If you have a 45nm cpu, you are aiming for the voltage given by the 0.635x multiplier, so to work out what to put for the the 0.667x GTL Ref (Y) values (it will be a negative number to make the resulting voltage the same as the 0.635x GTL Ref voltage), use the following eqn, but substitute the Vtt you use, in place of the example Vtt.
In the examples given, I have rounded off to 3 decimal places throughout for simplicity.
For me, Vtt = 1.14v
Equation:
Vtt x 0.667 + Y = Vtt x 0.635
1.14v x 0.667 + Y = 1.14v x 0.635
0.760v + Y = 0.724v
Y = 0.724v - 0.760v
Y = -0.036v
Y = -36 mv
So to verify it:
1.14v x 0.635 = 0.724v
1.14v x 0.667 = 0.760v
0.760v - 0.036v = 0.724v
or an easier, non algebraic alternative:
1.14v x 0.635 = 0.724v
1.14v x 0.667 = 0.760v
0.724v - 0.760v = -0.036v
= -36mv
Round up to the nearest selectable bios value, in Maximus II Formula bios this is -40mv
So now you set the 0.667x 'Y value' to -40mv
You have now made the 0.667x GTL voltage as close as possible to the 0.635x GTL voltage.
If you have a 65nm cpu, you are aiming for the voltage given by the 0.667x multiplier, so to work out what to put for the the 0.635x GTL Ref (Y) values (it will be a positive number to make the resulting voltage the same as the 0.667x GTL Ref voltage), use the following eqn, but substitute the Vtt you use, in place of the example Vtt.
For me, Vtt = 1.14v
Equation:
Vtt x 0.667 = Vtt x 0.635 + Y
1.14v x 0.667 = 1.14v x 0.635 + Y
0.760v = 0.724v + Y
Y = 0.760v - 0.724v
Y = 0.036V
Y = 36 mv
So to verify it:
1.14v x 0.667 = 0.760v
1.14v x 0.635 = 0.724v
0.724v + 0.036v = 0.760v
or an easier, non algebraic alternative:
1.14v x 0.667 = 0.760v
1.14v x 0.635 = 0.724v
0.760v - 0.724v = 0.036v
= 36mv
**EDIT**
Naming and meaning of the CPU gtl ref's, and land pin assignment names (Kindly provided by Mikeyakame):
CPU GTL Reference (0/2 Lane0/Lane2)
GTL Reference Data Strobe Input Buffer Middle/End Landing Pins 0 & 2. (GTLREF0/GTLREF2)
CPU GTL Reference (1/3 Lane1/Lane3)
GTL Reference Address Strobe Input Buffer Middle/End Landing Pins 1 & 3 ( GTLREF1/GTLREF3 )
For quad-core multi-die chips:
Middle Landing Pins (GTLREF0/1) act as input buffer for DIE0, End Landing Pins ( GTLREF2/3 ) act as input buffer for DIE1.
For dual core single-die chips:
Middle Landing Pin (GTLREF0/1) acts as input buffer for DIE0, End Landing Pin (GTLREF2/3) uncertain if used. Either unused and terminated to open drain Vtt or used as reference for middle pins as margin of error.
Things to note concerning adjustment:
Address strobe Pins (GTLREF1/3) will generally tolerate small variance with respect to reference voltage accuracy, and should in most cases be setup a little lower with respect to Data strobe Reference multiplier or voltage.
Why?
Probably the same reason as any other reference or input voltage for clock strobes, signal resonance or cross talking.
Data strobe pins (GTLREF0/2) will not tolerate being out by more than 0.5-1% below nominal, but will tolerate slightly more above nominal. Keep set values above Address Strobe GTL multiplier/voltage offset.
Why?
Works better in almost all situations, don't know why until I get a chance to monitor on a logic analyzer.
By How Much?
Varies according to FSB base clock, higher the FSB BCLK the smaller the difference will probably need to be at least with respect to the Diff Amplitude driving the clock.
Also not that when you are adjusting the GTL Refs with a board that using a fixed multiplier and a modifier (eg: +/- in mv) the resultant Vref will be slightly different for the data strobe lanes and the address strobe lanes, as shown by this example (Kindly provided by Seban):
Calculations made for a Vtt = 1.365v
GTL Ref 0 = +50mv = 0.9336025v
GTL Ref 1 = +10mv = 0.9381305v
GTL Ref 2 = +50mv
GTL Ref 3 = +10mv
GTL Ref 0 = +30mv = 0.9136025v
GTL Ref 1 = -10mv = 0.9181305v
GTL Ref 2 = +30mv
GTL Ref 3 = -10mv
As you can see, the resulting Vrefs are not quite equal, but they are close enough for the sake of GTL Ref adjustment. The aim is not to have the Vrefs all exactly the same, but to find the GTL Ref settings that give you the most stability.
**EDIT 2**
Attached to this post is a spreadsheet kindly made by Seban, allowing easy calculation of GTL Refs required, as a starting point fo your tweaking. Here is his description of it:
I got little present for every one for new year .
It is simple Excel file which is counting GTLs and NB GTL as well as new Multiplier for you and few other minor values for given Vtt. It is nothing special but saves time and allows to tweak GTLs without asking for help and getting too much into it.
How to use it:
First type in desired / currently used vtt voltage as in your bios (or rather the way it looks loaded in Everest for example - but bios one is also working fine)
Secondly put some correction numbers and watch how GTL values are changing (Max and Min values of GTL for given VTT are also shown there).
The goal is to figure out which GTL works best for you for one VTT - in most cases the same GTLs voltages will work for different VTTs or very similar ones (or sometimes not ). So when we switch Vtt we would need to adjust GTLrefs to values which will give same or close to same GTLs as working previous ones for different vtt.
Anyway have fun using it, I created it to make my life easier and it does the job.
PS. There are 2 sets of values to compare between different VTT.
PS2. This calculator is made for MF2 board but will work with all boards that got correction number in equation (like +40 or - 20 and so on).作者: fxj13920 时间: 2009-3-27 18:15
纯英文，看不懂啊:funk:作者: it998 时间: 2009-3-27 19:09
看得懂，译不顺！作者: 蒋PJ 时间: 2009-3-27 20:03 本帖最后由 蒋PJ 于 2009-3-27 20:13 编辑
如果你有一个45纳米的CPU ，你的目标的电压所给予0.635x倍增，所以什么工作提出的在0.667x的GTL编号（ Y ）个值（这将是一个负数，使由此产生的电压一样的0.635x的GTL参考电压） ，请使用下列eqn ，但替代的VTT您使用，以取代例如VTT电压。
在所列举的例子，我有四舍五入到小数点后3各地的简单。
对于我来说， VTT电压= 1.14v
方程：
VTT电压x 0.667 +为Y = VTT电压x 0.635
1.14v x 0.667 +为Y = 1.14v x 0.635
0.760v +为Y = 0.724v
为Y = 0.724v - 0.760v
为Y = - 0.036v
为Y = -36毫伏
因此，以验证它：
1.14v x 0.635 = 0.724v
1.14v x 0.667 = 0.760v
0.760v - 0.036v = 0.724v
或更简单的，非代数替代：
1.14v x 0.635 = 0.724v
1.14v x 0.667 = 0.760v
0.724v - 0.760v = - 0.036v
最多可选择就近的BIOS价值，在马克西穆斯二，这是一级方程式的BIOS - 40mv
所以，现在你设置0.667x ' Y值'到40mv
您现在已经作出了0.667x的GTL电压尽可能接近的0.635x的GTL电压。
如果你有一个65纳米的CPU ，你的目标的电压所给予0.667x倍增，所以什么工作提出的在0.635x的GTL编号（ Y ）个值（这将是一个正数，使所产生的电压一样的0.667x的GTL参考电压） ，请使用下列eqn ，但替代的VTT您使用，以取代例如VTT电压。