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 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 --- * Origin: a kind of gate (2:5020/299.100)

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