On 2013-02-28, Chris Adams <cmad...@hiwaay.net> wrote: > Once upon a time, Ralph Aichinger <ra...@pangea.at> said: >>Has anybody *two* GPIO PPS devices on different GPIO >>pins on their Raspberry Pi? Would a setup like this >>enable the ntpd to check the devices against eachother >>(or probably more likely give an idea of interrupt >>handling capabilities)? > > Remember: a man with two clocks never knows what time it is. > > If they are the same, everything's great; if they are different, what do > you do?
Since both are supposed to indicate exactly the same time to nanosecond accuracy, you know something is seriously wrong somewhere. Exactly where you may not know, but you know that you should not be trusting the times. Of course, you do have to take into account the fact that the sources are demanding recognition from your computer at exactly the same time, and your computer does not like that. Thus it is very possible that this will cause the two to give times that differ by many micro (not nano) seconds. If you can persude your system to service the interrupts on two separate processors, you may even be able to get them to agree to sub microsecond precision, but I have no idea if or how you could force your system to use two different processors to process the interrupts. Of course if the purpose is to provide redundancy, rather than testing, then two clocks are problematic for precisely the reason you give. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions