--- In "palito.farias" <> wrote:
>
> Dave,
> I've let the board collect ntp data for ~24 h and then using that data I was
> able to adjust for the constantan drift. (I'm using the following client
> http://doolittle.icarus.com/ntpclient/ that is based on xntpd), during the
> test runs I also gather other statistics that will give you a fairly good
> idea of how good your boards time is. If you follow their instructions I
> believe you are able to get to 10 ms accuracy, my board seems to be able to
> get close to that. I'm synchronizing two radars with 1000Hz prf hence I need
> sub millisecond accuracy so they can point at the same volume. But then read
> Ian's comment and it doesn't look promising for us?
You can do this with a small GPS: use NMEA sentences on the serial port plus a
PPS (Pulse Per Second) signal to either a GPIO or DCD line on the serial port.
(see my earlier post)
I'm surprised you could only get 10 mS or so. Is it better if you sync only to
a NTP server on your local LAN? That way the latency should be much better
than using a server out on the 'net.
I haven't looked into IEEE1588. Looks promising, but it's much less developed
than GPS NMEA + PPS in linux, and you still need a master on your local network
(master is disciplined to GPS + PPS).
regards, ......... Charlie
>
> Thank you all for your inputs, I appreciate them.
> /Polly
>
> --- In Ian Thompson <ian.thompson@> wrote:
> >
> > Dave
> >
> > The principal factors are the OSes and their IP stacks. You can't depend
> > on when NTP events are going to get handled if you are running a general
> > purpose OS. You might improve the accuracy if you code your NTP servers
> > and clients on an RTOS with a more deterministic IP stack.
> > There are efforts to put PTP/IEEE-1588 handling into the Linux kernel.
> > I'm not sure if this is to support specific PTP enabled hardware or to
> > move the handling of PTP events to early in the stack.
> >
> > Ian T.
> >
> > On 08/25/2010 09:02 AM, Dave Cramer wrote:
> > >
> > >
> > >
> > >
> > > On Wed, Aug 25, 2010 at 9:45 AM, Ian Thompson <ian.thompson@
> > > <ian.thompson@>> wrote:
> > >
> > >
> > >
> > > It's not considered possible to synchronize to better than ±20 mS with
> > > NTP,
> > >
> > >
> > > What are the limiting factors here ? I will have all of my boards on
> > > the same local network.
> > >
> > >
> > >
> > > hence the introduction of PTP [Precision Time Protocol or IEEE-1588] .
> > > To get microsecond accuracy with PTP one needs an Ethernet MAC with
> > > built in hardware support and a PTP master. I don't believe
> > > Technologic
> > > have any boards that include this as yet. I hope that they are
> > > considering adding it!
> > >
> > > What exactly is required ?
> > >
> > > Dave
> > >
> >
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>
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