Topic P5_CLOCK from CPU FAQ base
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— 299.COMP.IBM.PC.CH (2:5020/299) ———————————————————————— 299.COMP.IBM.PC.CH —
From : rwallace@world.std.com 2:5020/299.100 Mon 06 Mar 95 15:08
Subj : Unknown
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In article <60.2565.7218.0N1D3736@canrem.com>,
Donny Chan wrote:
>
>I also don't know which pin is the BF pin.
>
The BF (Bus Fraction) pin is at location Y-33 of the PGA array. The 1.5x
mode is selected by leaving the pin floating or connecting it to logic
"high". The 2x mode is selected by connecting the pin to Vss (ground). If
the motherboard does not have a jumper for this, the closest Vss pins are
X-36 and Z-36.
>
>I am testing a new motherboard from ACER for P54C CPU -- model: PI16, based on
>ALI M1449/M1451 chipset. There are jumpers block for 4 different external clock
>speed: 40/50/60/66Mhz PLUS a jumper block for SETTING THE CPU SPEED: either 3/2
>times the external clock speed or 2 times the external clock speed.
>
>I tested it with a P5-75 CPU external clock speed at 50. With the CPU speed
>setting for 3/2 times external clock speed, BIOS report 75Mhz during system
>boot up process. With CPU speed setting for 2 times external clock speed, BIOS
>report 100Mhz during system boot up process. I also verify both result with
>Norton Utilties 8.0's SYSINFO program.
>
Apparently, in anticipation of the 120 and 133 MHz P5 chips, many motherboard
manufacturers are including a jumper which controls the state of the BF pin,
although use of the jumper may not be described in the documentation for
boards supplied with the present 90 or 100 MHz processors. Rechecking the
processor manual, I found that BF "grounded" ==> 2x mode. (See above) Also,
since the earlier post, there was a report in alt.comp.periphs.mainboard.asus
from a user who had been running a _dual_ P5-100 Asus motherboard (Neptune
chipset) in 2x mode for over two weeks in OS2 2.11 and OS2 Warp. Norton SI,
which would probably use only one processor, was in excess of 400 [sorry,
don't recall version]. In that case, two out of two 100 MHz processors were
"overclocked" to 133 MHz. The present report from Donny Chan indicates that
the 75 --> 100 "overclock" is possible in some cases. Many people have been
successfull in "overclocking" P5-90's to 100 MHz in the standard 1.5x mode.
These results suggest that there is some significant design margin, with
regard to internal clock rate, in some of the 3.3V P5's presently reaching
end users. One could speculate that early "samples" of the 120 and 133 MHz
chips might even be drawn from present production runs.
As always with "overclocking":
1) Your "mileage" may vary -- a particular chip may or may not be
capable of operating with a significant increase in clock speed;
because of possible malfunction, the "experiment" should not be
done on any system that has high availability requirements
2) Greater than normal cooling will be helpful
3) Remember that the chip is NOT warranted for such operation, and
any damage to the processor resulting from "overclocking" would
be the user's responsibility. The financial consequences of
blowing the CPU are far greater with a P5 than they are with an
old 486-25.
Roger Wallace
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* Origin: a kind of gate (2:5020/299.100)