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From: gnu@hoptoad.UUCP
Newsgroups: sci.electronics
Subject: Re: time of year clock chips
Message-ID: <2239@hoptoad.uucp>
Date: Thu, 4-Jun-87 01:45:27 EDT
Article-I.D.: hoptoad.2239
Posted: Thu Jun  4 01:45:27 1987
Date-Received: Sat, 6-Jun-87 05:44:24 EDT
References: <16819@amdcad.AMD.COM> <8089@utzoo.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 45

The Sun-2 "TOD clock" was the National Semi MM58167.  It's a 24-pin
chip with .001 second resolution and a very slow 8-bit bus interface.  It
requires at least an external 32767Hz crystal and 2 caps.  Here's what
Tom Lyon and Joe Skudlarek had to say about it in "All the Chips that
Fit":

	"Another problem in chip design is the tendency to put
	functionality in the chip which could be done just as well, or
	better, in the software driving the chip.  In fact, users must
	sometimes program around incomplete solutions to create the
	required complete solution; this adds to the complexity of the
	code, costing in development time, and execution time and
	space.  An example here is the National Semiconductor MM58167
	Time Of Year Clock.  This is a low power clock used to keep the
	system time when the system is powered off.  The chip is
	apparently designed to appeal to users of microprocessors which
	can't do division and multiplication, for the chip keeps the
	time in separate registers for the month (no year), day, hour,
	minute, second, etc., rather than just providing a counter
	which counts at a known rate.  In addition, each register is
	kept in BCD format, which makes displaying the time very easy.
	Unfortunately, UNIX doesn't want to display the time, it just
	wants something from which to set its 32 bit counter of seconds
	since Jan. 1 1970, GMT.  An attempt to do a straightforward
	conversion from UNIX time to chip time fails because the chip
	has no support for leap years or daylight savings time.
	Ultimately, we ended up using the chip just as a bizarre
	series of modulo counters; the amount of arithmetic to convert
	chip time to UNIX time is disgusting."

As Henry mentioned, the Sun-3 system clock is the Intersil 7170.
As I recall, there were some problems with the early Intersil 7170's
used in prototyping the Sun-3.  The switch to battery backup was not
painless.  This may have been fixed in future chips, or it may be
that they didn't document how to wire it up well enough.  Check it twice!

The 7170 also provides a 100Hz interrupt, which is what the Sun-3 uses
for its Unix timing, so there is no other clock chip in the Sun-3.  I
find this a bit of a lose, since I like to time things to the
microsecond, but only having one clock chip on the board certainly wins
in other ways.
-- 
Copyright 1987 John Gilmore; you may redistribute only if your recipients may.
(This is an effort to bend Stargate to work with Usenet, not against it.)
{sun,ptsfa,lll-crg,ihnp4,ucbvax}!hoptoad!gnu	       gnu@ingres.berkeley.edu