Re: [chrony-users] Newbie requesting help

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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.


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