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.


Introduction to the Type 4 "Y" Pentium 90 Platform

The IBM Type 4 P90 complex is also often called the "Y" platform, because it showed up in a some late Model 9595-xYx machines. Originally it came with the Server 500 and was offered as an option for the Model 95 series. Preferably it should be used together with the 95A planar (2 serial and 2 parallel ports), which will offer optimum performance and support for all the platform features.

The codename of the platform was "Cubrun" (rumour says it's a small river), and the part and FRU numbers associated with it are:
   P/N 06H3729, FRU 06H3739 (with buggy P90)
   P/N 06H8589, FRU 06H7095 (with flawless P90)
The former is the original P/N used for the FCC application. These cards still may have a buggy Pentium 90 processor installed. Intel was (and still is) replacing them on demand.

Doesn't matter to us much, since we are going to replace the P90 CPU with something else anyway.

Location of the important components

IBM Type 4 Platform with Pentium 90
IBM Type 4 "Y" Platform with Pentium 90

All Type 4 boards have 256 KB IBM SurePath Flash ROM based BIOS.

The complex comes with a standard "Socket 5" processor socket. However it's the LIF (Low Insertion Force) type, rather than the more common and convenient ZIF (Zero Insertion Force) variant. The socket contacts are organized as a staggered pin grid array (SPGA), in contrast with the original Pentium P60/66 (and all 486) chips that used a regular pin grid array (PGA) sockets.

The board comes with the Intel Pentium 90 processor, which has a 60 MHz base clock and 2:3 bus/core ratio (1.5x multiplier).
The Pentium 90 processor is accompanied by an Intel 82497-60 Cache Controller and a set of 10 Intel 82492-60 Cache SRAMs for a total of 256 KB of Level 2 cache with parity (5 chips on each side of the card).
If you are lucky and got one of the platforms used on the last series of the IBM PS/2 Server 500, you may have 66 MHz rated cache RAMs and 82497 cache controller installed. However the base clock oscillator on these boards still ticks at 60.0000 MHz.

The P90 platform has 2 local voltage regulators. One of them - LT1084, 5 A, set to 3.3V - supplies the CPU with the core voltage (Vcore) and provides power for the 3.3 V I/O of the cache controller and the cache SRAM chips (Vcc3).
The other one - LT1085, 3 A, set to 3.75V - powers the highly integrated SynchroStream Controller (the SMD chip with ceramic base and silver heat spreader on top).

Warning: Please note that you shouldn't replace the 82497 cache controller with the older 82496 part from the P60/66 boards!
It may work, but probably not for very long. The controller used on the P60 platform is a straight 5 V part, the one on the P90 platform is a dual-voltage 3.3 V / 5 V chip. The P60 controller will short the 3.3 V supply to the 5 V rail! This may cause damage to the other parts that are using this 3.3 V supply - the processor and the cache SRAM chips. It will also stress the local 3.3 V voltage regulator.

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: 13 Sep 2021 - Changes & Credits | Legal Info & Contact