[time-nuts] Raspberry Pi tweaks and custom kernel, was RE: PPS for NTP Server - How Close Is "Good Enough"?

Bob Camp kb8tq at n1k.org
Mon Jun 15 21:01:59 EDT 2015


Unless you have fancy switches on your LAN (1588 stamping), PTP performance
will be dependent on load and the “goodness” of the switches you do have. These 
are pretty much the same (external)  things that impact NTP. On a LAN with variable 
loading (time to stream some movies …)  the advantage of PTP over NTP
may not be that great. 


> On Jun 15, 2015, at 10:15 AM, Chris Caudle <chris at chriscaudle.org> wrote:
> On Mon, June 15, 2015 1:23 am, David J Taylor wrote:
>> I don't think either system is good enough as a "microsecond" level
> server, but either is fine for "tenth millisecond" level.
> The BeagleBone Black has the advantage that you can run one of the timers
> from an external input, so you can use the 10MHz from your GPSDO to drive
> the timer (takes care of temperature variations of the system clock), and
> have the timer captured based on the PPS input (eliminates interrupt
> latency as a factor in capturing the timer).
> With that setup you should be able to get into the high nanoseconds
> stability range.  For transfer to other systems the BBB Ethernet supports
> PTP, so you should be able to get just under microsecond level
> synchronization between systems on the same LAN.
> Here is someone working on connecting a NavSpark GPS to a BBB.  I don't
> think I have seen his name on the 'Nuts list, but I don't recall for sure.
> http://blog.dan.drown.org/
> -- 
> Chris Caudle
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