(system time, gps, pcf50633 rtc, calypso rtc)

Timo Juhani Lindfors timo.lindfors at
Mon May 25 18:03:19 CEST 2009


is a simple program to compare different clock sources on
freerunner. It reads NMEA data from stdin. When it detects a new
second it logs the values of all clock sources. The resulting log file

shows[1] that:

1) pcf50633 rtc gains one second every 16300 seconds.
2) calypso rtc did not gain or lose any seconds during this test period
3) system time is fairly stable after I did "adjtimex --tick 9999
   --freq 3419747" which corresponds gaining one second every 20912

Are there any other time sources on freerunner that could be
used/tested? S3C has an RTC but according to larsc it is not

< larsc> +hm the s3c rtc is running way to fast
< larsc> +lindi-: hm, looks as if the rtc is not connected to a oscilator. so it's not usable

Also, is there some existing program that could read time from
pcf50633 rtc, compensate for the drift and write the result to system
time? hwclock --adjust --hctosys wants to write the adjusted value
back to rtc. I don't want to do this since it makes long-term
monitoring of the RTC drifting difficult.

best regards,
Timo Lindfors

[1] I used the following gnuplot commands to plot this:

set xdata time
set timefmt "%s"
plot "clock.log" using 1:($2 - $1) title "sys", '' using 1:($3 - $1) title "pcf", '' using 1:($4 - $1) title "calypso"

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