KUO@oregon.uoregon.edu (Shijong Kuo) (03/02/90)
There was some discussion about the asynchonous mode operation of 80387 a while ago, I don't remember what the conclusion was. I have a related question, perhaps someone would care to comment. 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 NUMC LK2 : CPUCLK2 is between .625 and 1.4. I know some of Micronics's MB is impplemented such that you can install a seperate crystal to supply this NUMCLK2. 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? Anybody care to enlighten me on this subject! kuo@oregon.uoregon.edu
dhinds@portia.Stanford.EDU (David Hinds) (03/02/90)
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
davidsen@sixhub.UUCP (Wm E. Davidsen Jr) (03/03/90)
In article <16675@oregon.uoregon.edu> KUO@oregon.uoregon.edu (Shijong Kuo) writes: | 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. If you mean that running the 387 at 20MHz with a 386 at 25 is in spec, that's true. But it doesn't mean that you can run a 20MHz part at 25MHz and be within spec. It will probably work that way, but in spec it's not. -- bill davidsen - davidsen@sixhub.uucp (uunet!crdgw1!sixhub!davidsen) sysop *IX BBS and Public Access UNIX moderator of comp.binaries.ibm.pc "Getting old is bad, but it beats the hell out of the alternative" -anon