[time-nuts] NIST time services

Chris Albertson albertson.chris at gmail.com
Sat Mar 22 15:54:21 EDT 2014

On Sat, Mar 22, 2014 at 12:24 PM, Chuck Forsberg WA7KGX <caf at omen.com> wrote:
> I can see a use for an inexpensive GPSDO with a built-in
> gigabit ethernet or USB3  port powering an NTP server.

Neither of those is a good way to transfer time to an NTP server.
Both Ethernet and USB are packetized.   The best way is with a simple
wire with a square wave pulse on it that pulses ones per second.
Nothing can be more simple or accurate.

The trick is to build an NTP server that can react deterministically
to the pulse.   I think an ARM based system could far outperform an
Intel based one.   ARM has two independent PRUs.  These are little
32-bit processes each with 4K of memory that are build right on the
same chip as the main ARM CPU.   The PRUs purpose built for real time
task and can handle nanosecond level timing.   In most existing system
the PRUs are ignored and everything is done using the ARM.

The other way to improve things even better is to not even bother to
have a link from the GPSDO to the NTP server.  Why not simply run the
NTP server software on the same processor as the GPSDO?   Just one of
the little PRUs is more than powerful enough to run a GPSDO.  They are
a 32-bit uP that runs at 200MHz, one instruction per clock.  The PRUs
don't run any operating system code but have access to all of the
ARM's memory and interrupts.  A PRU is way-overkill for a GPSDO.
Doing this eliminates the link cable from the GPSDO to the NTP server.
 If the ARM CPU can't handle 6 billion requests per day then buy many
copies the ARM based systems.  They are cheap.


Chris Albertson
Redondo Beach, California

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