albertson.ch...@gmail.com said: > Then use you Blue Tooth or whatever other short distance communications > system you have to support an IP network. TCP/IP over zBlueTooth works well > and is a standard now. Using this you can configure a NTP based network of > "peers". ..,
I'm a big NTP fan, but I wouldn't use NTP for this. (I assume we are talking about the standard NTP reference package from ntp.org.) It's too big and complicated and will be hard to debug if something goes wrong. The real problem is that it will do the best it can rather than scream and shout when something unexpected happens. On the other hand, I like playing with low level things like packets and clocks, and I'm not good at paying attention to the big picture when resources (manpower, time) are limited. It might work fine. It's probably easy to try it. If I was doing something like this (and I could be way off by what "this" is), I'd try roughly: Measure the clock drift rate on a sample of typical systems. These systems need a clock good to 1E-10. If that can be used by the OS time keeping code the drift should be hard to measure. If not, measure whatever the system will be using. Most likely, it will be way off, as in 10s to 100s of PPM. (The 100s covers software bugs in the OS and design errors in the hardware.) Linux (on a PC) measures the CPU crystal at boot time. There is lots of noise in that. You can bypass that noise if you build your own kernel and find that chunk of code and patch in a constant. (I'm thinking of one kernel build per set of boxes rather than one per box.) Now you have a good thermometer. Ballpark is 1 PPM per C. If you measure the drift on each system and save it in ROM/Flash/file-system, you can add some code to the startup area to tell the system how to tweak its clock. (I'll fish out the API details if anybody wants it.) Is that good enough? Do you even need to do the drift measurement and correction on each box? ... That's some combination of the requirements, how much drift is left and how well the boxes in the field track each other (temperature, manufacturing batch) and if you need the correct time or just closely tracked time. Are some boxes in the sun and others in the shadow? ... Another quirk to consider is that happens if you reboot a box half way through the battery lifetime. Does it get the time from its brothers or from the master PC? Does the PC drift track the others or track GPS or...? -- These are my opinions. I hate spam. _______________________________________________ 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.