[time-nuts] GPS for ntp
hmurray at megapathdsl.net
Tue Oct 21 15:20:44 EDT 2014
tvb at LeapSecond.com said:
>> You have to synchronise between the counter
>> value and what the OS understands is 'system time' in order to create a
>> retrospective timestamp for when the event occured.
> Also true.
> One solution to the problem is use two independent HW capture inputs. One
> for a GPS 1PPS and the other for your event.
> In this case the system clock does not need to be synchronized -- since it
> is used only to interpolate between the two events. The event timestamp is
> little more than adding the differential of the two most recent captures,
> which is a number from 0 to 1 second.
You haven't solved the problem yet. Now you have to synchronize the HW
You can probably do that with some simple but delicate initialization code.
Maybe copy the value from the counter used for the system time? At the
time-nut level, you have to worry about things like cache misses. (There may
be more fine print at that level depending upon the details of the hardware.)
You could also do it by calibrating out the difference: just feed the same
pulse into both input pins. You have to do that each time you (re)start
things. That's easy for a one-off project but adds another ugly step if you
want to do it in production. Collecting long term data moves a hobby project
a lot closer to production.
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