Desktop Pentium MMX has 2.8 voltage and that's it.

I don’t think a single cpu has ever been harmed by running too slow.

Try it at 100mhz next time 😉

rmay635703 wrote:

I don’t think a single cpu has ever been harmed by running too slow.

Try it at 100mhz next time 😉

I would be surprised if it would harm it, but it cost nothing to ask. My main concern really is about the stability.

Maybe I was not clear when I mentionned I did try the 1.5x ratio and would expect it to run at 100mhz. But then the bios tell me the cpu is back at 166mhz. I should run some benchmark too see if it's really at that speed.

Reading the jumper setup of this mobo did not allow me to understand how to set bus speed at 60 or 66 mhz. And I realized some undocumented jumper setup also give 133mhz speed. It's all those messed up configuration that made me worried about stability of the system at non standard rating.

rmay635703 wrote:

I would be surprised if it would harm it, but it cost nothing to ask. My main concern really is about the stability.

Actually, there are CPUs that can not run slower than what they were made for, the Z80 for example. So the question was totally legit.

Well thank you guys for your input. I've put back the cpu at 166mhz with correct settings and the computer seem more stable, anyway it did not crash of the evening. 😀

Maybe this oddball setting was responsible for the erratic behavior of that computer. And it would explain why running a p55c ppga below 166mhz is not a popular topic.

ElBrunzy wrote:

Reading the jumper setup of this mobo did not allow me to understand how to set bus speed at 60 or 66 mhz. And I realized some undocumented jumper setup also give 133mhz speed. It's all those messed up configuration that made me worried about stability of the system at non standard rating.

From the "Intel" section, I would say, that JP12 should have 1 & 2 closed (or a jumper between them?) for 60 MHz operation ..

H3nrik V! wrote:

From the "Intel" section, I would say, that JP12 should have 1 & 2 closed (or a jumper between them?) for 60 MHz operation ..

You are right H3nrik V! As soon as I short the jp12, the cpu went to 150mhz (how could I've missed that! 😊 ). I can now bring the computer down to 120mhz with 2.0x ratio. But again at 1.5x the cpu goes back to 150mhz.

I could confirm that this is true 150mhz with pctools System Info 9.0 as it score the same with 1.5x or 2.5x clock ratio. SI.EXE score unit is relative to IBM XT (IBM XT = 1) so at 150mhz it scored 175xt, at 120mhz it's 140xt and I could confirm the 1.1666 slope putting back the jp12 and benching 156xt at 133mhz.

I guess it's no problem playing with the bus speed as the video cards is pci and not agp. But still I think I will try to find something complex for my computer to do as I'm now worried about the system stability. I will have to find a dos or win98 stress test tool for that computer as I dont know any.

ElBrunzy wrote:

rmay635703 wrote:

I don’t think a single cpu has ever been harmed by running too slow.

Try it at 100mhz next time 😉

I would be surprised if it would harm it, but it cost nothing to ask. My main concern really is about the stability.

Maybe I was not clear when I mentionned I did try the 1.5x ratio and would expect it to run at 100mhz. But then the bios tell me the cpu is back at 166mhz. I should run some benchmark too see if it's really at that speed.

The P55C remaps 1.5x to 3.5x, the setting for 233MHz on a P55C is exactly the same as for 100MHz on a P54C. So with a P55C you can't get 100MHz that way. If you didn't get 233MHz it means some documentation is wrong (or the CPU is multiplier locked and not a P233MMX in the first place...).

Reading the jumper setup of this mobo did not allow me to understand how to set bus speed at 60 or 66 mhz. And I realized some undocumented jumper setup also give 133mhz speed. It's all those messed up configuration that made me worried about stability of the system at non standard rating.

You're thinking too complicated. The CPU just runs at a given multiple of the FSB it's fed with. It's rated to be stable up to at least the official speed, but under that it just runs cooler (as you already discovered running at 166MHz) and slower with zero negative performance impact.

Oddball FSB speeds can impact stability because they also mess with PCI and possibly ISA clocks, but 66MHz FSB is completely standard and the rest of the system is doing exactly the same as when the CPU is running at 166MHz or 233MHz.

dionb wrote:

The P55C remaps 1.5x to 3.5x, the setting for 233MHz on a P55C is exactly the same as for 100MHz on a P54C. So with a P55C you can't get 100MHz that way. If you didn't get 233MHz it means some documentation is wrong (or the CPU is multiplier locked and not a P233MMX in the first place...).

I guess you are right and it's really a 166mhz, I was wrong to assume I have a 233mhz in there. CPU-Z dont give me enough information but the revision mxa3 lead me to a 166mhz and that might explain that mysterious free p1-233mmx in my inventory 🤣 . I dont want to remove the cpu just for identification sake as I believe all ppga p55c will run at same level of warmth so I dont pay too much attention where they are. Am I correct to say that a 233mhz running at 120mhz will dissipate the same amount of heat than a 166mhz running at 120mhz ?

Thanks for the 1.5x ratio explanation, It puzzled me. Do you think it can be problematic to run the bus at 60mhz now ? This motherboard is quite crowded with pci and isa, but I fail to see how a standard bus value would not supported.

Set the FSB to 50mhz and 2x and see what happens.

ElBrunzy wrote:

dionb wrote:

The P55C remaps 1.5x to 3.5x, the setting for 233MHz on a P55C is exactly the same as for 100MHz on a P54C. So with a P55C you can't get 100MHz that way. If you didn't get 233MHz it means some documentation is wrong (or the CPU is multiplier locked and not a P233MMX in the first place...).

I guess you are right and it's really a 166mhz, I was wrong to assume I have a 233mhz in there. CPU-Z dont give me enough information but the revision mxa3 lead me to a 166mhz and that might explain that mysterious free p1-233mmx in my inventory 🤣 . I dont want to remove the cpu just for identification sake as I believe all ppga p55c will run at same level of warmth so I dont pay too much attention where they are. Am I correct to say that a 233mhz running at 120mhz will dissipate the same amount of heat than a 166mhz running at 120mhz ?

Thanks for the 1.5x ratio explanation, It puzzled me. Do you think it can be problematic to run the bus at 60mhz now ? This motherboard is quite crowded with pci and isa, but I fail to see how a standard bus value would not supported.

It'll be fine. Running my P233MMX build in everything between 100MHz (2x50) and 233MHz (3.5x66) at default core voltage, there's no instability whatsoever.

Only been playing around a little bit with undervolting the abovementioned build, but I seem to remember it booting and running stable at 2.0V underclocked to 166 MHz.

And yes, if we ignore small variations between individual samples, a 233MHz and a 166MHz P55C will dissipate the same amount of heat when running at the same frequency and voltage.

Thank you all guys for the information, I now sleep on both ears having that cpu running at 120mhz. You answered a lot of questions I had and removed worries.

Kaputnik : we both have a similar setup (and post #, soon we will be oldbies), you seem to have a very flexible motherboard, what's the model ?

I have another p55c ppga in my basement that I use on winter and I'm eager to downclock it with my new acquired knowledge.

Actually, 133MHz P55Cs were made in PPGA socket. But they are actually mobile parts. Whether or not it will work in desktop boards, I wouldn't know ..
The SSPEC SL27C, rated at 2.45 Volts: http://www.cpu-world.com/sspec/SL/SL27C.html

ElBrunzy wrote:

Thank you all guys for the information, I now sleep on both ears having that cpu running at 120mhz. You answered a lot of questions I had and removed worries.

Kaputnik : we both have a similar setup (and post #, soon we will be oldbies), you seem to have a very flexible motherboard, what's the model ?

I have another p55c ppga in my basement that I use on winter and I'm eager to downclock it with my new acquired knowledge.

It's nothing special, just a Gigabyte GA586HX, slightly modded with an extra pinheader for drawing 3.3V from the I/O plane, to enable the 3.0 and 3.5x multipliers with P55C.

Also got the FSB and multiplier jumper headers wired up to four SPDT switches mounted in one of the 5.25" bay covers, for easy manipulation without opening the case and fiddling around with jumpers 😀

FYI, I also get about 5% change in performance by tweaking memory settings up and down.

I have runned a passive cooled pentium 200 in 75Mhz. So absolutely no problem.

kaputnik wrote:

Also got the FSB and multiplier jumper headers wired up to four SPDT switches mounted in one of the 5.25" bay covers, for easy manipulation without opening the case and fiddling around with jumpers 😀

Have you given on the temptation of trying to change the cpu speed on the fly ? I guess it would react like those infamous turbo switch that worked fine on 486 but would only hang the cpu on pentium.

ElBrunzy wrote:

kaputnik wrote:

Also got the FSB and multiplier jumper headers wired up to four SPDT switches mounted in one of the 5.25" bay covers, for easy manipulation without opening the case and fiddling around with jumpers 😀

Have you given on the temptation of trying to change the cpu speed on the fly ? I guess it would react like those infamous turbo switch that worked fine on 486 but would only hang the cpu on pentium.

Done it by mistake a few times. Nothing happens at all 😀

The PLLs on the clock chip get programmed on startup. They don't change until reboot, so changing the switches should do nothing.