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From: dhinds@portia.Stanford.EDU (David Hinds)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 387 push ok?
Message-ID: <9714@portia.Stanford.EDU>
Date: 2 Mar 90 07:44:15 GMT
References: <16675@oregon.uoregon.edu>
Sender: David Hinds <dhinds@portia.stanford.edu>
Distribution: usa
Organization: Stanford University
Lines: 26

In article <16675@oregon.uoregon.edu>, KUO@oregon.uoregon.edu (Shijong Kuo) writes:
> 
> According to Intel's publication "Microprocessor and peripheral handbook
> p. 4-148.":  when pin CLKM=1, NPX387 works in synchronous mode, CPUCLK2
> is used to control the 387; when CLK1=0 (asynchronous mode), NUMCLK2 is
> used to control the 387, provided NUMCLK2 : CPUCLK2  is between .625 and 1.4.
... 
> Given all this, why should I bother to puchase a 25 Mhz 387 to work on my
> 25Mhz 386 MB. A 20 Mhz 387 is perfectly within the specifications published
> by Intel, unlike the 25Mhz push of 20Mhz 386 cpu. So reliability does not
> appear to be an issue, it seems to me. Granted, MIPS magazine had reported
> on 33 Mhz systems using 387-25 was slower when compared with 387-33. Other
> than that, are there other ramifications on using lower rated 387 on faster
> 386 MBs?
> 
    Well, yes, you can use the 20MHz 387 in asynchronous mode with a 25MHz
386.  Another crystal drives the 387 at 20MHz, so it will be 20% slower than
a 25MHz 387 running synchronously.  The speed limit on the 387 refers to
the maximum rating on NUMCLK2 for asynchronous operation, or CPUCLK2 for
synchronous operation.  The range is quoted to indicate that the 387 can't
be mismatched in speed by too much in either direction - either much faster
or much slower than the 386.  It is NOT meant to imply that you can drive
the 387 at 1.4* its rated speed.  

 -David Hinds
  dhinds@popserver.stanford.edu