[time-nuts] Beaglebone NTP server

Chris Albertson albertson.chris at gmail.com
Wed Dec 10 23:15:41 EST 2014

Those sub 1 u-second numbers are very good.  They argue for using the BBB
as an NTP server but I wonder if it really is the best.   I think the
numbers that matter are measures of the close on the computers who use your
BBB as a server.  In other words the goal is to synchronize a set of
computers.  Can The little BBB push accurate time out to a set of user
computers and keep then in sync better then some other NTP server platform?

On Wed, Dec 10, 2014 at 5:58 PM, Dan Drown <dan-timenuts at drown.org> wrote:

> Quoting Paul <tic-toc at bodosom.net>:
>> Using a PRU seems like overkill if all you want from the BBB is NTP.  The
>> standard pps-gpio should move the system clock precision below
>> system/network jitter (.5 to 1 microsecond).  The next step is using a
>> timer (TIMER4) which should get you into .1 microsecond offsets.
> As a note to people wanting to use the timer hardware on the BBB - I have
> a driver for it at https://github.com/ddrown/pps-gmtimer
> I wrote up the results in using it at http://blog.dan.drown.org/
> beaglebone-black-timer-capture-driver/
> The summary of it is:
> pps-gpio - 50% of the time local clock offset within +/- 0.07us, 98%
> within +/- 0.61us
> pps-gmtimer - 50% of the time local clock offset within +/- 0.04us, 98%
> within +/- 0.43us
> Also, if you're using pps-gpio, you might want to disable cpufreq and
> force your processor to 1GHz.  It'll help with interrupt latency and jitter.
> cpufreq ondemand, 300MHz-1GHz - http://dan.drown.org/bbb/run9/
> interrupt-latency.png
> 98% of interrupts handled 12.92us-23.21us after the event happened.
> cpufreq forced 1GHz - http://dan.drown.org/bbb/run8/interrupt-latency.png
> 98% of interrupts handled 6.04us-8.58us after the event happened.
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Chris Albertson
Redondo Beach, California

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