How and where (CPU, BIOS, software) is Minimum VID set?

onlinespending · Post by **onlinespending** » Sun Apr 25, 2010 9:50 pm

I'm trying to completely understand how and where the minimum VID is set since I'm having trouble being able to undervolt my E8400 and Q9450 to ultra-low values. As per the Intel specs and datasheets for these CPUs, the minimum VID is 0.850V (with a default VCore of 1.2V).

Then there's the concept of P-States. The P-States as part of SpeedStep may very well not have the "idle" P-State use the extreme minimum VID. Perhaps it's only set to some value slightly less than the default VID, such as 1.1V.

My microATX G33 motherboards only support a minimum manually set Vcore of 1.1V.

When I try to use either RMClock or CrystalCPUID to change the P-States, it does not allow a voltage below 1.1V. In CrystalCPUID there are many voltages with (X)'s next to them.

So my question is what exactly is the limiting factor here? Is it the CPU with the VID for its minimum P-State? Is it the BIOS enforcing some lower limit? Is it a compatibility problem with RMClock or CrystalCPUID (for instance if they don't "support" the i3,i5,i7 CPUs, would you then have issues overriding the P-States and setting a lower minimum VID)?

I'm hopefully it's the BIOS. Because if it is, then that means I could perhaps buy one that does not impose a high minimum VID limit. But that leads me to a question on another source of confusion. I've seen others mention that their BIOS allows a low manual VID setting (say 0.8V), but they are still not able to override the P-States and have RMClock or CrystalCPUID dynamically change the Frequency and VCore to have it run at that same low VID when it's idling. Ultimately that is what I am after; being able to dynamically change the VCore and Frequency with RMClock or CrystalCPUID but be able to have it idle at the lower end of VID range that is supported by the CPU (0.85V in this case).

I appreciate your help.

b_rubenstein · Post by **b_rubenstein** » Mon Apr 26, 2010 4:14 am

The minimum VID is a value programed into the CPU. All that can be set in the BIOS is Vcc (CPU voltage). There are three basic settings: Auto - BIOS determines the Vcc - tends to use higher than necessary voltage, Normal - Voltage level setting read from CPU (VID), Manually setting the Vcc.

Vcc will only change based on load and CPU speed in Auto and Normal modes. With C1E and EIST enabled and Vcc set to Normal, I get a Vcc of 1.08v with a Q9550 at idle, with a P35 MB. With a P45 MB, E8400 and C1E and EIST enabled, I get a Vcc of 0.97v at idle. Both PC are OC'd to run at 3.6 Ghz.

onlinespending · Post by **onlinespending** » Mon Apr 26, 2010 11:41 am

b_rubenstein wrote:The minimum VID is a value programed into the CPU. All that can be set in the BIOS is Vcc (CPU voltage). There are three basic settings: Auto - BIOS determines the Vcc - tends to use higher than necessary voltage, Normal - Voltage level setting read from CPU (VID), Manually setting the Vcc.

So if the minimum VID is programmed in the CPU (which may mean it's a register that can be changed, and therefore set by the BIOS), why doesn't RMClock and CrystalCPUID allow a value lower than 1.1V on my E8400? The processor supports a VID as low as 0.85V per the spec, so you would imagine that's the value set in the CPU. And it does appear others have been able to get lower Vcore values than 1.1V with the E8400.

b_rubenstein · Post by **b_rubenstein** » Mon Apr 26, 2010 12:39 pm

You should be able to go directly into the BIOS and set the CPU voltage as low as 0.85v. Just remember that that voltage value is for idle conditions when the CPU multiplier is x6. Undervolting will have very little effect on idle power draw.

I doubt that it's possible to change the VID in the CPU. Not everything is in re-programmable memory.

onlinespending · Post by **onlinespending** » Mon Apr 26, 2010 1:31 pm

b_rubenstein wrote:You should be able to go directly into the BIOS and set the CPU voltage as low as 0.85v. Just remember that that voltage value is for idle conditions when the CPU multiplier is x6. Undervolting will have very little effect on idle power draw.

I doubt that it's possible to change the VID in the CPU. Not everything is in re-programmable memory.

OK, well some motherboards, many of them in fact, don't allow a voltage lower or much lower than the default VID, mine included. So how then can I get RMClock or CrystalCPUID to use these lower VCore values? This brings me to my original question. What exactly is preventing those software tools from using the lower values? Is it the BIOS?