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

[mikeyakame]

[QUOTE=vox;3361757]

[I]

ok so NB gtl ref, as a optimal range of variation from 0.641-0.689? (0.666 -+ 0.025) x vTT= value to put on the NB GTL Ref field??

optimal nb gtl reference voltage, as with optimal cpu gtl reference are only relevant to optimal clock drive amplitudes for optimal clock steps. to understand what I mean by this all you need to know is this...

pci clock amplitude is driven with roughly 100mv which is in respect to it working best either side of a 33mhz step, nb clock (fsb) amplitude is driven with roughly 200mv working best either side of a 66mhz step, dram clock amplitude is driven with roughly 400mv working best either side of a 133mhz step, and lastely cpu clock amplitude is driven with roughly 800mv working best either side of a 266mhz step.

what all this means is when changing the frequency for any of the particular bus' is that if you stick to a window of say 25% either side of that bus' tuned frequency steps, ie for a fsb frequency, 15-20mhz either side of lets say 400mhz base clock (stepped at 66mhz to prevent cross talk with other bus') , 380 - 420mhz will give you the best performance and require the least amount of variance from optimal settings, and most of the time provided the vTT is set to a reasonable enough voltage for your vcc and vnb voltages, then you will find that you can work with 0.63-0.65x for cpu gtl ref, and 0.60-0.63x for nb gtl ref.

it's only when you go outside these boundaries you will find the need to use radical gtl reference, fsb termination voltage, and cpu phase locked loop voltage (pll) to stabilize the system properly. certain fsb frequencies are no go zones unless you have clock drive amplitude adjustment, and even with this you have to set the drive amplitude for the nominal frequency steps you require to shield the fsb clock waves from other external bus clock waves, ie 433mhz or 499mhz fsb drop you smack on with the pci bus clock steps (every other 33mhz step, ie 33mhz, 99mhz, 167mhz, 233mhz, 297mhz and so on..)...all this means is without changing the amplitude the fsb clock is driven at and simply changing the frequency within 10-15mhz of these bands will always wreak system havoc at worst or need ridiculous amounts of extra voltage supply to the nb / cpu and way out gtl references to stabilize the system. if you obey the window either side of each 66mhz step, you will find the system performs better, is more responsive and most importantly you don't need to fully understand the concepts behind the design to make even minor adjustments, just a concept of tuned performance zones and basics of why you change the values and when you might need to change them.