[ntp:questions] GPS clock for Linux

Uwe Klein uwe_klein_habertwedt at t-online.de
Sat Sep 13 10:13:46 UTC 2008


Hal Murray wrote:
>>>I assume it's a software bug/feature.  The problem is drift/wander
>>>in the offset.  The offset changes too slowly for normal
>>>jitter filters to work on it.
>>>  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
> 
> 
>>connected via serial/nmea or pps?
> 
> 
> Direct USB.
> 
> I have another one (using the SiRF chips)  that is serial
> via a serial-USB adapter.  Same thing.
There is a good chance that the direct USB thingy
has the serial2USB converter just integrated
( replacing the rs232 levelshifter ).
> 
> 
> 
>>that looks like some beat frequency effect.
>>But it is much too slow/time too big
>> to be sender and/or receiver uart clocking.
> 
> 
> Yes.  But I don't think it's a USB problem.  I get much better
> results with a Garmin GPS 18 USB.  (Of course, what I don't know
> about USB could fill ...)
I had my fingers in USB recently, but ..

> 
Hmm, you get one sentence per second?

I will have to read up if the usb2serial ics have some kind of fifo
included ( and enabled or not ). This could be an explanation.

uwe




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