Chris Albertson wrote:
Next how to get the PPS to be at the top of the second. Two methods
1) don't bother. Linux PPS will log the time of both the GPS's PPS
and the Rb PPS. Read the log file to determine the offset. This give
offset with uSec precision. To get better you need a hardware time
interval counter. I happen to have a few of those and can read
sub-nanoseconds but this is gross overkill. Microseconds are good
enough for NTP.
2) You can adjust the phase of the PPS by speeding up the Rb frequency
well above what it should be for some number of seconds and then
slowing it back to the "correct" speed. This exercise will move the
phase. Slowing it will work to move the phase in the other direction.
3) For gross adjustment of the phase a "big hammer" works. Power
cycle the Rb unit and chances are the phase will change by some
totally random amount. Repeat this process as required. Yes this
sounds stupid but on average you can get "close" with not to many
cycles. Then you apply ether of the two above methods. (I call
this "spin the clock hands". At you were to spin the hands on a real
clock like a roulette wheel they would land in the correct number one
in twelve times.)
Much simpler would be to use a fudge command to specify the current
phase offset between the Rb and GPS PPS signals, i.e. if the Db happened
to start out 367 ms off, just let ntpd know that these pulses are
supposed to have such an offset.
With the Oncore UT+ this is actually a feature: You can specify an
arbitrary offset from the top of the second, in order to avoid any
"thundering herd" effect from many other processes that also want's to
wake up every second, exactly on the second.
No matter how you solve this problem, I agree that having a very cheap
and extremely stable local timebase provides great gps backup/holdover.
Terje
--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"
_______________________________________________
questions mailing list
[email protected]
http://lists.ntp.org/listinfo/questions
