Underclocking?

[Norm Dresner]

It seems that, at least in some markets, 800 MZh P-3 CPU's are cheaper and
more readily available than 700 MHz ones. Although everything I know about
physics and electronics says that there's probably no real danger in
underclocking an 800 to 700, I thought I'd ask. Is it okay?

TIA
Norm

 

[Bob Knowlden]

Are you aware that PIIIs have locked multipliers?

There were two types of PIII 800 CPUs that I know of: 100 MHz FSB (front
side bus) with an 8X multiplier, and 133 MHz FSB (6X). The latter were
usually listed with a "B" suffix.

I'd expect a mainboard designed for a 700 also to be able to use an 800 -
they're both "Coppermine" CPUs. It might not be capable of the 133 MHz FSB,
though, so you might want the correct (non-B) 800.

Good luck.

Bob Knowlden

Address may be altered to avoid spam. Replace nkbob with bobkn.

 

[Norm Dresner]

Alas, it's not a motherboard but a Single Board Computer (ISA bus). And it
says 700 MHz which, I assume, means that the chip-set isn't qualified for
800 MHz.

Does the "Locked Multiplier" mean that it's the FSB that drives the CPU
frequency or that the CPU frequency is used to derive the FSB?

Thanks
Norm

 

[Tony Hill]

Alas, it's not a motherboard but a Single Board Computer (ISA bus). And it
says 700 MHz which, I assume, means that the chip-set isn't qualified for
800 MHz.

Does the "Locked Multiplier" mean that it's the FSB that drives the CPU
frequency or that the CPU frequency is used to derive the FSB?

Bus speed is used to drive the CPU frequency. Inside the CPU there is
a clock multiplier that clocks the internals of the chip at some
multiple of the bus speed. In almost all modern processors that
multiplier is locked at the factory with no possible way of unlocking
it (this is mainly done to prevent remarking, ie someone taking a
700MHz processor and rebadging it as an 800MHz processor).

 

[RusH]

Alas, it's not a motherboard but a Single Board Computer (ISA
bus). And it says 700 MHz which, I assume, means that the
chip-set isn't qualified for 800 MHz.

Chipset by itselfe has nothing to do with processor frequency. All it
cares about is I/O and FSB.

Pozdrawiam.

 

[Norm Dresner]

Bus speed is used to drive the CPU frequency. Inside the CPU there is
a clock multiplier that clocks the internals of the chip at some
multiple of the bus speed. In almost all modern processors that
multiplier is locked at the factory with no possible way of unlocking
it (this is mainly done to prevent remarking, ie someone taking a
700MHz processor and rebadging it as an 800MHz processor).

AHA! So if I set the board for 100 MHz FSB and put an 800 MHz CPU on
it it would run at 800 MHz. The lower FSB is 66 MHz which would get me a
tad over 500 MHz for the processor. Not what I'd ideally want, but it sure
beats the Socket 7 board with the 233 Mhz MMX I have running in the box now.
It would seem then that the proper course of action is pay the extra
money to get a real 700 MHz CPU which I can run with a 100 MHz FSB.

Thanks for the education.

Norm

 

[Norm Dresner]

Chipset by itselfe has nothing to do with processor frequency. All it
cares about is I/O and FSB.

Okay. That was a brain cramp.

It's been pointed out to me in a private e-mail that an 800 MHz CPU
would draw more current than a 700 MHz and perhaps overload on-board voltage
regulators.

Thanks for the correction.

Norm

 

[Anonymous Joe]

And

AHA! So if I set the board for 100 MHz FSB and put an 800 MHz CPU on
it it would run at 800 MHz. The lower FSB is 66 MHz which would get me a
tad over 500 MHz for the processor. Not what I'd ideally want, but it sure
beats the Socket 7 board with the 233 Mhz MMX I have running in the box now.
It would seem then that the proper course of action is pay the extra
money to get a real 700 MHz CPU which I can run with a 100 MHz FSB.

Thanks for the education.

Norm

The thing you have to be careful about is not just 1 thing, there are many
things involved in upgrading to a P3.

First, yes, the multipliers are locked, so it doesn't matter what the manual
says it supports, as long as it supports the proper bus speed.

Secondly, the board must be able to run a Coppermine CPU. You won't get a
board to run a Tualatin unless it was specifically made to do so, or unless
you are using a Poweleap adapter (which has its own set of power regulators,
AFAIK).

Third, the power draw must be able to be withstood by the board. If an 800
takes 70W, and your board can only do 65W, you're out of luck. (These
numbers are completly ficticious, BTW).

So, really, as long as your BIOS supports Coppermine, has the 100MHz bus,
can do the voltage of the CPU, and can give it enough current to support it,
then you shouldn't have any problems with an 800MHz.

Underclocking would be strange to do on an Intel, as you'd end up running an
87.5MHz bus, which is likely to throw off your AGP, and PCI speeds, thereby
making it even more risky.

At the very least, it's a good thing you did ask and not go it alone.

 

[RusH]

It's been pointed out to me in a private e-mail that an 800
MHz CPU
would draw more current than a 700 MHz and perhaps overload
on-board voltage regulators.

1-3(hmm maybe 5) Wats more wont do you any harm


Pozdrawiam.

 

[Nate Edel]

So, really, as long as your BIOS supports Coppermine, has the 100MHz bus,
can do the voltage of the CPU, and can give it enough current to support it,
then you shouldn't have any problems with an 800MHz.

It might very well work even without explicit BIOS support for Coppermine;
I've done that (P3/800-100fsb, very early revision ABIT BH6), and although
the BIOS-reported speed was wrong, it otherwise worked wonderfully, and the
processor ran at the right speed.

 

[Nate Edel]

Okay. That was a brain cramp.
It's been pointed out to me in a private e-mail that an 800 MHz CPU
would draw more current than a 700 MHz and perhaps overload on-board voltage
regulators.

It's possible.

It's also possible that 700mhz was simply the fastest processor available
when they validated the board, and that the 800mhz will work fine. There's
very little way to be sure.