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000005 - Model 70 PowerBoard Overclocking
Note: This is for Model 70
PowerBoard not for 8580.
40MHz base clock setting for Model 70 PowerBoard.
Originally written by F355 (Psudenym in FIBMJ) on 1999.04.26. Translated by
Sandy on 2000.05.10
Please be advised that F355 and I are both not responsible for any damage or
inconvenience on your system with following script.
You need to solder off just *one* of the pins of PLLIC. Search PLLIC
marked IMI SC-427BYB atop on the chip. It's a tiny SSDP type chip with 10 pins
on each side. It locates around pin #1 of big blue translucent chip, just
between OSC marked H14318 and flash ROM socket. Heat up pin #9 of PLLIC, solder off the pin from the board. the target
pin is "pulled-up" internally in the chip and if you float the pin from the
board, electrical status of the pin will turn to *High*. Set J11 jumper to the position for 25MHz operation (J11= ON).
Now the output frequency of the PLLIC is 40MHZ. Keep in mind, Cyrix
100/120/133 in x4 multiplied speed and DX4-100 may not work. Only AMD 5x86 x4/x3
will run on the board (on WinXX). (Errm... There *IS* such CPU, Cyrix 5x86
GP120 x4, actually.)
Pin out of IMI SC-427BYB
_____________
| o |
Xin = |1 20| = 14.1318MHz
Xout = |2 19| = VDD
Vss = |3 18| = PD2
7.159MHz = |4 17| = VCOin2
S0 = |5 16| = 16NHz
VCOin1 = |6 15| = 32MHz
PD1 = |7 14| = VSS
VDD = |8 13| = MCLK2
S1 = |9 12| = S2
MCLK1 = |10 11| = TS
|_____________|
"o" is orientation marking.
S1 (pin #9) should be soldered off from the planar.
Combination of Pin Logic and Output Frequency
| Input |
Output |
Jumper |
| S2 |
S1 |
S0 |
CPU Base Clock |
J11 |
| 0 |
0 |
1 |
25MHz |
ON |
| 1 |
0 |
1 |
33MHz |
OFF |
| 0 |
1 |
1 |
40MHz |
ON |
** S1 is internally high and physically grounded (pulled down) to Vss in
original circuit. So it is always *LOW*. S0 is pulled up to Vcc through R255 on
the board, It is always *HIGH*. Changing base clock between 25MHz and 33MHz is
done by J11. If the jumper set is ON, S2 will be grounded *LOW* (=0), while it
is OFF then S2 will be *HIgh* (=1). To get 40MHz, set S1 *HIGH* (this is done
by soldering off the pin #9 from the board), and set S2 *LOW* (J11=ON).
Attention to you all. The PLLIC is very little and pins are very close to
each other. Furthermore 14.318 OSC locates very close to PLLIC. Be careful not
to break off the pin, nor to make a bridge between pins by your soldering iron.
Otherwise your system will not work on 25/33MHz anymore. It may cause fatal
errors on your expensive PowerBoard.
DO IT WITH YOUR OWN RISK! NO ONE WILL BE RESPONSIBLE BUT YOU.
and
TAKE CARE!
If anybody wants to amend above method to *CORRECT* English, please do it.
If you can connect lead wires to floated pin #9 and it's solder land each, and
set jumper pins on the other end, you can change back to normal setting with
shorting the jumper pins.
About Mod.80 Powerboard. PLLIC used in Model 80 board is IC DESIGNS
2028SC-B. I have no pin-out for it.
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