FSB 133 CPU on FSB 100 mobo?

  • Thread starter Thread starter Norm Dresner
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Norm Dresner

Disregarding the decrease in speed and any cost considerations for a moment,
is there any technical reason why a CPU that has a FSB designation of 133
MHz wouldn't run on a 100 MHz FSB motherboard?

Background: For the curious, these are not normal motherboards but single
board computer cards that plug into passive ISA or PICMG backplanes. In the
industrial embedded world, speed of the CPU is often not the most important
factor but rather having something that's appropriate to the task. I have a
few SBC cards that are limited to P3 CPU's up to either 600 MHz or 800 MHz,
depending on the particular model and in the current market, it's actually
easier to locate 133 MHz FSB CPUs than 100 MHz ones.

I assume that the effective speed of a 133 MHz FSB CPU run on a 100 MHz FSB
card would decrease by 25% (100/133) so that an 800/133 MHz CPU would appear
as a 600/100 MHz CPU. Is this correct?

TIA
Norm
 
Disregarding the decrease in speed and any cost considerations for a moment,
is there any technical reason why a CPU that has a FSB designation of 133
MHz wouldn't run on a 100 MHz FSB motherboard?
Click to expand...

There shouldn't be, though odd-ball BIOS compatibilities issues or
voltage regulator issues could potentially crop up.
Background: For the curious, these are not normal motherboards but single
board computer cards that plug into passive ISA or PICMG backplanes. In the
industrial embedded world, speed of the CPU is often not the most important
factor but rather having something that's appropriate to the task. I have a
few SBC cards that are limited to P3 CPU's up to either 600 MHz or 800 MHz,
depending on the particular model and in the current market, it's actually
easier to locate 133 MHz FSB CPUs than 100 MHz ones.

I assume that the effective speed of a 133 MHz FSB CPU run on a 100 MHz FSB
card would decrease by 25% (100/133) so that an 800/133 MHz CPU would appear
as a 600/100 MHz CPU. Is this correct?
Click to expand...

That is correct. Again, it should work just fine, the CPU should just
be recognized as a 600MHz processor, but incompatibilities are always
a potential issue. I'd test it out with one chip first and see how
that goes before buying a whole batch. I would guess you've got at
least 90% chance of success, but it's definitely not a sure-thing.
 
Tony Hill said:
There shouldn't be, though odd-ball BIOS compatibilities issues or
voltage regulator issues could potentially crop up.


That is correct. Again, it should work just fine, the CPU should just
be recognized as a 600MHz processor, but incompatibilities are always
a potential issue. I'd test it out with one chip first and see how
that goes before buying a whole batch. I would guess you've got at
least 90% chance of success, but it's definitely not a sure-thing.
Click to expand...

Since right now the cost of older FSB 133MHz CPUs is running about 50% of
the cost of 100MHz ones [when they can be found], I think that 90% is a
worthwhile gamble.

Thanks
Norm
 
Norm Dresner said:
Disregarding the decrease in speed and any cost considerations for a
moment,
is there any technical reason why a CPU that has a FSB designation of 133
MHz wouldn't run on a 100 MHz FSB motherboard?

Background: For the curious, these are not normal motherboards but single
board computer cards that plug into passive ISA or PICMG backplanes. In
the
industrial embedded world, speed of the CPU is often not the most
important
factor but rather having something that's appropriate to the task. I have
a
few SBC cards that are limited to P3 CPU's up to either 600 MHz or 800
MHz,
depending on the particular model and in the current market, it's actually
easier to locate 133 MHz FSB CPUs than 100 MHz ones.

I assume that the effective speed of a 133 MHz FSB CPU run on a 100 MHz
FSB
card would decrease by 25% (100/133) so that an 800/133 MHz CPU would
appear
as a 600/100 MHz CPU. Is this correct?

TIA
Norm
Click to expand...

Technical reasons, no, none at all. This of course would be in a normal
situation, which seems to not be the case here.

Nonetheless, if the device, which I assume has a BIOS and VRMs, can handle
the voltage on the CPU (which should be the same as for the 100MHz variants,
assuming you meant 100MHz bus Coppermines) and can recognize a Coppermine,
then there isn't any reason it shouldn't work.

To be more literal, if you buy an 800MHz (133x6) CPU, what you're buying is
really just a Coppermine P3 that has a 6x multiplier built-in, and has been
tested to run at 800MHz successfully. Whether you run it at 800MHz, 600MHz,
or 400MHz (66Mhz bus), doesn't matter to it; it is merely designed to use a
6x multiplier, and that is all it cares about.

If these 600MHz things are sufficient enough, you can consider other things,
depending on the physical location of them, such as quieter fans. If this
is some sort of server room, which I am suspecting, then likely it is
uninhabited by people often, and noise would not be a factor.
 
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