How to set up GTL Ref Values for 45nm & 65nm

[mikeyakame]

Heres a stab in the dark as to why there appears to be a small difference between X48 and P45 chipsets for GTL, it's just a shared thought.

P45 chipsets have a feature called CPU Margin Enhancement at least on Asus bios', what Intels official name is I don't know. From the little I know and I can figure out, the Margin Enhancement is a feature to address the electrical and physical constraints the front side bus had reached for the X48 based chipsets. These are most likely preset values either by OE or Intel that set up the signal skewing for the FSB/CPU clock waves to minimize clock jitter and ringback on the receiver, slew rate compensation, etc. Why you ask? AGTL+ is designed with limit of 400MHz on BCLK and beyond that integrity of the system can't be guaranteed. So P45 being mainstream and last ditch effort on FSB chipset to go beyond theoretical limits, this was added.

Now the point of the previous paragraph is, these presets most probably compensate for a percentage of signal problems that occur with high FSB frequencies that on the X48 chipset we we're using varying GTL Reference Points to handle compensation for clock signal integrity loss.

This is just a shot in the dark at why. None of us will ever know for sure, just theories and speculation.

You'll probably find that Asus' option names, Compatible / Optimal / Performance actually are presets for FSB frequency ranges. Compatible might be < 400MHz, Optimal 400-500MHz, Performance 500MHz ->. Thats just an example as its not that simple, but there will be ranges of frequencies that can be probably be compensated for with a margin of accuracy before those preset values go out of range.

If there is one thing that sucks though, it's that Asus, DFI, etc don't document what these settings and values do because they want to keep their design edge on their competitors products. This unfortunately has the consequence of us the guy who buys the product and uses it having to figure out what "insert corny performance name" option actually does and how it effects every other setting. I love having such flexibility, but some day the price of it without documentation that at least outlines its use it'll be too much for anyone.

[Ghostleader]

Thank you CryptiK and mikeyakame

But how do I know what the default value for CPU GTL ref are when I calculate, lets say Q6600 on a P45 board and I can set the GTL for all cores.

Are the default (auto) like this on a P45 board

GTL Ref 0 = ,635
GTL Ref 1 = ,667
GTL Ref 2 = ,635
GTL Ref 3 = ,667

And if I want ,667 on all cores I hade to ad (+) some (how much is depended on the VTT value) for 0 and 2 and let the 1 and 3 left at auto so they all are at ,667.

[CryptiK]

Thank you CryptiK and mikeyakame

But how do I know what the default value for CPU GTL ref are when I calculate, lets say Q6600 on a P45 board and I can set the GTL for all cores.

Are the default (auto) like this on a P45 board

GTL Ref 0 = ,635
GTL Ref 1 = ,667
GTL Ref 2 = ,635
GTL Ref 3 = ,667

And if I want ,667 on all cores I hade to ad (+) some (how much is depended on the VTT value) for 0 and 2 and let the 1 and 3 left at auto so they all are at ,667.

I can say for sure the Maximus II Formula has the GTL Ref options as:

GTL Ref 0 = ,635
GTL Ref 1 = ,667
GTL Ref 2 = ,635
GTL Ref 3 = ,667

But with other boards it may be different, also some boards tie two GTL Refs together (ie: 0/2 & 1/3) so you only have two adjustments possible, not four. In this situation 0/2 usually = 0.635x and 1/3 usually = 0.667x but unless they state what it is in the BIOS or in the manual it's anyone's guess.

EDIT - as Mikey said though there is often adjustments going on behind what you are doing and as such are beyond your control, so don't focus on what they are originally but through trial and error establish what settings give you the best performance and stability.

[Ghostleader]

Thank you again CryptiK and mikeyakame

I get the picture.


Run Prime95 small with a little lower vcore the stable and see which core that fails and then adjust that GTL for that core and run Prime again.

Is this a good way to tune GTL?

or how do you guys test which GTL are most stable?