[time-nuts] Beaglebone NTP server

[Dan Drown]

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.