Path: utzoo!attcan!uunet!samsung!usc!ucsd!nosc!crash!pnet01!jca
From: jca@pnet01.cts.com (John C. Archambeau)
Newsgroups: comp.sys.ibm.pc
Subject: Re: changing AT's clock
Message-ID: <3076@crash.cts.com>
Date: 9 Jun 90 20:16:04 GMT
Sender: root@crash.cts.com
Organization: People-Net [pnet01], El Cajon CA
Lines: 38

silver@xrtll.uucp (Hi Ho Silver) writes:
>In article <9207@hubcap.clemson.edu> snugroh@hubcap.clemson.edu (Anto) writes:
>$I just bought a used NEC AT-compatible.
>$It has 80286-8MHz.
>$When I opened it, it looked like all memory (ROM and RAM) are 
>$for 12 MHz. So I wonder if I can just change the CPU and clock to 12 MHz to
>$get better speed with cheap price.
>
>   It probably won't work.  There are other items in your system besides
>the ROM, RAM and CPU which may be affected.  Chances are all the "glue"
>chips will be able to run at the higher speed, but keep in mind that the
>I/O bus that feeds the expansion slots probably takes its timing directly
>from the CPU's clock, so by speeding up the CPU to 12 MHz, you'd also be
>speeding up the expansion slots to 12 MHz and chances are at least one
>of whatever cards you have plugged in will not work at that rate.  There's
>another discussion in this group on that topic; you might want to read it.
>
>$There are several x-tals in the mother board (the motherboard has serial and
>$parallel port), but the one that close to CPU is 24 MHz.
>$Anyone know about clock divider by 3 ?
>
>   I'm not familiar with the various chips used to drive 80286s, but I
>know on some other microprocessors the crystal frequency was divided by
>three.  The 24 MHz crystal is probably the correct one.

On a a 286, it's the clock crystal divided by 2 to get the clock speed of the
CPU.  On the math coprocessor, it's the crystal divided by three or four.
 
     // JCA

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