| Re: [chrony-users] Newbie requesting help |
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William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
On Mon, 21 Mar 2016, Stuart Maclean wrote:
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?
You do need it as an initial "vote" to determine the whole second (or you can
use the GPS for that) but after that, assuming your system does not suddenly
go and loose whole seconds of time for some reason, you only need the PPS,
since the system can keep time well enough over a second to make sure that it
can supply the second, and just needs the PPS to get the micro/nano second
right.
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.
Good answer. So yes, you do need something to track the time there.
Are you sure that your clock is better than the system time itself? Your clock
is also freewheeling. So, you could use your PPS to get microseconds, and your
clock C to get the seconds. So sure, you can use two shm, but the second will
not get used probably except at startup to get the system time seconds on
track.
Stuart
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