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Clock Multiplier

Posted on 1999-01-10
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Last Modified: 2008-02-26
Hi all.

I've just bought a TX 2 Pro Main board and an IBM 686mx PR300 CPU to upgrade an old Olivetti P66.

I am trying to set the board up so that I can get as close to 300MHz on the CPU as possible, however IBM states that the fastest clock multiplier that I can use with this CPU is X 3.5. This means that I would have to use an external clock of 83MHz.

Now I only have 72 pin SIMMS and I was told that they would'nt be able to cope with a bus speed of 83MHz. I was also told it would be better to set the external clock speed at 66MHz with a clock multiplier of X 4.5, however this exceeds what IBM have stated.

Can anyone give me some good advice here.

Thanks in advance

Suzi
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Question by:SuziW
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by:jhance
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The IBM/Cyrix chips don't have the 4.5 x multiplier like the AMD K6 and Intel Pentium MMX chips so you can't do what you want.  I'd go ahead and try the 83MHz bus configuration, however.  Chances are good that the RAMS you have will work at 83MHz.  Most will.  Failing that, replace the Cyrix chip with an AMD K6-2 and you'll have a lot more clock multiplier options.
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by:SuziW
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Thanks for your answer, however I am rejecting it at this stage because I don't want the question locked out so soon. This allows me to get some feedback from other experts.

If it remains the best answer then I will ask you to resubmit it.
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by:netmage
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If you over-clock, you either get lucky or an unreliable computer.

You have a better chance of getting good reliablity with an overclocked CPU by installing pc-100 ram.

I don't think that SIMM modules will work too well above 66Mhz either.

I've used different successful configurations of clock speeds on cyrix(IBM) cpu's with different motherboards for fun but it depends on the quality of your m/board and ram whether it works or not.

Try the 4.5X option and be careful that the voltages don't change to the config of another type of CPU.

If it works, good luck.

Netmage
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by:SuziW
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Hi Netmage,

As far as I'm aware, I am not trying to overclock the CPU. If it is rated for 300MHz then it should run at this speed without overheating.

Jhance has already stated that the CPU will not accept a clock multiplier of X4.5 but the ram should run at 83MHz.

However you have said that I can try a X4.5 multiplier and that the Ram won't run at a bus speed of 83MHz which is the opposite.

So what do I do. If I run the CPU at X4.5 with a bus speed of 66MHz, will it damage the CPU. Or if I run the CPU at X3.5 with a bus speed of 83MHz will it damage the RAM. What is my best option.

If someone say's run the CPU at X3.5 with a bus speed of 66MHz then that is a solution that I have already figured out, but it will not get the best performance from my CPU.

I can't afford to damage any components on my main board, and I certainly can't afford to buy another processor that accepts X4.5 clock multiplier. I just wan't the minimum risk solution that will get the best performance from my CPU rated at 300MHz.
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by:harrys
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SuziW,
the PR300 in the processors name is its performance rating and not the maximum clockrate support by the CPU, it just means that cryrix rates the CPU to be as fast as an (nonexistant) pentium 300.
If it is a real 686mx-PR300 like you state above it will run at 266MHz (66x3.5, 75x3.5 or 83x3) 75x3.5 would be the best option if your RAM is fast enought and your motherboard is able to clock the PCI bus asyncronly with 33MHz when using Boardspeeds over 66MHz (check your manual for details). Otherwise use 66x3.5, higher multipliers are not possible since the cpu doesnt support BF2...
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by:rmarotta
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SuziW,
I'd be surprised if the CPU will work on the P-66 motherboard.
Have you run the new CPU yet?
Regards,
Ralph
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by:jcarlo
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What speed is the CPU suppesed to be set to?  I think it's supposed to be a lower setting than 300, like 233 or 266, as harrys mentioned, and IBM/Cyrix claims it's as fast as a pentium 300 (whatever that is).  Whatever speed IBM ways to use, you should use.  I have heard of many dead Cyrix/IBM chips from overclocking - those chips are basically nothing but space heaters, which are manufactured and then marked to the absolute highest rating they can stand up to.  About your memory...  If it is new EDO there should not be much of a problem getting it to 75 and probably also 83 MHz.  If it is old memory (from the P66) it might do 75, but 83 is doubtful.  If you cannot run the memory reliably at 83, you should buy SDRAM DIMMS (preferably the PC100 type, although PC66 should work just fine at 83).  Just a note for the future, buy an AMD K6-2 if you want to go the Socket 7 route.  They have fewer problems like this and cost about the same.

regards,
Jeremy
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by:rmarotta
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P-66 Socket 7 ???????
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by:SuziW
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Ralph,

Thanks for cheering me up. I'd be pretty suprised aswell if the CPU ran on a P66 main board. I stated in my original question that I bought a TX 2 Pro board along with the CPU to upgrade my old Olivetti P66 (computer).

Harrys, you are right. I have finally tracked down a link to the Main Board manufacturer, and they state that for the CPU in question, I should set 66MHz and X3.5 . The PR300 is a performance comparison number, and not the actual clockspeed. But why do they do that. I went to the IBM site and they gave all the specs for the CPU including core voltage of 2.9v and I/O voltage of 3.3v and everything else, but they never gave the speed that it should be clocked at. If I had Dimms instead of Simms, then I would have clocked it at 83MHz X3.5 and probably have destroyed the CPU.

Thank you everyone for your help. If Harrys could post his comment as an answer, then I will award the points.
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by:jhance
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ftp://www.national.com/cyrix/6x86mx/docs/6xmx-ch3.pdf

Here is the reference to the Cyrix technical document which details the clock multipliers available to this part.

If you look at page 3, you will see that 2, 2.5, 3, and 3.5 are the only ones available.

You should try your RAM at 83MHz and see how it works out.  If the system is unreliable, then you can fall back to 75MHz.  If all else fails, you can get PC-100 RAM and run it a 83MHz.  You can also try overclocking the CPU and try for 350MHz (100MHz x 3.5).
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by:rmarotta
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SuziW.
Sorry, I guess my mind went foggy when I read "P-66"............
Ralph
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by:SuziW
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Ralph, no apologies needed. As I said, you cheered me up. My late husband left me a legacy of 4 PC's wired up throughout the house, and I've been trying to figure out how they work eversince, just so that the kids dont think that I'm dumb when they ask all sorts of questions.

jhance, someone has already mentioned that overclocking IBM CPU's will only end in tears. I think it might be worth sticking to what the Main Board manufacturers website recommends, even though IBM did'nt give a recommended clock speed for their CPU. What do you think?


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by:swwelsh
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You're right that the Cyrix/IBM PR rating is confusing, but they are just trying to make their chip competitive. This follows the accepted computer industry practice of making up a new standard if your product doesn't measure up to the existing one :) As for all the rumors about Cyrix chips frying, I can tell you this is mostly unfounded. The earliest 686 chips did run very hot, and I did fry one after about 18 months, but it was because of a failed processor fan. Their newer chips run much cooler, but I would suggest that the best thing to do with these or any other chips is to replace the processor fan every six months or so. They are only about 5-10 bucks and are easy to clip on. You can also download some freeware programs (Waterfall, Rain) that will make the chip run cooler - I have had no problems with these cpu cooler programs. Just my $.02
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harrys earned 70 total points
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and the answer is ;-)
the PR300 in the processors name is its performance rating and not the maximum clockrate support by the  CPU, it just means that cryrix rates the CPU to be as fast as an (nonexistant) pentium 300.
If it is a real 686mx-PR300 like you state above it will run at 266MHz (66x3.5, 75x3.5 or 83x3) 75x3.5 would be the best option if your RAM is fast enought and your motherboard is able to clock the PCI bus asyncronly with 33MHz when using Boardspeeds over 66MHz (check your manual for details). Otherwise use 66x3.5, higher multipliers are not possible since the cpu doesnt support BF2...
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by:jhance
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Gosh harrys, you're just right on top of this, aren't you?
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by:harrys
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jhance,
I was asked to post it as an answer, not ment to offend anyone ...
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by:SuziW
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Thank you Harrys
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