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From: hundt@wind.bellcore.com (tom hundt)
Newsgroups: comp.sys.ibm.pc
Subject: turbo systems (how to toggle; reply)
Message-ID: <1799@bellcore.bellcore.com>
Date: Tue, 4-Aug-87 11:18:20 EDT
Article-I.D.: bellcore.1799
Posted: Tue Aug  4 11:18:20 1987
Date-Received: Thu, 6-Aug-87 01:38:48 EDT
Sender: news@bellcore.bellcore.com
Reply-To: hundt@wind.UUCP (tom hundt)
Distribution: world
Organization: Bellcore, Morristown, NJ
Lines: 42

THEORY
In a every XT there is an 8284 clock controller chip.  This chip has
a couple of unused inputs:  F/~C and EFI.  Unused, that is, unless you
have a Turbo clone.  The F/~C (Frequency/not Crystal select) chooses
the clock input from one of two places:  a crystal on X1-X2 inputs, or
an External Frequency Input on EFI.  A one-speed motherboard will 
always select F/~C low (selecting the crystal) and get its clock from
the X1-X2 inputs.  A two-speed board will have the F/~C tied to the output
of one of the 8255s on board, and will have maybe a 24 MHz oscillator
going to the EFI.  (Note:  The chip has a built-in divide-by-three
on these inputs, so 24 MHz in gives 8 MHz processor clock.)
The BIOS controls the clock speed by toggling the appropriate 8255 output.


PRACTICE
In my, and many other, XT clones the following does the job:

OUT 61H, 40H  for slow speed,
OUT 61H, 4CH  for fast speed.


COMMENT
The clone companies give you really lousy schematics (like, you get
a non-turbo motherboard drawing with your turbo machine...) and
the way I found this info out was by using Debug, doing I (INPs) from
all the 8255 registers, and seeing what changed when I hit CrtlAlt-
to toggle speed.


QUESTIONS
Does the above work on AT clones also?
Does anyone have a clone XT that works differently?
You could presumably replace your 24 MHz oscillator with a 30 MHz one,
so as to get 10 MHz operation.  Anyone tried this?  (I've been too lazy
to take apart the *whole* machine to get the m-board out...)
Caution:  Don't try this with a 4-layer motherboard!
Did anyone get the *real* schematics with their system?

Happy trials.
 /-^-\  Thomas M. Hundt / BELLCORE Morristown NJ / hundt@bellcore.bellcore.com
 |   |  {seismo|ihnp4|ucbvzx|decvax|ulysses|allegra|clyde}!bellcore!hundt
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