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.
-Dave
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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