[ntp:questions] setting the time using gps-time and pps without ntpd?
nb at komeda-berlin.de
Thu Apr 24 14:28:17 UTC 2008
> nb at komeda-berlin.de (Nicola Berndt) writes:
>> recently I tried building a microsecond precision timeserver using a
>> gps-device with pps signal.
>> The gps-data is transmitted via usb, the pps signal via the rs232
>> carrier detect line.
>> I tried several drivers and they all worked, but I had to figure that
>> obviously my mainboard (via epia en12000EG) has terrible jitter. - unusable!
> Are you sure that it is not the reading of the time that is the problem.
> What gps do you use? Are you sure it really has and is using PPS?
> And what do you call "terrible"
I use a Conrad USB-Receiver that contains a U-Blox board. I took the
PPS-Signal from the according pins inside the device myself and tested,
if it sends a pulse every second and it does. So that looks ok to me.
"Terrible" I call that I can watch the jitter with ntpq -p jump from -50
to 50. I assume those are microseconds, so +/-50 ms makes 1/10 seconds.
I can set any clock that precise using my bare hands ;)
The next problem I actually stumbled across is the long time ntpd needs
to settle. The system I inted to build has to be ready right after the
start, because the pc lies in a shelf, off, and is switched on right
when needed. There is no way to let it run, since it is battery-driven.
>> To solve the problem I think about simply setting the time every n
>> seconds according to the collected gps time and the pps-signal. My
>> system will run off for far less than a microsecond in an interval of
>> let's say, 20 seconds or so. Like this I would implement a constantly
>> jumping time, but the amount of jumps would be extremely small.
> Just use ntp to discipline your clock. No need to try to bacly reinvent
Oh, I happily would, I just didn't get it to work and gave up at some
point (after days..)
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