[time-nuts] GPS clock to RT-Linux for pulsar machine

David Andersen dga+ at cs.cmu.edu
Fri Jul 1 17:05:35 EDT 2005

If your A/D card will do the timestamp for you, that's the best way.  
Then you shouldn't need astounding precision from NTP, which you 
probably won't get.  50 microseconds is pushing it.  Various A/D cards 
like the NI M-series DAQs will do hardware timestamps from an external 
timer source (up to 80Mhz, which more than exceeds your 50 microsecond 
requirement).  You'll be much happier not trying to fudge your computer 
into doing this job and having to worry about all of the possible 
sources of delay for the interrupts, etc.

If you've got the card counting both the 10mhz and the 1pps, then your 
timestamps will be in absolute time already, modulo the second 
conversion.  That level of accuracy is easy to achieve with NTP.


On Jul 1, 2005, at 4:45 PM, David Forbes wrote:

> Hello all,
> As part of my radioastronomy job, I'm being asked to devise a low-cost 
> pulsar detection machine for very high frequencies (~80 GHz), which 
> requires many tens of hours of integration time staring at the pulsar 
> while maintaining absolute timing accuracy of ~50 microseconds.
> I have GPS time and a maser available for absolute and relative timing 
> respectively. I expect to be taking the data via a low-cost A/D card 
> clocked by the maser at about 100 us per sample. I plan to use RTLinux 
> unless I can be convinced there's a better way, as I already use 
> RT-Linux successfully for our various spectrometers.
> My question is: What's a good way to synchronize the RT-Linux OS clock 
> to the GPS clock? I am considering the use of the A/D card's interrupt 
> with the 1PPS from GPS. I expect to use NTP to get the correct second.
> Does NTP provide a way to tie an interrupt from some user-provided 
> hardware to sync up the fast clock to ~50 microsecond accuracy, or is 
> there some other method?
> Thanks in advance for any help.
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