Q6600 differences in voltage?

[MurderCityDevil]

I've been overclocking my q6600 for a while now, and I'm confused about the differences between the voltages set and displayed in bios and the ones that cpu-z gives me. For instance, I just overclocked it to 3.0ghz, and set the voltage in bios to 1.325. I have the evga 680i mobo, and in the bios the "actual value" column displays a lower value than whatever i set. If i save and reboot, and then go to bios again, the value will be lower than what I set it as a second ago. Also, just now I checked cpu-z and my voltage is reported at 1.280v. OC'ing is really difficult because I have to guess what voltage I need to set it as to get the actual value, so I might have to set 1.4 just to get 1.35v. Why does this happen and what can I do to correct it?

 

[BillParrish]

It is called Vdroop and is designed in as protection to the CPU, as the current through the CPU voltage regulation circuitry changes with the load put on the CPU the voltage drops across the components of the CPU voltage regulator changes (V = I x R) the voltage regulation circuit then tries to adjust output voltage to compensate for this increased voltage drop, it cannot do it instantly, and so there is "overshoot" during this automatic adjustment process. The voltage going to the CPU will have a momentary "over voltage spike" until the CPU regulation circuity stabilizes. Because of this all Intel designs certified by Intel for their CPUs have a built in buffer of a few tenths of a volt to keep the spike from damaging the CPU. This is critical at very high CPU voltage settings. You cannot "correct" it as there is nothing wrong. You can negate it by just setting a higher setting in the bios so that the actual voltage is what you want. At very high Vcore settings there is some risk to processor life.

http://download.intel.com/design/processor/datashts/31327807.pdf

section 2.6.

The difference between reported values at one bios setting between no CPU load (idle) and under load is called Vdrop and represents the inability of the CPU voltage regulation circuitry to be exactly accurate under all situations in compensating for the increase in voltage drop across the circuit components due to increased current. This is more of a design quality issue of the CPU voltage regulation circuity.


If you were willing to pay $100 extra for a board these issues could be eliminated by a very high quality and expensive CPU voltage regulation circuity design and components. However in design, everything is a compromise.


cpu-z should not be trusted with voltages. It is a sofware program written by a 3rd party and ties very hard to correctly read the sms buss sensor chip and interpertet the data. Somethings it does well and sometimes it does not. Even the bios can be in error, but you have to go with something unless you stick a DVM on some board connections/cpu and so I recommend the manuf bios or sofware for voltages unless you have information that they are wildly inaccurate which does sometimes happen but is usually fixed quickly with a bios update.

 

[MurderCityDevil]

Wow, thanks for the explanation, that's some interesting stuff. But what about the vdroop mod? Doesn't that get rid of the problem?

 

[Markyip1]

The pencil mod you are referring to will eliminate vDroop from your computer if done properly, but understand (as Bill made clear in his post) that vDroop is not a problem. It's a perfectly normal operating aspect of your PC.

The pencil mod remains popular for many over clockers, especially those targeting a very high OC. If you find that you need to specify voltage in the range of 1.5v for stable operation, you might consider performing the mod. This will allow you to specify a lower voltage in the BIOS for your chip, as the voltage you specify will be the voltage used for both idle and load processing states. The net effect is that the chip's voltage at idle and partial-load can be reduced (in comparison to no mod,) resulting in a lower operating temperature in these states.

However, take note that performing the mod will void your motherboard's warranty (although you can simply erase the graphite on the resistor should you ever want to "undo" it.) It may also shorten the life of your motherboard, as performing the mod causes the voltage regulator to perform at a reduced state. Lastly, your processor will not be protected from the voltage spikes which occur between idle and load states (as discussed in the previous posts.) If you set your processor's vCore to a high voltage (say well in excess of 1.5v) with the mod in tact, you risk damaging your processor from said voltage spikes.

Mark.

 

[leftheaded]

yesterday, i read on some forum that the difference between vdroop and vdrop is ...

vdrop = the voltage drop between BIOS settings and cpu-z
vdroop = the voltage droop in cpu-z when the cpu is under a load

i'm no EE, so take my reading with a grain of salt

 

[MurderCityDevil]

ok, well I would want to get a stable 3.6ghz overclock on my q6600. from what I've seen, 1.5 seems like a good voltage for this frequency, and the only thing keeping me back from it is cooling at the moment (still waiting for my lapped ultra120extreme). Do you think it's a good idea for me to do the mod?

 

[Markyip1]

Every chip is different as to how much voltage it needs for a given clock frequency. You never want to over-estimate, as extra voltage just equals extra heat. Many G0 quads can hold a 3.6ghz OC stable at well under 1.5 volts. The best chips can even go under 1.3, although that's a one-in-a-million chip. Assuming you have a G0 Quad, you should look for stability between 1.3 and 1.45 volts, although some will need 1.5 volts (set in the bios) or more.

Mark.