>> 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 > capture counters. Hi Hal, Nope, there's no need to synchronize HW counters (against "system time" or UTC or something). That's the beauty of time-stamping or capture counters: they are free-running (at some internal CPU frequency) and share a common clock counter register from which the capture/snapshot is taken. > 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.) No, again that's the beauty of H/W capture counters. You completely avoid the OS or software rat's nest called system time. Only the capture registers keep perfect "system time" by virtue of their continuous h/w counting, unaffected by software, locks, interrupts, cache, TLB, or microcode latency issues. > 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. The modern CPU's with capture/compare registers I've seen use a common N-bit timer register as the capture source, so there's no issue with intra-capture synchronization. What is still critical, to align with UTC for example, is inter-clock synchronization. And that's why two h/w capture counters are needed -- one for the event (LAN packet, for example) and one for the GPS/1PPS timestamp. /tvb _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
