75 and 83.3 MHz FSB

[radman3d2]

When ever I try to use the 75 or 83.3 MHz FSB to overclock my computer it eventually locks up. I'm not even pushing the CPU to much. My ram is PC100 SDRAM so that is not the problem. I have great controll over everthing with the award bios, so I even lowered setting there but eventually the computer locks up. It dosn't crash (no blue screen of death) it just locks up. My MB is Chaintech 5SIM, CPU is 380 AMD, 128 RAM. The computer runs great at 400MHz (66*6) and at 450Mhz (75*6) it runs OK for a while but... Any ideas would be greatly received.
Radman

[RADAR1797]

Radman,

I tried getting info on your MB, but alas Chaintech did not have a 5SIM listed (that I could find). 75 and 83 MHz FSB run your PCI bus out of specifications--37.5 and 41.5 MHz respectfully over the 33 MHz PCI spec. Now if you have a well implemented MVP3 chipset on your MB (unknown b/c I couldn't get the MB info) then there should be an option to run your PCI bus asynchronously to the FSB for 75 and 83 MHz. I am making the assumption that because you are not running a 100 MHz FSB, that your MB is older and does not have the MVP3 chipset. Also do you have an AGP slot? If you have an older PCI video card, it may not like the heightened data rate and lock the computer solid.
Now it could also be that your chip does not like going above 450 MHz. Try 5 * 83 = 415 Mhz and see how that works. Have you raised the voltage at all? Also use the highest FSB available to you, it will increase your MB's L2 cache speed and improve overall performance. Hope that helps some.

-RADAR

** Ok I found the quick refernce guide for this MB, it looks you can only go down to 2.8v, so raising the voltage is not viable. You also cannot run your PCI bus asynchronously, so your only recourse is to find the offending peripheral (the one locking up your system) and replacing it with a more modern card.

-RADAR

[This message has been edited by RADAR1797 (edited 09-28-2000).]

[radman3d2]

I have a Voodoo3 2000 (PCI, no AGP). The board does have the async setting, which is set at 75 or 83.3. The chipset is SIS 5582 and I have core voltage settings of 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9 and 3.0. I have tryed all possible settings 5*83(2.4v), 6*75(2.6v, some sucess), 5.5*83(2.6v) and many others but it always locks up sometime or another. I have come to think it has something to do with the async setting, but I'm not sure because I'm not even sure what it does.

[RADAR1797]

Cool, the quick refernce pdf on Chaintech's site was less than forthcoming. It is good that your MB supports the voltages for the core, does it support changing the voltage for the IO? If so, try moving the Vio up to 3.5 and see if that makes it more stable. Same principles with the CPU applies to overclocking a bus too. If you do not have a way to modify Vio, then take everything out except your barebones (ie MB, RAM, CPU, Video card, etc) and run it for a while and see if it still crashes. It will be your lucky day if all the problems were attributable to an old sound card or NIC.

-RADAR

[Win_98]

I think you have overclock your CPU way too much for it to be stable. To test if your 83mhz bus is really stable try 83 x 4 for 333mhz, and 75 x 4 for 300mhz then you know for sure if it the FSB or +70mhz is a little too much! Usually it not a good idea to run at 37.5mhz pci using 75mhz, or 41.5 using 83mhz bus because PCI which consist of your harddrive, modem, are more likely to lock think up. so go to http://www.sisoftware.co.uk/ and get sandra millenium and it can tell if your PCI is running at right speed or not.

[This message has been edited by Win_98 (edited 09-28-2000).]

[Peter M]

That SiS chipset can run the PCI bus asynchronously at 75 or 83 MHz CPU bus.

Older revisions of the SiS 5581 chipset (and its identical twin 5582, as well as the 5597/5598 with integrated VGA) seem to have a CPU bus timing issue with AMD processors at 75 or 83 MHz. There are other boards that don't work in that mode. At the time that chipset was made, only Cyrix had 75 MHz processors, and noone had 83 MHz ones. Cyrix processors have the most critical signal timing adjustable, and the SiS boards' BIOS all set that to "slow". This might be the reason why AMD K6 don't work at those speeds. Later revisions of the chipset seem to fix that - at least there are much later boards that are officially rated 75/83 MHz capable, and do work well with K6 at those speeds (like PC-Chips M571LMR, which must be the last SiS 5598 board that is still being made).

Regards, Peter