AOH :: 8MHZ-PC.TXT|
Upgrading a 4.77 MHz PC or XT to 8 MHz! How it's done!
COMPUTER PARTS GALORE INC. * * HARDWARE TECHNICAL
316 COLLEGE ST. TORONTO * * BULLETIN
928-2161 * * 1/2/86
-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
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=
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
GOOD LUCK AND HAPPY COMPUTING
COMPUTER PARTS GALORE
316 COLLEGE ST. TORONTO
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