the reference clock's vote for the 
time at all?  Then how do I know what absolute time any PPS signal means?
> Only one vote is needed. And you only have one vote-- both times are
> equivalent-- they come from the same source: your clock.
So the Linux system time doesn't get a 'vote'?  Its time is not an 
'input' to chrony at all????  Sorry I am not understanding this.
> GPSD grabs and interrupt, which tells it when the time on the exact 
> second.
> And it grabs the second source to tell it which second that is. You 
> already have that.

I do?  I am not following.  Could you try to map by reference clock C to 
a GPS device, in terms of its value and pps outputs?
> A ntp packet is four timestamps. When the request was sent, when the 
> request
> was received by the remote clock, when the time was sent out by the 
> remote
> clock, and when the time was received by the local system. In your 
> case the
> remote system and the local system are the same. ntp ( which chrony 
> also uses) determines the offset of your local system by 
> (t1+t4-(t2+t3))/2 Thus t2 and t3 should be the "on the second" times, 
> and t1
> and t4 the system time when the pulse comes in.
>
> >
> This will give a delay of 0, and an offset which is how much the local 
> time is
> off from UTC.
So i do or do not need 2 shm structs?  If I do, how do I fill in the 
clock and receive members of the two?

> Your accuracy will now depend entirely on the accuracy of that clock 
> chip.
>
> Are you sure you cannot just use gps time?
Quite sure.  I am under water for hours at a time.  My GPS device isn't 
one that works under water.  Maybe a more expensive model does.

Stuart


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