C47

AM2 Overclocking Guide - Single Core

This guide is for single cores only, although I doubt is dual core will be any different.....

After searching alot on the internet for an overclocking guide for an AM2 motherboard, I found no results. So after much reading of different 939 guides and my motherboard manual I decided to compile this 'draft' and share my knowledge. Getting straight to the point, AM2 systems have a distinct advantage over 939 because of DDR2 support. As we know the max FSB for AMD systems, is 200Mhz, and in 939 systems 200Mhz was the max for the memory as well (DDR400), which meant to get a successful overclock the memory had to be of good quality, so that it could handle bus speeds greater than 200Mhz. Luckily for AM2 users, their memory is already running at higher bus speeds (266Mhz for DDR2-533 and so on) while their processor is running at 200Mhz FSB. So if we can reach a maximum of 266Mhz FSB, we can leave our FSB:RAM ratio as 1:1 and go all the way up to 266Mhz knowing out cheap RAM wont cause any problems. Obviously people with DDR2-667 can go way above 266Mhz upto 333Mhz, but here the motherboard,CPU (HTT) will become a limiting factor (assuming 1:1 ratio).More on this later. Otherwise, a computer running at 266Mhz FSB with some other ratio that makes the memory run at something higher than 266Mhz and less than 333Mhz will definitely be faster than the computer with 266Mhz FSB with 1:1 ratio. This is the main difference between any 939 and AM2 overclocking guide.

I had an option to go for and Opteron 146 939 or AM2 3800+. Im glad is chose the latter, because the Opteron overclock would have required good (expensive) memory.

AMD Athlon(tm) 64 Processor 3800+ (Orleans Single Core 2.4Ghz)
Gigabyte nforce410/430 GA-M51GM-S2G (with the stupid ITE 8716 sensor that gives my CPU temp as 9-15C!!!!)
TwinMOS 512MB DDR2-533 (at 277Mhz it gives errors)

So here is my system...as you can see I was really low on cash, which is why I chose 512MB instead of 1GB and got the mATX version (S2G) instead of the ATX (S3G).

Why to buy an AM2 system:
1) Has Quad Core support , 939 doesn’t
2) Has DDR2 support, 939 doesn’t (where I live DDR400 is more expensive than DDR2-533 and DDR2-667)

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Note : This Gigabyte motherboard is not recommended for overclocking because it has no options to set Vcore, VDimm, HTT Multiplier, CPU/RAM ratio, uptil the most recent BIOS release (version F8). And it also reports wrong temperatures, so if the guy at newegg is happy that his CPU is at 16-18C....his real temperatures are (39-41C).....there is a -23C thermal offset.

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Note : Gigabyte GA-M51GM-S2G users : If you want to decrease the HT multiplier, flash to BIOS version F7(which has the HT options only and no FSB/CPU/Memory options) change the multiplier to 4X, save, then flash back to version F8. More on this later....

The HyperTransport

Keep in mind that HyperTransport will be the limiting factor for going above 250Mhz. AM2s run at 1000Mhz(HT Freq) which is, 200 (FSB) X 5 (HT Multiplier). Increasing the FSB will also mean increasing the HT Freq, and overclocking the HT Freq can also lead to system stabilyity problems, thereby increasing our 'causes for instability' to three
1.CPU cant run that high
2.Memory can run that high
3.CPU cant handle HT Freq

...and we don’t want that. We therefore make the HT multiplier to 4X, which gives 200X4 = 800Mhz HT Freq. We know the CPU can take 1000Mhz HT Freq so we can go all the way upto 250Mhz FSB, knowing the HT Freq shouldn’t cause any problems. (250x4 = 1000MHz).

Now some people might say, why not decrease the HT multiplier down to 3X, since that should give us a max of 333Mhz FSB. I personally think its not a good idea, because with the 3X multiplier the max HT Freq will be 250X3 = 750Mhz. (I am assuming here a standard motherboard which can handle ~250Mhz FSB). So even though our CPU and RAM maybe running at super speeds, our HT Freq will bring down overall performance because of its low frequency. Besides, for my processor 250x12 = 3000Mhz is the max, and if I can get to 2.8Ghz without going above 250Mhz then its better because I don’t want to push my motherboard too hard.

Lemme explain:
With standard HT Freq 1000Mhz, we run at 2000Mhz HT (it is double pumped) as advertised on the AMD box. If the HT bus is 16bit wide (2 bytes) with dedicated upstream and downstream, we have
2000 Mhz * 2 bytes = 4000MB/s (in one direction)
which is 8000MB/s (upstream and downstream) total bandwidth between the CPU and inbuilt Northbridge.

At 750Mhz, we have (1500Mhz HT)
1500 * 2 = 3000MB/s
which is 6000MB/s total bandwidth between the CPU and inbuilt Northbridge.

Normally all the data that comes from the memory arrives that the inbuilt Northbridge before going to the CPU. So we have two buses, in a way. One is between the CPU and the inbuilt Northbridge( 8000MB/s at 2000Mhz HT Freq), and the other between the inbuilt Northbridge and the memory(~3500MB/s on DDR2-533, or somewhere around ~5000-6000Mb/s in dual channel mode). As we can see the inbuilt Northbridge (or the CPU) is not overburdened with the incoming memory data.

Running at 750Mhz HT Freq, with an overclocked DDR2 memory (lets say DDR2-900) in dual channel mode (which almost doubles the bandwidth when compared to single channel), the memory bus bandwidth will definitely be more than the HT bus bandwidth (which is at 6000MB/s). So the CPU/Northbridge is at 6000MB/s and the Northbridge/Memory is at 7000-8000MB/s. What we have is a 'bottleneck' or in other words the CPU cannot handle the data coming from the memory, because of the CPU and inbuilt Northbridge bus speed limit (HT Freq which is at ).....and the performance decreases. And assuming this is the max FSB reachable on our standard motherboard, it is a problem.

Keeping things at 4X multiplier is better, because our standard motherboard can handle 250Mhz, bringing back the HTFreq to good old 1000Mhz giving us 8000MB/s bandwidth.

With all these things in mind, there is one more option. My motherboard had an option to make the HT bus 32bits wide, changing quite alot of things (mostly the HT bus bandwidth). I tried it with Central Brain Identifier and it didnt work, probably because my CPU doesnt support it.

The Overclock

This section is for people with Gigabyte's GA-M51GM-S2G motherboard (I really narrowed it down here....LOL)...with a single core Athlon64 CPU, with DDR533 or greater.

Software Required:
@BIOS (to flash BIOS in windows)
CPU-Z
Central Brain Identifier (CBiD)
Everest 2.20 (to check memory bandwidth)
Memtest (to check for memory problems)
SiSoft Sandra or PCMark05 (to check CPU performance - PCMark05 score list can be obtained from tomshardware.com)
OCCT

The problems faced by the owners of this motherboard:
-Wrong temperature figures
-No Vcore, VDimm, FSB:RAM ratio options in BIOS

Step 1: Change the HyperTransport
Using the @BIOS utility, flash to version F7. Go to the BIOS, press Ctrl+F1 to reveal 'hidden' options. Go to Advanced Chipset and set the HT multiplier to 4X. Save and exit. CBI and CPU-Z should read 800Mhz for HTL.

Step 2: Reduce the CPU Speed
In @BIOS, check clear DMI information and flash to BIOS version F8. Go to the BIOS. Then Advanced BIOS options, and disable AMD Cool n Quiet. Go back to the main menu. Press Ctrl+F1 to reveal 'hidden' options. Go to Advanced Chipset and set the CPU multiplier to 5X and set the DDR speed to DDR400(this makes the FSB:RAM ratio 1:1). Save and exit.

Step 3: Start playing......
No I dont mean games......start playing with the settings. Go the BIOS and change the FSB to 250Mhz. Save and exit. Go into windows and run CBiD. From there go to 'mobility' and slowly increase the CPU multiplier. After an overclock run OCCT torture test and Memtest for stability. If everything is ok, go to the BIOS and set the multiplier to that value and save. That’s it.

One Option : (if you have a 3800+ with 533Mhz ram)
In BIOS, set the following:
FSB:250Mhz
DDR: DDR400
CPU Multiplier : 10X

This should give you 2.5Ghz even if you have sub-standard ram. You can play on with the settings and try other combinations. I was able to reach 2.6Ghz(217x12) and 2.7Ghz(225x12). But when I tried 2.8Ghz, i got a blue screen.....so i stopped at that....anyway for me its stock speed is good enough...

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WARNING : Since you have no control over Vcore and Vdimm, keep and eye on these two from either CPU-Z of CBiD. The above will automatically increase your Vcore to 1.312V from 1.26V as shown in the pictures below.

Normal


250Mhz FSB


2.5Ghz (250x12)


2.6Ghz (217x12)


2.7Ghz (225x12)


I hope I did my service to all AM2 Single core owners......any mistakes in the guide....feel free to correct me.....

C47 Signin off.....