David Lord <[email protected]> writes: >Unruh wrote: >> David Lord <[email protected]> writes: >> >>> David J Taylor wrote: >>>> David Lord wrote:
>.... >> >> I get an average of 2usec from pps driving the parallel interrupt ( with >> "myown" interupt service daemon). I think much of that is a) ntp's >> handling of rate fluctuations, and b)natural variation in the interrupt >> handling due to computer issues. My gps 18 has about a 20m run to the >> computer, and is simply a "bar wire ( well one of the Cat5e twisted pair >> wires) connection. >Thanks for that Bill, to both you and David, I'll still make up >the pcbs but will first try connecting with straight through cable. >.... >> Putting in a thermistor on top of the clock crystal and read the >> temperature and use it to predict the rate variation. That will probably >> give far better results than using a temperature controll. Also using >> chrony ( which now has an shm refclock input at least in beta). will >> give better results than ntp. > From my searches I had opposite impression, that patches to ntp >for temperature compensation were good but not anywhere near as >good as keeping crystal at constant temperature. >Add to that a pps source probably takes system clocks largely >out of the equation. >I'm very happy to give chrony another try. I used it for a few >years without problems when on dialup. I'm thinking of >notebook rather than servers but I'd be happy to try on one of >servers (webserver uses ntpd but has chrony-1.20nb3 which I >guess is old). Yes, that is old. The latest is 1.23 but Miroslav Lichvar has since then put in a patch for giving chrony an shm refclock driver I am not sure if I can post the location of the patch, but his email is mlichvar at redhat dot com This is beta software, so report to him if you find bugs. It needs a line in chrony.conf refclock SHM 1 poll 4 offset 0.0 refid GPS1 (poll, offset, refid are optional) The "1" refers to the shm port number (gpsd uses 1) Note that he finds that chrony is about a factor 20 better than ntp at poll level 4 (I think there is something wrong there) and about 2-3 times better at poll level 3. I suspect this depends crucially on the thermal behaviour of the computer. _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
