Still Crazy

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.

  1. 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.

  2. 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*.

  3. 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.


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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Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

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