Why is software so terrible at measuring rails?

[njdevilsfan87]

So, I decided to see how my HX520 would do an overclocked Q6600 system at 3.2GHz, with 4x1GB running at about 850MHz, and a single 7900GS volt modded to 1.55V running at 700/900 clocks.

System powers up, I'm seeing 12.00 on the multimeter exactly. Impressive...

I load up P95 large FTT (for max power consumption), ATI Tool with artifact scanner running, and Windows Media Player just for the hell of the sound card & HDD to be going at the same time.

Here are the software readings (Easy Tune 5 on a P35-DQ6) versus the multimeter.


Software reading | Multimeter Reading
---------------------------------------------
11.92 idle | 12.00 idle
11.79-11.85 load | 11.97-11.98 load

So why is this software reading so off? Does software just "assume" that the 12V will drop when the system is put under heavy load? Is there actually anything built into motherboards that attempt to read the 12V rail?

 

[PurdueRy]

That's really not that far off.

 

[mpilchfamily]

Software is pulling the readings from a chip on the motherboard. Generally referred to as the Winbond chip because thats the company that is most widely used for the chips. This chit is taking its reading from the 24 pin connector but may be on the opposite side of the motherboard. So the resistance given in the traces of the motherboard are affecting the readings. as you can see different boards will have a different margin of error. This same winbond chip is where programs like Speedfan get there temp information. At least there Temps 1,2,3,and 4 while the Core reading comes directly from the CPU.