How Can I Find Out, Which Bus Speed My Motherboard Supports?
To be honest, the easiest and most sensible answer to that is: TRY IT OUT !!! Switch the multiplier to the lowest setting and then put the bus speed jumpers in all the different configurations. For non mathematical people, there are 2 to the power of the number of jumpers configurations. 4 configurations for two jumpers, 8 configurations for 3 jumpers, and so on. Just boot up to the DOS prompt and run ctcm, or a similar program which will tell you the CPU speed. You then only have to divide it by the multiplier setting to find your bus speed. Again, if your motherboard uses SoftMenuTM BIOS, you only have to look in your BIOS setup menu to find all of the different bus speed settings from which you can choose.
There is also, however, a different approach:
- Motherboards with the PLL chip PLL52C59-14 can run at up to 75 MHz and they also support the 'turbo frequency' feature, which increases the bus speed by 2.5% (officially approved by Intel's CPU specifications).
- For 75 MHz
Pin 8 via 2.2 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!)
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 VFor 68 MHz, the 'turbo frequency' for 66 MHz
Pin 8 via 2.2 k Ohm to 0 V
Pin 12 via 10 k Ohm to 0 V
Pin 13 via 10 k Ohm to 5 VFor 61.5 MHz, the 'turbo frequency' for 60 MHz
Pin 8 via 2.2 k Ohm to 0 V
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 0 V
Measured on the Abit boards IT5H, IT5V, PR5, which all use the PLL52C59-14. The PR5 also comes in a version with the PLL52C61-01, however, the setting below seems to set the board at 61.5 MHz instead of 83 MHz.
- Motherboards with the PLL chip PLL52C61-01 can run at 83 MHz bus speed as well and also theoretically support the 'turbo frequency'. There obviously is, however, a way of configuring this chip (circuitry), so that it would not run at 83 MHz. I'm working on that.
These are the conditions: - For 83 MHz or 61.5 MHz 'turbo frequency' for 60 MHz. Unfortunately this depends on the circuitry on the motherboards.
Pin 5 via 10 k Ohm to 0 V ('0 V' means 'ground' and NOT 'disconnected'!)
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 VFor 75 MHz
Pin 5 via 10 k Ohm to 5 V
Pin 12 via 10 k Ohm to 0 V
Pin 13 via 10 k Ohm to 5 VFor 68 MHz, the 'turbo frequency' for 66 MHz
Pin 5 via 10 k Ohm to 5 V
Pin 12 via 10 k Ohm to 5 V
Pin 13 via 10 k Ohm to 5 V
I have measured this on the Asus P/I-P55T2P4 rev. 3 board and was able to verify the exact same settings on the FIC PA-2006 board. The FKI SL586VT II or Magic Pro MP-586VIP board also use this chip, but you can't get to 83 MHz. Instead you get the pathetic speed of 61.5 MHz.
In terms of jumpers, this means that you will have to find out which of the three jumpers is connected to the particular pins. It's circuited via the pull up/down resistor of 10 k Ohm. In case you only have jumpers with ON/OFF positions instead of 1-2/2-3 positions, the ON condition is the condition for 0 V, and the OFF or open condition is for 5 V.
This information is only for real freaks who want to get their boards to 75 or maybe even 83 MHz by all means. This way, you might be able to use the higher bus speeds on boards that only have 4 bus speeds, as long as this board uses one of these PLL chips. You do the necessary soldering at your own risk and should only attempt this if you know exactly what you are doing!!!
There are several motherboards that use one of the two PLL chips, depending on what was available when the motherboard was assembled. The Abit PR5 is such a fellow (of course I was unlucky enough to receive a board with the PLL52C59-14) and it seems the shuttle HOT 557 is another. There are a few reports of 83 MHz bus speeds with these boards, but most simply can't run at this faster bus speed. This is a real pity, because both of these two boards can use SDRAM - the best RAM for the 83 MHz bus speed.
