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				    * *
	COMPUTER PARTS GALORE INC.  * *	  HARDWARE TECHNICAL
	316 COLLEGE ST.	TORONTO	    * *	    BULLETIN
	928-2161		    * *			1/2/86
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	-PC and	XT 8 MHZ CLOCK UPGRADE -

These modifications will allow you to easily upgrade the clock speed
of your	8088 based IBM or compatible to	speed execution	of
programs by up to 60%. We have tried and tested	this
modification on	several	boards and found it to work well with
most software and hardware but please be aware that soom
incompatibilities will arise. Also we have found that the
closer to 8 MHZ	you run	your board the more component level
problems you will encounter. These kind	of problems will take
the form of erratic behavior. So be advised , the faster you
want to	run your machine the more quality and performance of
the individual components will count.

The first requirement is a 8088-2 replacement processor. (While
your at	it you may concider an 8MHZ V20, then you could	squeeze
another	10% out	of your	box, but thats another articule).

The second requirement is another xtal to produce the higher
clock speed. This can be accomplished by one of	two ways. The
simplest is to purchase	an oscillator package, the value of
which is three times the frequency of your targeted running
speed. That means, if you want an 8MHZ processor clock then you
need a 24 MHZ oscillator package. Similarly, if	you want a 6.6
MHZ processor clock then you need a 20 MHZ oscillator package.
Please note we said another xtal so don't get excited and rip
out your good old 14.318 MHZ xtal, you'll need it before we're
finished. The second way to do this is with a xtal and an
oscillator jig.	A quick	look at	the motherboard	schematic will
yield a	suitable design	for the	jig.

The third requirement is a small double	pull double throw
switch.	Preferably one that will easily	mount on your present
computer case.

The fourth requirement is some solder, wire wrap wire, a bit of
proto board, a component carrier, and preferably a spare 8284
Intel clock chip.

The fifth requirement is a suggestion for the super keen money-
is-no-object guys. We think that even better more reliable
performance can	be achieved by using HCMOS logic chips where
ever possible on the board, especially bus drivers,decoders, and
multiplexers . Also Intel has announced	but we haven't seem a
line of	8MHZ HCMOS 8088	support	chips most notably the 82C84A ,
82C88 ,	82C37A-5. We did not use these chips on	the boards we
modified but any of the	problems we encountered	indicated that
the higher spec	chips are the way to go.

Ok lets	begin. You'll be glad to know that Intel has made the
whole thing easy for you by the	way they designed the 8284 clock
chip. They designed it to run with an internal or external
clock. The way your present XT or clone	is set up, you are most
likely using the internal clock	configuration with a 14.318 MHZ
clock (	picked for the virture of being	able to	supply the
color burst frequency when divided by 4	). Now the beauty of
the way	Intel set things up is that if you instead chooze to
use the	external clock option on the 8284 and speed up the
CLK and	PCLK outputs of	the 8284 the chip will still use the
14.318 xtal to produce the OSC output which goes to the
peripheral slots where it is used by the color graphics	card.
Please note that the CLK is one	third the base frequency fed
into the 8284. The PCLK	is one half of the CLK.

The way	to select the internal or external clk input for the
8284 is	by the logic level of the F/C (	Frequency/ Crystal
Select ) pin 13	of the 8284. When strapped LOW F/C permits the
processors clock to be generated by the	crystal	(14.318). When
strapped HIGH, the CLK is generated by EFI pin 14 8284 (EFI=
External Frequency).

So the first MOD is to be able to pull pin 13 of the 8284 high
or low.	We found that the easiest way to do this without
damaging the motherboard was to	remove the 8284	from its socket
and mount it on	top of a chip carrier letting all legs except
pins 13	and 14 continue	thru as	before to the motherboard. Pin
13 must	be wired so that it can	be either grounded or pulled
high through a 1K resistor. This is what one side of the DPDT
switch is for.

The second MOD is to feed the appropriate external frequency into pin 14
of the 8284. If	you were smart and got your self an oscallator
package	this is	easy. An oscallator package is the same	size as
a 14 pin dip I.C. with Vcc at the top left hand	corner
and ground at the bottom right hand corner. The	 output	is
at the top right hand corner. The bottom left hand pin is not
used and is usually marked by a	dot indicating that it's pin #1.
If you are using a xtal	you will have to make an oscillator
with a 74LS04 and some components. Remember that the 8284 will
divide by three	whatever frequency you feed it !

Now comes the third and	final step. We found that the 8237, the
DMA controller did not want to run at 8MHZ. The	way we handled
this was to feed it the	PCLK instead of	the CLK	when running at
8MHZ. You use the second half of the DPDT switch to switch the CLK
with the PCLK. The DMA clk input is pin	12 of the 8237.	We bent
pin 12 of the 8237 up out of the socket	and ran	a wire from it to the
center pole of the switch and with the CLK and PCLK from the
8284 connected to the other two	poles. Since you have both the
switch for the internal	and external clock and the switch for
the DMA	clk speed on the same switch you must be sure to wire
them up	so that	pin 13 of the 8284 is pulled HIGH at the same time
that pin 12 of the 8237	is connected to	the PCLK clock.

Well thats all there is	to it. We also found that 150 ns memory
worked better that 200 ns at 8MHZ. You will notice that	most of
the commercially offered turbo XT boards being flogged in town run
with a 20 MHZ clk for the high speed option. We	believe	this is	to
save running in	to spec	problems with the LS and S logic used
on the boards, so you might try	this first and move up to 8MHZ
if all goes well. If this makes	sense to you then make your Mod
so that	you can	replace	the oscillator if you want to. Try four
Auget socket pins.

We at COMPUTER PARTS GALORE hope that this proves useful
information to those of	you adventurous	enough to want to
experiment with	your computer. Rest assured that if you	work
carefully and think about it you will be able to implement the
above modifications in such a way that no permanent damage is
done to	the motherboard.

This technical note has	been composed for the purpose of
distributing via bulliten board	for the	use of the general
public.	Please feel free to upload to any and all such forums.
We would appreiciate if	the credits were left attached to all
such disemination of the above material. Due to	the method of
transmission we	have not included any schematics of the	mods.
We feel	that the simplicity of the mods	makes this no great
disadvantage.

GOOD LUCK AND HAPPY COMPUTING

					Glenn Tooth
					COMPUTER PARTS GALORE
					316 COLLEGE ST. TORONTO