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From: rfutscher@pbs.org
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: AMD Math Coprocessor again.
Message-ID: <1990Dec20.130855.11093@pbs.org>
Date: 20 Dec 90 18:08:54 GMT
References: <1990Dec19.121014.3338@hellgate.utah.edu>
Organization: PBS:Public Broadcasting Service, Alexandria, VA
Lines: 34

In article <1990Dec19.121014.3338@hellgate.utah.edu>, cyang%peruvian.utah.edu@cs.utah.edu (Cheng Yang) writes:
> 
> Hi, everybody.  I am very thankful that I have got a bunch of helpful
> replies from you regarding to the AMD 80C287.  I just called the dealer
> and I have another problem.
> 
> On the manual of my motherboard, it said that "  To determine which speed
> 80287 is needed, purchase the one that can operate at 2/3 the maximum speed
                                                       ^^^^^
> of your motherboard.  The IBM AT computer is designed to use the pin on the
> 80287 that devides the crystal frequency by 3.  The 80286 devides the crystal
> frequency by 2 to obtain the system clock speed. .... "  and it recommend I
> use 80287-10 to work with my 16MHz 80286.  Actually the crystal frequency
> on my motherboard is 32 MHz.  So the coprocessor will get 10.67 MHz.(not
> exactly 10 MHz)  (My motherboard is bought over one year ago and there
> was few 12 MHz coprocessor at that time, I think.)
> 
> The dealer I talked to said that I need an AMD 80C287-12 which is $139.00.
> And he said that the $99.00 AMD 80C287 is 10MHz and it is for 12 MHz 286 or
> less..
> 
> My question is:  Who is right?  My manual or the dealer.  What will the result
> be if I put that 10MHz AMD 80287 on my motherboard, will the 10.67MHz hurt
> the 10MHz 287? Or the chip won't work totally?    Do I really need a 12MHz
> coprocessor???
> 

    I purchased a ITT 287, and it would not pass the test that came with it.
A replacement ITT 287 failed the test the same way. Like your motherboard
the clock for it was also an uneven division, which made the duty cycle
of the clock 1/3, 2/3. If the duty cycle was 1/2, 1/2 I am sure that 
it would pass the test. I am sure an Intel 287 would pass the test with 
a duty cycle of 1/3, 2/3. With clone 287 you need to be aware of duty cycles
that are not 50/50.