| Re: [chrony-users] Newbie Help Needed |
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William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/
On Fri, 12 Jan 2018, Stuart Maclean wrote:
On 01/12/2018 01:08 AM, Miroslav Lichvar wrote:
The code looks good.
If the string has a constant length and you are effectivelly time
stamping the transmission of the \n character, I guess that could
explain the stability. I'm not sure about the small offset to the PPS
though. It's interesting.
I got intrigued by your observation that my read of the CSAC's 'seconds since
the epoch' string over a serial line was so stable. I realized I had been
doing my chrony testing with my system essential idle, which may have been
the reason for those serial line reads being so 'predictable'.
I ssure would not expect to be stable, and would expect it to have a large
offset. It takes time to send a serial string. (at least number of digits x
9/baudrate) and if the number of digits changes, it will vary.
So I then got my embedded system to 'run' for real, which involves reading
from four other serial lines at between 1/5 and 1/2 Hz. This gives the kernel
more I/O channels to juggle. My chrony output is now
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
#? CSAC 0 4 377 12 +969ns[ +693ns] +/-
30us
I really have a hard time believing this above because of the above.
#* PPS 0 4 377 10 -1055ns[-1334ns] +/-
30us
210 Number of sources = 2
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
==============================================================================
CSAC 6 3 80 -0.131 0.299 -2640ns 2307ns
PPS 18 12 274 -0.000 0.050 -1ns 4416ns
I am still just learning all these terms, but isn't 'Frequency' column
tracking the the variability in the 'offset' field of the sample SOCK message
that I send to chronyd when I get the CSAC string over serial?
Frequency is the offset of the system clock from the 1 sec/sec rate of the
UTC.
I am having trouble with all of the above. But maybe I just do not understand
what the CSAC is.
If yes, it suggests that stressing my system with other serial reads does
indeed increase this frequency.
Is 'jitter' the same thing? i.e. the variability in the offset between a
refclock's time and local system time??
Stuart
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