Re: Clock Synchronization

["Stanley R. Allen" <Stanley_R_Allen-NR@raytheon.com>]

"Dr. Michael Paus" wrote:
> 
> 
> We are running a distributed application on three PowerPC boards
> which are connected via VME-Bus. The application is written in
> Ada 95 (GNAT) and we use LynxOS as our RTOS. The problem we are
> facing is that we need highly synchronized (~1ms) clocks on
> each board. We are sending messages with time-stamps back and forth
> and so it is important that each part of the application has the
> same understanding of the time. Does anybody have an idea of how
> to accomplish this without putting too much extra load on the
> system? Does anybody know of a hardware solution for this problem
> (a common clock which can be read from all CPU-boards)?
> 

http://www.truetime.com
http://www.datum.com

These companies provide time cards that can be synchronized using
an external signal.  You'll need to have a central source for the
time to distribute to each machine, probably a GPS receiver that
can produce output in IRIG B format.  The cost can be fairly high
for a complete solution.  Your applications will have to use the
time card's clock instead of the motherboard clock.

Another technique is to use the Network Time Protocol (NTP), which
requires that a daemon be running on each machine (ntpd).  One
node is designated the 'server'; its daemon sends it's concept
of time to the 'clients'.  The daemons in the client will adjust
the local CPU clocks, so you can use the local clock.  The NTP
daemons talk over an ordinary LAN, so there's no need for extra
IRIG B wires.

NTP is said to be good for applications needing about 1 millisecond
resolution.  For approx. 1 microsecond resolution, use the time cards.

-- 
Stanley Allen
mailto:Stanley_R_Allen-NR@raytheon.com