Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!ogicse!decwrl!public!thad
From: thad@public.BTR.COM (Thaddeus P. Floryan)
Newsgroups: comp.sys.3b1
Subject: Re: RTC precision?
Keywords: RTC system clock microseconds
Message-ID: <1988@public.BTR.COM>
Date: 2 Mar 91 09:56:36 GMT
References: <1991Feb27.195148.4122@sci.ccny.cuny.edu> <12768@helios.TAMU.EDU> <1991Feb28.160950.7042@sci.ccny.cuny.edu> <1991Mar1.040542.14965@ceilidh.beartrack.com>
Organization: BTR Public Access UNIX, MtnView CA, Contact: cs@btr.com 415-966-1429
Lines: 26

In article <1991Mar1.040542.14965@ceilidh.beartrack.com> dnichols@ceilidh.beartrack.com (DoN Nichols) writes:
>[...]
>	The resolution of the clock is 1 second, and the timestamps are
>derived from the system clock.  If you ran it long enough, you'd get a 999mS
>range.  Till we get some extra hardware, a counter run from the 10MHZ system
>clock, divided down, and readable by the system, we aren't going to get much
>more resolution unless we dedicate LOTS of cpu cycles to it.

Actually, there IS a 1/60 sec timer on board, and it's the one used by the
"time" command and also the one I used for the BSD-equivalent gettimeofday()
I posted last year for use with some of the BSD "Tahoe" networking software
suite I ported to the 3B1.

And, though I don't have the data sheets at hand, I "believe" the RTC could
resolve to better than 1 second but there are no "hooks" in the 3B1 to allow
access.  I use a similar clock in one of my own products for datestamping
"events".

And if you want real precision, I have a Heath GC-1000 WWV clock connected
to one of my serial ports and that sucker provides a very accurate time
reference traceable to the NIST (formerly NBS) via "aethernet" (radio :-)
along with another output at PRECISELY 3.6 MHz which could be divided-down
to provide 0.000000277 second accuracy if you're concerned about leaving
the popcorn in the microwave too long.  :-)

Thad Floryan [ thad@btr.com (OR) {decwrl, mips, fernwood}!btr!thad ]