Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!wuarchive!mit-eddie!uw-beaver!fluke!vince
From: vince@tc.fluke.COM (Craig Johnson)
Newsgroups: comp.sys.3b1
Subject: Re: RTC precision?
Summary: Toshiba 8250 RTC chip resolves only seconds
Keywords: RTC system clock microseconds
Message-ID: <1991Mar5.193756.25135@tc.fluke.COM>
Date: 5 Mar 91 19:37:56 GMT
References: <1991Feb27.195148.4122@sci.ccny.cuny.edu> <12768@helios.TAMU.EDU> <1988@public.BTR.COM>
Organization: John Fluke Mfg. Co., Inc., Everett, WA
Lines: 24

In article <1988@public.BTR.COM>, thad@public.BTR.COM (Thaddeus P. Floryan) writes:
> 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".

Sorry Thad.  I checked the Toshiba data sheet and found that there are
only readable registers allowing resolution to 1 second.  There is a
programmable output that can be made to run at greater than 1 Hz rates,
but I didn't see a way to monitor the state of the output from within the
RTC chip.  I haven't checked my schematic to see if that output is
actually used anywhere in the Unix PC.

I once had a look at some early schematics that showed an input register
that tracked the value of the video display address register.  They
apparently had ideas about implementing a light pen.  That could have been
useful for doing some fine resolution timing.  Too bad though, apparently
this register never got implemented in actual production units since the
address it was at instead got used for the P5.1 upgrade.

---
	Craig V. Johnson		...!fluke!vince
	John Fluke Mfg. Co.			or
	Everett, WA			vince@tc.fluke.com