On Jan 19, 6:54 pm, Unruh <[email protected]> wrote:
> "your" gps clock does not drift. Your HW clock may drift.
> The time sent out by GPS is accurate to a few nanoseconds. Your hardware
> clock is not.
Of course. I agree.
> >hw clock (the clock that I would have obtained without GPS).
> >My final goal is to convert a series of gps timestamps to the
> >equivalent hw timestamps (with microsec precision).
>
> No idea what it is that you are trying to do.
I want to compare the outputs of an application when this app is fed
with GPS input & when it is fed with HW input
HW input=series of ("HW timestamp" + event)
GPS input=series of ("GPS timestamps" + event)
I want the same list of event. Only the timestamp can differ in the
inputs.
So I have to run the exeperience only once (i don't want to run an
experience with GPS and an experience whithout GPS)
The only way to do that is to invent a way to convert GPS timestamps
to HW timestamps
(i.e. the timestamps that would have been generated if the GPS was
disabled)
>
> >1)My first idea was to recreate (off line) the hw timestamps by
> >-inverting the NTP kernel clock discipline algorithm
> >-feeding it with the loopstats statistics and my gps time series
>
> ??? It is a dissipative system. going backwards is highly unstable.
It's just a conversion function F that I was thininking about: F: GPS
timestamp -> HW timestamp.
After conversion, the HW timestamps=F(GPS timestamps) should grows
monotonically as the GPS ones.
However after having read, an old post (on this mailing list) about
the ntp disipline,
I came to the conclusion that is too difficult to found out F.
(So let's forget that).
> >3)Is there a simple way to do that? It seems very simple but I'm not
> >from the domain.
> >Has a hw clock driver clock already been implemented to do such an
> >operation?
>
> What operation?
The operation to read the HW clock and to print its stats (especially
offset!) in
the peerstats file.
>From all these written offsets, i will have the drift and the job is
done!
However, it is not as simple as one can think,
because it seems that ntp sync the hardware clock each 11 minutes:(
This sync must be disabled.
Is there a way to do that?
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