'Locked' PCI Bus speeds?

[Dputiger]

Ok, I've posted stuff on this before, but I was dumb and REVERSED the tables. My mobo lists the following settings:

Host Frequency/PCI Bus Frequency
60 Mhz 30 Mhz
66 Mhz 33 Mhz
75 Mhz 30 Mhz
83 Mhz 33 Mhz.

Now, my system is unstable at a 75 Mhz FSB unless I pour a lot more cooling into it than is required to cool it when I OC the clock multiplier on the chip. I'm hoping someone can support or smash my theory on why this is:

My system is stable at 60 Mhz (30 Mhz PCI) and at 66 Mhz (33 Mhz PCI). Is it unstable at 75 because the PCI slots are moving too slowly? Since cooling the system down makes it faster, is this why I have to use MORE cooling? Will raising to an 83 Mhz FSB (and 33 Mhz PCI) fix the problem?

[otheos]

I think you've got something wrong in your table. the PCI bus divider is 1/2 so you table should be:

60 -> 30
66 -> 33
75 -> 37.5
83 -> 42

and I see no way out of it, so clocking it to 75 gives you a high(er) PCI bus that your cards can't take. Unless this motherboard uses a 2/5 multiplier for 75 and 83 I must be correct (sorry if I am not), but this will be then a really weird motherboard.

As for your theory, I think if your PCI cards run @ 33 ok they run @ 30 ok too. If the problem is heat then something is taking too much of speed and this could be only if it is overclocked not underclocked. I personally go with the 1/2 divider theory. What m/b is it anyway?

[This message has been edited by otheos (edited 06-03-2000).]

[Dputiger]

Otheos--sorry. I posted this one, than realized the numbers were wrong, and reposted. So people, just disregard this please--sorry about that.

[wtp]

hey, don't post too many new posts that are the same exact questions. this actually confusing me. I replied a lot, and it was unnecessary to make that "to WTP" post since i'm not really familiar with your problem, i'm just saying what i know, and i think u should just make one post, and not any direct posts. Direct posts should be sent over e-mail. Don't do it again. http://sysopt.earthweb.com/forum/smile.gif

wtp http://sysopt.earthweb.com/forum/smile.gif

[Dputiger]

Yeah, I apologize for that. My first question was just supposed to be "How/Why do I have to run my machine so cold" and my second was "what about these bus speeds" but I got confused myself, screwed things up, etc. http://sysopt.earthweb.com/forum/smile.gif Sorry to anyone I confused--I get confused myself sometimes.

[Peter M]

otheos,

PCI divider isn't 1/2 by law of nature as you seem to think. That's just Intel's restriction on the matters. Other people's chipsets are and have been a lot more flexible here.


DPutiger,

you got a VIA or ALi chipset there, don't you?

If so, then the only things that run at 75 MHz are the CPU, the L2 cache if it's a socket-7 system, and the DRAM array. Try backing down on DRAM speed. If that doesn't help, you're stuck at 66.

All the other stuff runs off the PCI clock, and by your testing the problem isn't hiding there.

Regards, Peter

[Dputiger]

Yah--it's an ALi Aladdin IV chipset. That table that I posted up above came off of my motherboard information sheet from IBM and so it looks to me like the PCI bus speeds are 'locked' at either 30 or 33 Mhz.

I guess what I'm trying to figure out is this:

Is my machine unstable at 75 Mhz because of voltage, or because the PCI slots aren't running fast enough to keep up with a 75 Mhz FSB? I know this board was designed to take a Cyrix chip which DID run at 83 Mhz, so I'm tempted to bump the FSB up to there and see what happens.

How do you back down DRAM speed? THe only way I have to do that is to use my Shareware version of TweakBIOS.

BTW: wtp and Bruno have already weighed in on this one in the "for wtp" post.

[Spencer]

What kind of RAM are you using? PC66/PC100/PC133? If your using PC66, that could be your problem. You're overclocking you're RAM along with everything else. Try getting some PC100 RAM and see if that helps.

[Peter M]

Aladdin IV does run perfectly fine at 75 and 83. (Typing this on a PC-Chips M560 Aladdin IV mainboard at 75 MHz :-)

The speed difference from CPU bus to PCI bus is not important for stability - the ratio is much higher with today's 100 and 133 MHz CPU bus mainboards where PCI bus is still at 33.

So if you can improve stability by better cooling, you got the culprit already - it's the CPU. If that's something that wasn't designed for 75 or 83 MHz bus, then it might very well run into signal timing problems on the CPU bus.

Regards, Peter

[Dputiger]

Ahhh...so you think the CPU has been the problem all along?

My RAM is not the problem--at least not at 75 Mhz. If I take all PCI and ISA's out, the machine does NOT crash. That's why I thought it could be the PCI slots...I don't understand why its the CPU.