PPS over USB

[Hal Murray]

dan-ntp at drown.org said:
> Here's 7 days worth of data:
> https://dan.drown.org/rpi/usb-vs-gpio-pps.html 

Interesting graphs.  Thanks.

What sort of setup were you using?  I assume the HAT graph was using data 
from a system using the HAT to set the time.  Was the USB graph using the 
USB-PPS for timing or also running on the HAT system when it was using the 
HAT for timing?

If you are using a device for timing, it's easy to get a histogram of offsets 
that will show the shape of the curve, but if you are using a device to set 
the local clock, you can't tell if your clock has an offset.

For something like a PPS over USB there are two ways to see the offset.  One 
is to run on a system that has a better way to get the time, for example a 
HAT.  The other is to monitor using NTP from another system that you trust.


esr at thyrsus.com said:
> Dan and Hal, how do you interpret those results? I would naively suppose
> that the listed fudge should be the mean of each distribution (assuming it
> looks Gaussian) and thius the arithmetic average of those pars - am I
> missing anything? 

If you are using a device to set the local clock, that throws away any 
systematic offset.  You need something else to see that.  That's why I keep 
bugging you to setup a PC with a good PPS.  That will give you a place to 
stand and look at other systems.

In this case, plugging the USB-PPS device into a Pi will let you see the 
offset.  In that case, you are "standing" on the Pi.  The USB Ethernet won't 
get in the way.




-- 
These are my opinions.  I hate spam.