Stock voltages: reference vs. mobo BIOS

[monsieurrigsby]

CPU reviews/guides look at overclocking with respect to the stock VCore, and generally state this value per CPU. However, I'm aware that different mobo manufacturers can set their stock voltages differently for the same CPU (in my case, my Asus M3N78 Pro sets a Phenom II X3 720 at 1.4V with the default 200MHz FSB, or at least that's what the BIOS reports in its HW Monitor function; whereas, all the stuff I've read point to a 1.325V stock for this Heka CPU).

Now, presumably AMD provides some reference curve of voltage vs. frequency (or multiplier), which mobo manufacturers then take and may tweak according to their own testing and the specifics of their mobo hardware/BIOS. I'm trying to understand three things:

(i) Where are these reference values published (if anywhere)? Having a look through all AMD's official tech sheets, they only seem to give the voltage range: the "AMD Family 10h Desktop Processor Power and Thermal Data Sheet" gives 1.15 - 1.425V for P0 at 2800MHz (for my CPU).

(ii) What are reviewers really talking about: the reference values (in which case where do they get them: point (i)?), or the mobo's default values?

Is it just a special case that this Asus board is "wrong"?

(iii) Can it be that the BIOS still understands the reference values but then applies its own modifier? I ask this because, using Phenom MSR Tweaker, I have to set the VCore to 1.325V in its UI to get CPU-Z to show the c.1.4V that match what it shows in a vanilla configuration (i.e. no CnQ, no use of Phenom MSR Tweaker).

Unless it's a coincidence that I have to set the "real according to articles" stock V to get what I want, this would suggest that the BIOS "understands" the reference curve but applies an offset anyway. Or am I barking up the wrong tree here?? (I'm assuming this isn't a coding error in the Tweaker: I presume there's a fairly standard Windows interface to set these which it would be hard to give the wrong value to.... I know CPU-Z sometimes shows incorrect voltages if it doesn't fully support the monitoring chipset, but in this case it matches the BIOS monitor values, and Asus' PC Probe II also confirms these voltages).

Any advice/info. appreciated. Sorry for the long post D:

 

[miahallen]

Go to page 49...your CPU is the right most column "HDZ720WFK3DGI" and look at the "dual-plane" specs.
http://support.amd.com/us/Processor_TechDocs/43375.pdf

Notice where VID min & max are listed there is a "note 9" which reads:

9. Variable voltage, any valid voltage between VID_VDD Min and VID_VDD Max is allowed.

The min and max are listed as 1.15V and 1.425V respectively. This means that within the specified temp range....any of these voltages are acceptible.

This is the BIOS developers guide which AMD provides to the motherboard manufacturers:
http://support.amd.com/us/Processor_TechDocs/31116.pdf

2.4.1.3 MinVid and MaxVid Check
The allowed limits of MinVid and MaxVid are provided in [The COFVID Status Register] MSRC001_0071.
Prior to generating VID-change commands to either the PVI or SVI, the processor filters the InputVid value to
the OutputVid as follows:
• If InputVid < MaxVid, OutputVid=MaxVid.
• Else if (InputVid > MinVid) & (MinVid != 00h), OutputVid=MinVid.
• Else OutputVid=InputVid.
This filtering is applied regardless of the source of the VID-change command.

I'm not an electrical engineer, maybe someone else can assist more? But to me it looks like the bold line I marked says that the motherboard should feed the maxVID allowable which would explain why you're chip is defaulting to ~ 1.4V

hope this helps

 

[VirtualLarry]

No wonder AMD systems aren't as power-efficient as Intel's, if their published mobo specifications say to max out the voltage by default.

I was wondering why my Athlon II X4 630 had a default VID of 1.3-something, and my mobo was defaulting to 1.425v on AUTO setting.

 

[busydude]

So does it mean that even if I overvolt my CPU to 1.425V, it is still operating under specifications?

Does that mean I can operate it at 1.425V all the time and yet not damage it?

 

[miahallen]

So does it mean that even if I overvolt my CPU to 1.425V, it is still operating under specifications?

Does that mean I can operate it at 1.425V all the time and yet not damage it?

As long as it stays within the rated temperature range....YES

 

[monsieurrigsby]

OK...... very weird, thanks for pointing it out miahallen. But, if this does mean what it says (!), then clearly several mobo manufacturers aren't doing it. Look at our beloved AnandTech's review on another Asus board (M4A78T-E: http://www.anandtech.com/show/2721/11). Clearly 1.325V stock, so I guess my question still remains.....

I still can't believe that there isn't AMD supplied data on stable voltages (at least for some representative chip sample). Those voltage ranges are really only saying "you can't do irreversible damage if you stay within these ranges for that frequency (and don't bust the temperature envelope)", aren't they? There's no way an X3 720 would boot anywhere near 1.1V at the default P0 settings....