[time-nuts] raspberry pi, adafruit gps & ntp

Dan Drown dan-timenuts at drown.org
Mon Jun 17 11:47:59 EDT 2013


Quoting folkert <folkert at vanheusden.com>:
:
> +127.127.28.1    .PPS.            0 l    4    8  377    0.000     
> 2.231   0.005
:
> I'm surprised that the jitter goes down to 0.005 as I'm now measuring
> the PPS from userspace. My program runs with "real time" scheduling and
> maximum priority but still the kernel needs to do a context switch etc.
> when it receives the pps pulse.


I got similiar results with measuring the PPS timestamp in a userland  
real-time (SCHED_RR) process.  All my hardware is different from  
yours, but it sounds like our software is close in architecture.   
Here's the graph of jitter over two days:   
http://dan.drown.org/stm32/run5/time-variance.png

I'm using the setting "minpoll 4 maxpoll 4" for a 16 second cycle and  
sending the median of those 16 offsets to ntpd.  I'm also collecting  
the max and min offsets, and graphed them here:  
http://dan.drown.org/stm32/run5/usb.png

As a comparison, I tried timestamping in the kernel and put the  
min/max graphs for kernel vs userland side by side:  
http://dan.drown.org/stm32/run6/usb-compare.png

The jitter for the kernel timestamping run is improved as well:  
http://dan.drown.org/stm32/run6/time-variance-compare.png

More info on the userland/kernel comparison:  
http://dan.drown.org/stm32/run6.html



More information about the time-nuts mailing list