From: S_JUFFA@iravcl.ira.uka.de (|S| Norbert Juffa)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Can I use a 387DX 33MHz on a 386DX 40MHz
Date: 22 Jul 1992 12:21:57 GMT
Organization: University of Karlsruhe (FRG) - Informatik Rechnerabt.
Distribution: world
NNTP-Posting-Host: irav1.ira.uka.de
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In <4689@unixhub.SLAC.Stanford.EDU> johnw@jupiter.SLAC.Stanford.EDU writes:

> 
> Question for a friend who owns a 386DX 40MHZ motherboard.
> 
> 
> He wants to add a math coprocessor which is rated at
> 33MHz to his 386DX 40MHZ motherboard (AMD CPU) with 64k cache.
> 
> Is this possible???  or will he need to decrease speed
> to 33MHz  by changing a crystal on the motherboard ????    
> 
> Thanks in advance for any help you can give us.
> 
> -Jonathan
>

Yes, you can run a 387-33 coprocessor with a 386-40 CPU. However, you have to
have a mother board that supports this. You need a mother board that has two
sockets for a quartz oscillator (one for CPU and one for the coprocessor) and
a jumper that selects between synchronous operation of the coprocessor (copro
runs at same speed as CPU) and asynchronous operation (copro run at different
speed as CPU). Note also that only the Intel 387DX coprocessor really runs
asynchronously in such a configuration. The clones from ULSI, IIT and Cyrix
*do not* run asynchronously. 387 chips have two clock inputs: CLK is the main
(CPU) clock, CLK2 is the alternate clock which allows the coprocessor to run
at a different speed than the CPU. CLK is always needed as the coprocessor has
to pass information to the CPU and vice versa. CLK2 is optional and only needed
to drive the coprocessor asynchronously with respect to the CPU. The 387 clones
mentioned above *ignore* CLK2 input, even if it is supplied. So they will
always run at the speed of the CLK input signal which is the same as the CPU
speed (in your case, 40 MHz). I own a 33/40 Mhz motherboard and the Cyrix, IIT,
ULSI, and Intel 387 coprocessors, and have experimented with the various
combinations. I have verified that only the Intel 387DX will run
asynchronously. The Intel data sheet for the 387 states that CLK2 input may
between 70% and 140% of the CLK input, so it seems safe to run a 40 MHz CPU with
a 33 MHz 387DX. But why not install a 40 MHz coprocessor from one of the clone
makers. They all offer 40 MHz versions at about the same price as Intel sells
the 387DX-33 for (typical US$ 99). From my tests done on the 387 coprocessors,
I would advise you to go for the Cyrix 387+. Overall this is the fastest of the
387 compatibles and it is the most accurate when it comes to transcendental
function, too. BTW, do *not* try to install a 33MHz coprocessor with your
40 MHz CPU synchronously. While there is usually no problem with the additional
heat generated, the coprocessor might start to err in its computations if run
at a speed higher than specified by the chip maker. I had this problem with 
a Cyrix 83D87-33 (predecessor to the 387+) which I experimentally run at 40
MHz. It passed all the test programs supplied by Cyrixand third parties fine, 
but it failed due to multiplication errors in the Whetstone and Linpack 
benchmarks I compiled. Hope this helps.

Norbert

-------------------------------------------------------------------------------
Norbert Juffa       email: S_JUFFA@IRAVCL.IRA.UKA.DE         Live and let live!