P90 Platform Tuning

Table of Contents

[-0-]  Version History, Disclaimer & Legal Info
[-1-]  Precautions and Warnings before you start
[-2-]  Introduction to the Type 4 "Y" Pentium 90 Platform
[-3-]  The importance of cooling certain components
[-4-]  Modification A) Using a Pentium Overdrive 180 / 200
[-5-]  Modification B) Hardwiring the BF0 / BF1 Pins for different Bus / Core ratios
[-6-]  Modification C) Changing the Base Clock from 60 to 66 MHz
[-7-]  Modification D) Using a Pentium MMX 233 MHz with an Interposer
[-8-]  Problems, Workarounds and other stuff

Content by Peter H. Wendt (original HERE). Modified by Major Tom.


Modification D) Using a Pentium MMX 233 MHz with an Interposer

Let's start with some CPU basics.

Do I have a MMX or Classic (non-MMX) Pentium?

First check the writing on the top side of the CPU:

   Classic (non-MMX) Pentium chips are labeled Intel pentium ixxx.
   MMX Pentium chips are labeled Intel pentium w/ MMXtmtech.

Where xxx is the speed rating (i.e. 166 or 200).

Sometimes it's difficult or impossible to read the writing on the top (glued heatsinks etc.), in which case you can identify it by checking the underside:

   Classic (non-MMX) Pentium chips are labeled intel pentium FV80502xxx
   MMX Pentium chips are labeled intel pentium w/ MMX tech FV80503xxx

Where xxx is the speed rating. Part number starting with 80502 signifies a non-MMX chip (P54C/CS/CQS), and 80503 signifies a MMX part (P55C).

Most MMX chips came in the newer plastic package (PPGA, black base with silver heat spreader), but some of them used the ceramic package (CPGA) typical for older processors (classic Pentium, 486, 386DX...). But the "MMX tech" marking should always be clearly visible - on the top as well as on the underside.

Please note that some later non-MMX chips also used the black plastic package - namely Pentium 166 and 200.

Warning: Never try to install a Pentium MMX CPU on the P90 platform directly!

The MMX chips use a split-voltage design where the processor core runs at a lower voltage (i.e. 2.8 V) than the I/O portion of the chip (that runs at 3.3 V). Using the chip on a straight 3.3 V platform is not a good idea. The chip will produce excessive heat and the overvoltage may degrade or destroy the chip! You need to use a special adapter - so called interposer - with a secondary power regulator, which reduces the core voltage to an appropriate level. This regulator isn't powered through the CPU socket on the motherboard/processor card, but directly from the main power supply. This is necessary to satisfy current requirements of the higher-clocked chips. Many older Socket 5 boards were not designed for this kind of power draw.

One of these adapters is the Madex 486007 kit. It consists of two parts:

1. The socket adapter for the CPU itself

The socket adapter

2. An external switchmode regulator

The core regulator

The two boards are interconnected with 4-pin plugs (blue wires) and the regulator board is then connected to the main PSU via a standard 4-pin 3.5" FDD power plug ("small molex"). The regulator board has 4 small SMD switches that can be used to set the desired core voltage. The "large" golden heatsink could be ordered separately and is not part of the base kit itself.

The interposer has a very thin and fragile pins on the bottom and has to be carefully installed on the platform socket. You may notice that not all pins are present on the underside of the interposer - that's intentional, these are the pins that carry the lower core voltage. As mentioned before that voltage is supplied by the additional voltage regulator that is powered directly from the main PSU.

Once you have installed the interposer, CPU, regulator and CPU heatsink, the system should come up as normal. There are jumpers on the interposer that can be used to set the appropriate core/bus ratio (multiplier). On the 60 MHz (unmodified) platform a 1:3 ratio will bring you to 180 MHz. For a test run I would suggest to leave the pins open and try out the combo at the usual 90 MHz processor speed. If that works reliably, start working your way upwards...

YARM - Yet Another Regulator Module

I've once bought this MMX interposer module together with some non-IBM motherboard. It has a local regulator for the core voltage - yet another member of the LT-108x family, that is once again powered directly from the main PSU via a standard 4-pin 3.5" FDD power plug. It has jumpers to set the correct core voltage and bus/core ratio. But the whole design is a tad awkward as you can see from the pictures below.

YARM - view onto the jumpers
View of the DC-input and the jumpers...
YARM - view around the edge
...and how the wire runs around the edge.
 
YARM - view onto the lower side
View from the MCA side upwards
YARM - view with regulator
Voltage regulator with its own heatsink

It has a couple problems. There are no catches on the CPU socket that could be used to fix the CPU cooler. The jumpers are also placed awkwardly, right where the heatsink clamp would normally be, which further complicates heatsink installation.
To fix the heatsink in place I've come up with the contraption you can see above - two pieces of wire and one zip tie. It's not ideal as the wires are pushing against the interposer PCB and barely fit inside the socket hooks. This also causes the heatsink to be pushed off-center. I would be a bit concerned about reliability of this solution, if I was to run this thing for a long time (which I don't, to be honest).

Take this as an experiment only. The complex refused to POST properly with this combo. As always it first complained about a platform change, but then during the refdisk load it dumped into a 0129 1500 error. I'm not 100% sure, but I think the MMX processor I've used here was previously running in a clone with a 3.3 V core voltage for some time... so it may be damaged. Not sure if that's what caused the problems here, but it's possible.

Whatever, in theory it should work and you might get a similar looking result if you manage to find one of those interposers.

Content created and/or collected by:
Louis Ohland, Peter Wendt, William Walsh, David Beem, Tatsuo Sunagawa, Jim Shorney, Tim Clarke, Kevin Bowling, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 19 Nov 2021 - Changes & Credits | Legal Info & Contact