so by default your MB overvolts it ? what is the main benefit of doing this ?
The motherboard checks the processors [VID] when the chip is installed and sets the vCore according to this info . . . the [VID] is programmed into the chip by Intel® . . .
If you download
CoreTemp you can check what [VID] your processor has . . . the voltage you see in CPU-z will almost always be less than what the [VID] says it should be . . .
If your [VID] says 1.3500v then you will most likely see approx 1.3000v in CPU-z . . . the difference between the [VID] and the actual vCore you get is known as vDr
op . . . its quite common but causes a few rOOkies some confusion as they expect top see the same voltage that is set in BIOS appear in CPU-z . . . to further confuses matter once the chip and associated power circuitry is placed under heavy load the vCore can drop even further . . . so if the [VID] says 1.3500v and you see 1.3000v in CPU-z with the PC not under heavy load you may well then see this voltage figure drop to 1.2750v or lower once the heavy load begins . . . this is known as vDr
oop . . .
Depending on the quality of ones motherboard and PSU the effects of both vDrop and vDroop have a negligible effect on proceedings and the PC 99.9% will run perfectly stable . . . however once you start overclocking and running the chip out-of-spec then you can run into stability problems if the vCore drops below a certain point whilst under heavy load . . .
Anyways regarding the [VID] and the board overvolting the chips . . . I believe Intel® play it safe and build in a margin of error by adding a bit too much voltage instead of a bit too little voltage . . in the case of the former the chip will always run stably albeit sucking up a little too much juice . . . in the case of the latter the chip will run a bit cooler and not use as much voltage but under extreme load could become unstable . . . of course Intel® don't want a user to have any stability issues with their products as it would "damage" their brand reputation!
So all the above is mainly background info but in the o.p you asked this question:
i have CPU voltage set to auto i would like to set this to a fixed amount could you help me on this
If you follow the instructions you have been given you will be able to determine the "exact" voltage your processor needs at any given frequency . . . all you need to do is change the vCore from [Auto] to what it says in the [VID] info and then carefully start testing and lowering the vCore until you run into stability issues . . . I always begin this voltage testing with the processor at stock myself and then work upwards . . . this is known as Voltage-Scaling-Testing and gives you a map of what vCore your processor needs at various frequencies and helps determine what is the processors "Sweet-Spot" . . . i.e the optimum balance of voltage vs frequencies . . . you tend to find that after a certain point the chip starts to need a huge boost to its voltage to gain a few meagre MHz which means you have past the optimum spot . . .
Regarding other voltages . . . as long as you stay at 400MHz-FSB or under there is no real need to play with any of the other voltage settings . . . its only really vCore that needs playing with . . . however once you venture past 400MHz-FSB you are entering No-Man-Lands outside of official Intel® specifications so as setter mantioned you will most likely need to "tinker" with other voltages if you run into any stability issues! . . .
Hope this post helps you (and any other LGA775 rOOkie Clockers reading) and good luck with your mission . . . have a nice xmas!
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