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Modification B) Hardwired BF0 / BF1 Pins for different Bus / Core ratio
Now, what options do you have if you can't find one of the fairly rare Pentium Overdrive chips? Let's see what have we here. A processor board offering a single-rail 3.3 V DC supply and 60 MHz base clock. Why not install let's say a Pentium 120, 166 or even 200? It should just fit in and run, right? Eh, note quite...
Most Clone Boards have a set of jumpers or switches that can be used to change the base clock between 50, 60 and 66 MHz, and select different bus/core ratios (clock multipliers). Usually accompanied by a silkscreened list of all the possible settings.
The P90 platform is different. It has only two 3-pin jumpers, and they serve a completely different function... nothing related to the CPU clock. So, there is no convenient way of changing these settings. There is a single CPU clock oscillator - a 60 MHz one. And the two pins on the processor socket that control the bus/core ratio are left unconnected.
But don't lose hope just yet! We can still tie the two unconnected pins to whatever level we need: either logic "1" - 3.3 V, or logic "0" - ground. To make things easier, the processor pulls the unconnected pins to logic "1" internally. So, with both pins (BF0 and BF1) open, we get binary value "11" - which according to the documentation means a 2:3 clock ratio. And that's exactly what we would expect on a P90 board with a base clock of 60 MHz. A ratio of 2:3 is the same as 60:90 (MHz).
Okay, so where are the two darned pins anyway?
BF0 is the 8th pin left in the 5th row from the top - just above the "5" in "R157".
See the picture below:
Position of the BF0 / BF1 pins on the Pentium socket
Next you need a list of the
possible combinations that can be achieved with the various processors.
You should keep in mind that we only have 60 MHz base clock
available. If you plug in processor that was designated for operation with
50 MHz external bus, you will be overclocking it, which often works if you
go up just by "one notch", but the system may encounter stability issues or
may not POST at all. Damage to the chip is unlikely, as long as you give it
sufficient cooling that is...
Here's an example on how to "hardwire" a Pentium 166 processor on the
P90 complex. The P166 uses a 2:5 bus / core ratio - according to the
list above this requires both pins to be at logic "0" = ground. You simply
solder one wire from the BF0 pin to the unmarked end of capacitor C162 and
another wire from BF1 to the unmarked end of capacitor C210. You could also
tie the ends together and solder them both to any of the two capacitors,
or solder a single wire from ground to BF0 and then to BF1... whatever you
Wiring BF0 and BF1 to GND for 2:5 bus / core ratio
After the modification, installing the P166 CPU, and starting up the operating system you may run a speed check tool (like the one included in QCONFIG - tool that comes with IBM PC-DOS 6.1 and later). It will determine the processor speed at about 150 MHz - which is what you get when you multiply 60 MHz by 2.5x...
Note: Please keep in mind what is said in the cooling chapter - you will need a good processor heatsink and you better upgrade the 3.3 V regulator heatsink as well. Check Jim Shorney's recommendations and examples HERE.
The current draw of a P166 processor is 4.25 A at 3.3 V at 66 MHz and 166 MHz internally. One might suppose that it behaves like a real P150 processor when underclocked to 60/150 MHz. Which will result in a 3.85 A draw - 0.9 A more than the original P90 processor, which takes 2.95 A @ 3.3 V. That is well within specs of the regulator (5.0 A). Increase of 0.9 A is roughly an increase of about 1.5 W of additional thermal load, which is really pushing capabilities of the small heatsink. Therefore improved cooling isn't a luxury - it's essential.