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From: davet@oakhill.UUCP (Dave Trissel)
Newsgroups: net.arch,net.micro.mac,net.micro.68k
Subject: Re: timing loops
Message-ID: <647@oakhill.UUCP>
Date: Mon, 17-Feb-86 01:08:44 EST
Article-I.D.: oakhill.647
Posted: Mon Feb 17 01:08:44 1986
Date-Received: Tue, 18-Feb-86 04:29:37 EST
References: <156@motatl.UUCP> <530@hoptoad.uucp>
Reply-To: davet@oakhill.UUCP (Dave Trissel)
Organization: Motorola Inc. Austin, Tx
Lines: 33
Xref: watmath net.arch:2546 net.micro.mac:4710 net.micro.68k:1490

In article <530@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
>
>People who design microprocessors, who don't want software to depend
>on the timings of individual instructions in particular systems,
>should provide a system-independent way to delay for a specified
>amount of time.  We use whatever you give us, guys!
>

I grew up with the early IBM 360 and it's built-in interval timer.  Later
models and the 370's had a time of day clock as well.  I have often
yearned to have the same common system-independent timing facilities
in micros as well.  But how do you accomplish that without forcing
every system designer to hook up a constant frequency clock to every
microprocessor in the family?

Of course, the problem is that the basic clock frequency driving the chip
is variable depending on the system.  If we implemented on-chip a clock
or timer register from where would it derive its frequency?  Having an
"adjust divisor" register setup by the system to factor the system clock
would just push the problem right back into the hands of the O.S. coders
where it is now since code somewhere would have to then setup the proper
divisor.

We currently have one customer running the MC68020 at 15 Meghertz, so we can't
assume that the "standard" test frequencies of 12.5 and 16.6666 will be used.
Customers are already preparing for the 20 and 25 Megahertz versions but
there is no way to know now what their exact frequencies will end up.

If you or anyone else has any suggestions on how to do this give a yell.

  --  Dave Trissel  Motorola Semiconductor, Austin, Texas
	{ihnp4,seismo}!ut-sally!oakhill!davet
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