Re: [chrony-users] Newbie Help Needed

[Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>]

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'.

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
#* 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?
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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