I wrote a kernel driver for the BBB timer hardware. It produces pps events for
things like ntpd to consume. The source for it is at
https://github.com/ddrown/pps-gmtimer
A side effect of this driver is it measures the interrupt latency and jitter on
the BBB hardware. The attached file interrupt-latency.png contains an interrupt
latency histogram and cumulative distribution function. This interrupt latency
should be similar to what the pps-gpio driver experiences on this hardware. The
difference is the timer hardware driver can remove the effects of the interrupt
latency.
The graphs of offset over 7 days worth of samples don't show a huge difference.
The *-timer.png file is from pps-gmtimer and the *-gpio.png file is from
pps-gpio. The stddev of the offsets are 244ns from the gpio run and 143ns from
the timer run.
The BBB hardware supports hardware IEEE1588/PTP timestamps, so people might be
interested in that as well. The driver for it is already in Linux, but not
enabled in the default BBB kernel config.
Let me know if you have any questions.
Unrelated to the PPS driver: all the feedback on the software frequency
temperature compensation has been useful. 90% of the time, the local clock is
within 173ppb. 98% of the time, it's within 218ppb. I think I can narrow this
range even further with a temperature rate of change variable. All the graphs
are at http://dan.drown.org/bbb/latest/
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