Resolved Can a CPU be run at a lower fsb?

My motherboard (GA-945P-DS3/S3) has a fsb of 1066/800 but can be overclocked with software supplied by manufacturer (Gigabyte) to 1333 BUT it states that if this is done then you must only use DDR2 533 and I have DDR2 667 (so I would have to change the ram if I overclock).

I am looking at upgrading the Core 2 Duo CPU to a faster one.

What happens if I try and run a CPU (on the supported CPU list for the motherboard) which has a fsb of 1333 WITHOUT overclocking the motherboard ie running a CPU with a fsb of 1333 at 1066?

Apart from running slower can any damage be done?

Lastly will a CPU with a fsb of 1333 but running at fsb 1066 give a (benchmark) speed as a CPU OF THE SAME SPEED (eg 2.8Ghz) which has a fsb of 1066?

 

You can safely run a CPU at lower FSB, but why would you ?

 

But why would you?

Well I would because -
1 The motherboard's 'native' fsb is 1066. 1333 only by overclocking - can't this put strain on other components?
2 I would need to upgrade the bios - there is always the chance that the upgrade will go wrong
3 I would have to replace my memory (from 667 to 533) - stretches my budget.

 

All the 3 things indicate a mismatch between your CPU & board. If a supported CPU is put on a board, the board should auto configure everything for that CPU.

 

The motherboard supported cpu's state (for CPUs with a fsb 1333
-
CPU Model Frequency L2 Cache FSB Since BIOS Version

Core™ 2 Duo E8600 3.33GHz 6MB 1333 F6i (by Overclocking)
Core™ 2 Duo E8500 3.16GHz 6MB 1333 F6i (by Overclocking)
Core™ 2 Duo E8400 3.00GHz 6MB 1333 F6i (by Overclocking)

The user manual states

Front Side Bus Supports 1066/800/533 MHz FSB (notice 1333 not mentioned)

also

The BIOS SETUP states in the page titled-

MB Intelligent Tweaker(M.I.T.)

O.C. FSB1333 Core. 2 CPU
Enable use of a CoreTM 2 CPU with 1333 MHz FSB through overclocking. To enable this feature,
you must install DDRII 533 memory module(s) and a CoreTM 2 CPU.
Disabled - Disable the use of a CoreTM 2 CPU (1333 MHz FSB). (Default value)
Enabled - Enable the use of a CoreTM 2 CPU (1333 MHz FSB).

Any comments?

Will a E8400 CPU (fsb 1333) be recognised by the motherboard if it is at fsb 1066 ie NOT overclocked to match the CPU - THIS IS AN IMPORTANT QUESTION

Would a E8400 CPU (Core 2 Duo 3Ghz 6Mb L2 cache 1333fsb) running at fsb 1066 be NOTICEABLY faster than say a E7400 (Core 2 Duo 2.8Ghz 3MB L2 cache 1066fsb) running at fsb 1066?

 

I have seen this often done in HTPCs - PCs integrated into home theater systems where the PCs are configured for totally passive (no fans) cooling. When you spend $10,000 plus for a set of speaker, the last thing you want to hear is your HTPC fans humming. So these systems are often underclocked to keep them running cool.

 

I may be missing something here but surely where jparnold reads must install 533 memory modules it simply means a minimum of 533 and 667 would simply slow down to the system requirements?

I may be wrong but it does seem odd that you'd have to replace higher spec RAM with lower when you are overclocking.

 

Well I didn't make that up it is written in the manual.

MB Intelligent Tweaker(M.I.T.)

O.C. FSB1333 Core. 2 CPU
Enable use of a CoreTM 2 CPU with 1333 MHz FSB through overclocking. To enable this feature,
you must install DDRII 533 memory module(s) and a CoreTM 2 CPU.
Disabled - Disable the use of a CoreTM 2 CPU (1333 MHz FSB). (Default value)
Enabled - Enable the use of a CoreTM 2 CPU (1333 MHz FSB).


I still have NOT got my question answered "can a CPU run correctly at a lower fsb than the CPU can run at "
Or doesn't anyone know the answer?
All I'm getting is "why would you want to do that" etc.

I am thinking that the fsb speed published is the FASTEST fsb the CPU can run at and therefore it can run at a lower fsb - a bit like installing DDR2 800 ram onto a motherboard which can only handle DDR2 667 ram. AM I CORRECT?

Does anyone know the answer to my question?

 

Why don't you try it & give us the answer ?

I think I answered it in the first post itself.

 

Thanks

 

And I answered it too when I said this is often done in HTPCs.

 

Thanks again (Bill)

I have just received a reply (from a technician) to a message posted to GIGABYTE's web site which stated that with the E8400 it CANNOT BE USED WITHOUT OVERCLOCKING this motherboard (increasing the fsb to 1333) and reflashing the BIOS to version F6i

So (in this particular case - for the E8400) we are ALL wrong - interesting?

Reading the supported CPU list I have now noted that the E6850 (also 3Ghz and also fsb 1333) CAN be used BUT only with the motherboard fsb of 1066!

Hopefully this puts to bed this question and also what everyone thought! BE CAREFUL

So I am now looking for a E6850.