FAQ: How do I run the ram at DDR-XXX speed?

[Hans]

There are two ways you get the potential out of that sweet DDR-400 module on your machine, via FSB overclocking or asynchronies (async) mode. Before we begin, it's a good idea to check out the potential of your module here (http://www.overclockers.co.nz/ocnz/misc/ddrguide/1.shtml) . Note: This guide is aimed at AMD DDR platforms, however the same concept applies to other platforms as well.

Let us have a look at the easier way, asynchronies mode. In this mode, your DRAM and FSB are running at different speed. The DRAM clock is usually faster than FSB. You can do it other way round but what is the point?

The DRAM clock is controlled via the DRAM multiplier. Not all of the AMD DDR chipsets can handle DRAM multipliers though. Thus, I've listed the possible modes for the popular chipset below.

[Hans]

Nvidia

nForce2 SPP/IGP

The nF2 chipset is capable of running memory at 2 times the FSB speed, i.e. 2*FSB.


VIA

KT266/KT266A:

FSB: 100 MHz
Possible memory speed: 100 or 133 MHz.

FSB > 133 MHz
Possible memory speed: Memory is synchronised to FSB speed (You can run the memory lower than the FSB speed, which I see no point in doing so).

KT333:

FSB: 100 MHz
Possible memory speed: 100, 133 or 166 MHz.

FSB: 133 MHz
Possible memory speed: 133 or 166 MHz.

FSB > 166 MHz
Possible memory speed: Memory is synchronised to FSB speed (You can run the memory lower than the FSB speed, which I see no point in doing so).


KT400:

FSB: 100 MHz
Possible memory speed: 100, 133, 166 or 200 MHz.

FSB: 133 MHz
Possible memory speed: 133, 166, or 200 MHz.

FSB: 166 MHz
Possible memory speed: 166 or 200 MHz. (Some MB will limit the memory speed to 166 MHz when overclocked past 166 MHz mark)

Ali

AliMagic Rev C:

FSB: 100 MHz
Possible memory speed: ??

FSB: 133 MHz
Possible memory speed: 133 or 166 MHz.

FSB > 166 MHz
Possible memory speed: Memory is synchronised to FSB speed (You can run the memory lower than the FSB speed, which I see no point in doing so).


I don't know much about Sis chipset.

As for the AMD chipset, well, it only supports 100(FSB)/133(Memory) mode.

[Hans]

Before you all get crazy and turn up the multiplier, take a note of the following. In AMD's platform, there is virtually no point in running the memory faster than the FSB speed, as the system bus can not provided the bandwidth required. Here is a more in-depth info (http://www.overclockers.com/tips00157/).

Be warned, some boards will be less stable in async mode. It may take quite a few BIOS tweaks and experiments to get the things right. Some boards will NEVER work in async mode. In that case, you will have to resort to FSB overclocking

Below is a demonstration screen shot on how you can change the memory speed. (Abit AT7 MAX2)

[Hans]

Some people might notice there is an option called ¡§SPD¡¨. SPD stands for serial presence detector. (Have a look at the picture below). This chip stores all the memory timing and memory speed data (written by manufacturers). When selected, the motherboard will communicate with SPD to get the ¡§correct setting¡¨ for the respective module. Only use this setting if you are NOT sure the operation characteristics of your module.

[Hans]

What can we do to counter the problem noted above? Well, the quickest way is to overclock the FSB. Hence, we have the following.

In order to get the most out of your expensive ultra fast ram, you need to run them in synchronise mode. (for the AMD platform with EV6 bus) Thus, we will want to raise the FSB as much as possible, in order to achieve the highest memory speed. However, FSB overclocking is more complicated when compared to async mode. You need to worry about unlocking the CPU, PCI & AGP clocking, and motherboard overclockabilty.

Why do we need to unlock the CPU? Well, it is near impossible to run an AMD CPU with an FSB of 180 MHz + and default multiplier. Even the XP-1600+ has a multiplier of 10.5X. If you try to run that CPU at 180 MHz then you are trying to get it run at near XP-2300+ speed. Imagine how often that happens. If we can lower the multiplier to 9 or 8, then the CPU would only be running at 1620 MHz or so. (A much probable case). Use www.google.com to find info on unlocking the CPU. Alternatively, have a look at this site, http://www.ocinside.de/index_e.html.

[Hans]

When the FSB is overclocked, you are overclocking the PCI and AGP's speed as well. We can also include the plain old ISA, USB, etc as well, but I will leave them out for the time being. Now, AGP speed is not that much of concern these days, as a good modern AGP card can operate around 100 MHz mark, the default AGP clock is 66 MHz.

However, it is a different story with PCI bus though. Your network card, if installed, would probably crap out around 43 MHz mark. Your HDD? Well, that purely depends on your luck. It is a well known fact that high PCI speed, > 40 MHz, kills the HDD or worse the data on it. (Time is all it takes). The PCI clock is determined by the PCI divisor which in term is generated via the PLL and the chipset. I have listed below the possible PCI divisor on the popular chipsets.

[Hans]

Nvidia

nForce2:

nForce2 chipset is capable of locking PCI clock at 33MHz and AGP clock at 66MHz, regardless of FSB used. Hence, one can push the FSB as far as the board and ram allow.


VIA

KT266/KT266A:

Possible PCI divisor: 1/3 and 1/4.

KT333:

Possible PCI divisor: 1/3, 1/4 and1/5 (Some boards are limited to 1/4)

KT400:

Possible PCI divisor: 1/3, 1/4 and1/5 (Some boards are limited to 1/4)

Ali

AliMagic Rev C:

Possible PCI divisor: 1/3, 1/4, 1/5 and 1/6.

I don't know much about Sis chipset.

Not too sure about AMD chipset as well but I think it¡¦s limited to 1/4.

Apparently, nForce chipset has a step-less PCI divisor which keep the PCI bus very close the original spec.

I would not recommend going pass 166 MHz on a KT-266/266A motherboard and 200 MHz for a KT-333/400 MB. Up the memory and I/O voltage will be useful at high speed.

[Hans]

Generally speaking:

Nvidia nForce2
For all boards based on Nvidia nF2 chipset, the PCI bus is automatically locked at 33MHz.

Abit: The multiplier adjustments can be found under SoftManual III.

Asus: The multiplier adjustments are automatically set by BIOS.

Albatron: The multiplier adjustments are set via FSB jumpers.

Chaintech: The multiplier adjustments are set via FSB jumpers.

Epox: The multiplier adjustments are set via FSB jumpers. (automatic when FSB > 133 MHz)

Gigabyte: The multiplier adjustments are set via FSB jumpers.

IWill: The multiplier adjustments are set via BIOS (XP333)

Soltek: The multiplier adjustments are set via FSB jumpers.

Soyo: The multiplier adjustments are automatically set by BIOS.

You can work out the current PCI speed by multiplying FSB with the divisor. (why we call it divisor still puzzles me).

For example:
FSB = 180 MHz
PCI divisor = 1/4
PCI speed = 180 * 1/4 = 45 MHz

You can set the PCI divisor via BIOS (generally tweakers' board), FSB jumpers or the board is going to set it for you. Generally speaking, you would want to maintain the PCI bus lower than 40 MHz.

After you had read through all of the above, you are ready to change the multiplier and FSB settings to get the most out of your setup. Remember, lower the multiplier and up the FSB. Below are some BIOS screen shots from Abit AT7-MAX2.

[Hans]

Lower the multiplier

[Hans]

Up the FSB

[Hans]

Set the appropriate PCI divisor

[Grrr!!]

Very useful tutorial ... thanks Hans.

[ozzy]

That was really good, thanks

[the OC shrink]

Hit the link to check out how different makes of ram compare in the FSB rankings


www.theocshrink.co.uk/t5.htm

[Tribalgear]

I'm using a SY-P4X400 dragon and this post worked on my board now my 3200 DDR is running at (400) not 266 like it was. thanks for the Info. :D

[Rabbi]

Nice sticky Hans!

I'd like to add a few tips in FSB overclocking, on an nForce2 chipset :D

Before I start tho... remember that overclocking the FSB is not recommended by the manufacturer so will void your warranty... if they find out. I won't be held responsible for broken stuff.

Like Hans explained, on an nForce2 chipset there is a PCI/AGP lock which keeps their frequencies near enough to 33/66. This makes FSB overclocking less of a risk, but there is still a lot to know.

1) Your RAM will be a major limiting factor if you have a good overclocking board. But there are a few ways to squeeze the most out of your RAM.
Loosen the timings. In the BIOS you will find 4 different RAM timing options, where you will find them in BIOS depends on your board. If your system has reached it's FSB oc limit and you think it's the RAM at fault loosen the timings a little and see if that helps. I would looses CAS first by 0.5 increments at a time, then test again and see if that helped.

2) VDimm, or RAM Voltage. Upping the VDimm 0.1v-0.2v will often yield at least a few more MHz. If you have RAM with Winbond BH-5, Winbond BH-6, or Winbond Ch-5 to a lesser degree, it will clock very nicely with more juice! :D:D
The new DFI Infinity/Lanparty boards have VDimm options up to 3.3v!!! I would NOT recommend this to anyone that's a beginner, but at least 2.9v is safe in my opinion. I have never heard of 2.9v damaging a DDR memory module.

3) Some boards have a Vdd (Chipset voltage) option in BIOS. This is basically the voltage that is fed to your North Bridge. Not all boards have this option, and not all boards respond the same, but this setting can be very useful in FSB OC'ing.
The principle is similar to pure CPU OC'ing, more voltage = more stability... well... not always. My old NF7 liked to clock better with 1.5Vdd while this one likes the max option of 1.7Vdd... unfortunetly OC'ing is not an exact science due to all the variables so the best way to know what works best for yourself is to experiment with different settings.


This is only a brief run thru but hopefully some find this helpful :)

Peace
Rabbi
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