P4: reduce FSB from 133 to 100

[E.d.g.a.r]

Hi everybody!

I want to reduce FSB of my Pentium 4 Northwood 2.66 GHz CPU from 133 MHz to 100 MHz (running with 133 MHz at default).

The Intel datasheet describes the BSEL[0:1] pins are responsible for the system bus speed.

The BSEL-Table describes how to get 100, 133 or 200 MHz FSB:
BSEL1 = L, BSEL0 = L: FSB: 100 MHz
BSEL1 = L, BSEL0 = H: FSB: 133 MHz
BSEL1 = H, BSEL0 = L: FSB: 200 MHz
BSEL1 = H, BSEL0 = H: FSB: reserved

So I wonder if it is possible to shorten the two pins to get 100 MHz instead of 133!

I read the article from Matt Watras where he describes how to overclocked intel Xeon CPUs from 100 to 133 MHz FSB. Maybe he can help.

Greetings from Germany,
Ed

 

[NiTrO bOiE]

One question, why would you want to do a mod that reduces the CPU speed?

 

[E.d.g.a.r]

Hi again.

The Mainboard i am using (Gigabyte GA-8IEXP v1.2) can support CPUs with 100 MHz and 133 MHz FSB.

The problem: When using a 100 MHz CPU, I can select a maximum multiplier of 2.66 for memory speed in relation to FSB. That gives a speed for the Memory of 266 MHz.
But: When using a 133 MHz CPU the maximum multiplier in the BIOS is 2.0. So I can reach a memory speed of 266 MHz at FSB 133.

I have Kingston HyperX 3200 RAM (DDR 400, 2-6-2-2) and I want to get the memory some faster than 266 MHz.

The CPU can be overclocked to 150 MHz FSB without problems. (Resulting in 20*150 MHz = 3.0 GHz)

Raising the FSB with the 133 MHz CPU to 150 MHz I can reach a memory speed of 2.0 * 150 MHz = 300 MHz!

Raising the FSB with the 100 MHz CPU to 150 MHz I can reach a memory speed of 2.66 * 150 MHz = 400 MHz!

So: with the 100 MHz CPU overclocked to 150 MHz I get 100 MHz more memory speed!

That's all for now,
Timo

 

[jdmcnudgent]

just set the fsb to 100 in the bios.

 

[E.d.g.a.r]

Hi again.

just set the fsb to 100 in the bios.

Of course I tried this.

It doesn't matter what FSB I select in the BIOS. The Mainboard recognizes the 133 MHz CPU and because of this the RAM-multipliers maximum is 2.0.

Thanks for your reply.

Greetings,
Timo

 

[Kingdom]

first of all, if u did make ur cpu a 100fsb chip, then you wouldnt be able to reach 150fsb, look at it this way.

Would u be able to overclock ur system with 50fsb as it is now ? thats some 183fsb, ur saying no, and im guessing its the processor thats holding u back, so if u had it running 2.66ghz at 100fsb wich sounds impossible, cus u would probably make it a slower processor alltogether then, well then u'd have to overclock it with 50fsb to get what you want.

Insted you should find the weak link, probably buy a mother board with better memory multipliers or so.

 

[Albuquerque]

You guys are obviously not understanding what he is saying: his motherboard dictates the maximum memory ratio depending on the hardwired FSB indicator pins on the chip. If the motherboard sees a 100FSB chip installed, it gives him the option to go for a 2.66x memory ratio... If it sees a 133FSB, it only gives him the option for a 2.0x memory ratio.

His 2.66ghz processor, unmodded will do 3ghz on a 150FSB. His maximum memory speed would then be 300mhz DDR.

His 2.66ghz processor, modded so that it reads as a 100mhz FSB, would STILL DO 3ghz on a 150FSB. Remember, Intel locks the multiplier, so it's still 20x whatever your FSB is. But since his motherboard reads it as a 100mhz FSB processor, his maximum memory speed (thanks to the higher available ratios) is now 400mhz DDR.

 

[E.d.g.a.r]

Hi again!

First thing to say: It's working!

I tried to tie the two BSEL pins together using a very small piece of a very fine wire. Bue either my fingers are to thick or I am to fidgety to do this.
After half an hour I stopped the frickling work and inserted a piece of this wire into the corresponding holes on the mainboards cpu socket.
After knocking the CPU in the socket and turnung on power the BIOS reports a P4 2.0A CPU at 2.0 GHz!

Whoa!

And now, as Albuquerque mentioned above I can use the 2.66 multiplier! Overclocking to 150 MHz FSB works very well at 400 MHz DDR-Speed!!!

Thanks guys!
Lets see how far the CPU can go....
Greetings,
Ed

 

[DJ Switch]

props on the intelligence man . . . very good observation!!

Kyle

 

[FIZZ3]

This trick would also work on 845PE boards, wouldn't it? Don't have one here myself to look at it, but the Gigabyte 8pe800u manual shows similar ratios for 100 vs 133Mhz BSEL CPU's.

http://europe.giga-byte.com/MotherBoard/FileList/Manual/manual_8pe800u_e.pdf

Then, an 845PE board, modded for BSEL selection in the same way, could run RAM at 178DDR (356Mhz) @ 133Mhz FSB, providing more bandwidth to approach the 533Mhz FSB than 166DDR does.
This is simply a way of using the FSB:RAM setting of 3:4 (meant for 100Mhz FSB & 133Mhz DDR) at 133Mhz bus speeds, instead of the default 4:5 setting (133:166).