SOmething along those lines. If you change the vTT voltage and use the same GTLREF multiplier you would be increasing GTLREF also since it follows vTT.
If you are using 0.67x multiplier for example or 67% of a 1.30v vTT, that gives a GTLREF voltage of 0.87v. Lets say you increase vTT to 1.60v, since GTLREF follows vTT, you would now have a GTLREF voltage of ( 1.60 * 0.67 ) or 1.072v. The problem with this is that GTL reference voltage needs to be generally between 0.8-1.0v max. ANy higher and you generally end up with a situation where the NB can't figure out what is and what isn't a valid clock wave. In other words the end result worst case scenario is, no valid clock wave = no post since the NB doesn't understand what is and what isn't a clock signal.
So say I was aiming around 0.95v GTLREF at 1.60v vTT, I would need to reduce the Multiplier down to 0.59x or decrease the voltage by 0.12v or -120mV to get back to that point. This is just an imaginary value and generally you wouldn't need 1.60v vTT unless you were using over 1.60v vCore and 1.60v vNB.
THe idea of adjustable GTL Ref voltage is to be able to find a fine balance between the relationship of vTT, vCC / vCore, vNB, and most importantly to be able to raise or lower the voltage ranges where a valid clock wave occurs for both logical high and logical low waveforms. If you don't understand those it's not all that important, just remember that clock waveforms occur when you apply power to the circuit, then one clock later when you switch the circuit back to ground. Sometimes theres jitter, noise, cross chatter, and other electrically related phenomenons that disrupt or deform the clock waveform. Changing the voltage at which the reference for all other voltages in the system follow can move the clocks waveform into a region where previously the clocks waveform deviated from its wave path, you move the points at which the readings are taken. It's something like telling day and night apart during long days. The only way to differentiate is by the sun and moon but since there is heavy cloud where you are standing and you have no watch for all you know it might be night even though it looks like day. You move yourself from where you are to where the clouds arent, and doing this you know if its the sun or moon in the sky, thus if its night or day. GTL reference voltage adjustment isn't all that more complex than that for somebody who doesn't need to understand all the science and maths behind it. Moving it enough sometimes you get a clear enough sky to operate normally!
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