Pentium M for the Desktop: AOpen i855GMEm-LFS & DFI 855GME-MGF

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A. Thermal Testing and Noise Analysis

Each system was first tested with its default heatsink and fan, with the fan running at 12V and the CPU running at the default Vcore. The little 40x10mm DFI fan at 12V had that annoying, high-pitched whine that is common with most small fans. There was also a fair amount of air turbulence noise. It wasn't as bad as many small fans that I've heard, but it was definitely too loud to live with. The 80x15mm AOpen fan, on the other hand, wasn't too bad at 12V. It sounded more like a decent 92mm fan. It had a sort of low-pitched mechanical whine and a bit of air turbulence noise. Overall temperature rise under load was pretty low.

Next I tested each heatsink with the fans running at 5V. The DFI fan lost a significant amount of its whine at 5V but there was a noticeable clicking noise now present. And it obviously wasn't putting out much airflow as thermal performance became awful. The AOpen fan was surprisingly quiet, and basically silent from more than 2' away, but a bit of metallic clicking noise was more evident now that air turbulence noise was reduced. Performance was adequate, especially given the low noise. I could live with this fan at 5V, and it might be the perfect HSF to use in tight spaces (think HTPC or SFF) with its low overall height of only 1¬Ĺ".

Temperatures with Stock Heatsinks, CPU at Default Vcore (1.324V)
Board / HSF voltage
°C rise
°C/W (TDP)
°C/W (MP)
AOpen - 12V
DFI - 12V
AOpen - 5V
DFI - 5V

°C rise refers to the rise in temperature over the ambient at load.
°C/W - TDP calculations: Intel's TDP of 21W was used.
°C/W - MP calculations: CPUHeat & CPUMSR Projects' estimate of 26.8W was used.

B. Thermals and Noise with Reduced Vcore

Both motherboards have user-friendly BIOSes with lots of adjustable parameters. In particular, CPU voltage is adjustable from the default setting of 1.324V down to a minimum of 0.875V.

Overclockers won't be so happy as neither board allows any Vcore overvolting . This may not be a big issue, as other web sites have reported 20-30% overclocks of their Dothan P-M processors at stock Vcore. We tried some overclocking with each board: Both were able to run ~ 20% overclocked at the default Vcore. The DFI board is a little more OC-friendly as it has a larger selection of dividers to keep the memory and PCI/AGP bus speeds under control when the FSB of the CPU is increased.

Lowering the voltage of the CPU while keeping the CPU speed at default reduces CPU heat output greatly without any performance penalty. Less heat equals less need for cooling, which allows for slower, quieter fans, or maybe even no fan at all!

So it was with great anticipation that I approached this part of the testing. I ran a series of tests to determine the minimum stable CPU Vcore. This was accomplished by lowering the default Vcore in 0.1V increments, then running Prime95 for about an hour. If the system passed, I lowered the Vcore by another 0.1V and repeated the process. I kept dropping the Vcore till errors started started showing up in Prime95. At this point, I increased the Vcore in small increments until Prime95 would run for 48 hours with no errors. Both boards gave me the same, 1.1V Vcore minimum threshold for this particular 2.0GHz Dothan.

Temperatures with Stock Heatsinks, CPU at 1.1V
Board / HSF voltage
°C rise
AOpen - 12V
DFI - 12V
AOpen - 5V
DFI - 5V

With the AOpen, CPUBurn load temp dropped by 11°C! That's an 11°C temperature drop with no loss of performance and no increase in fan speed. It's like getting a cooler CPU for free, and will actually save money due to lower power requirements over the life of the CPU. The DFI board got almost the same temperature reduction as the AOpen board, an impressive 10°C.

Lowering the CPU fan speeds to 5V was the next step, and these results were even more impressive. At the lower Vcore, the AOpen load temp dropped from 54°C to 39°C, a 15°C drop. With the tiny DFI heatsink, it went from 73°C to just 45°C! That's a 28°C drop, just by lowering the Vcore by 0.22V!

So how much of a power drop effected such thermal reductions? To estimate power dissipation of the Pentium M 2.0 Dothan at the reduced 1.1V, we used the nifty utility CPU Power (v2.1) by Kostik. Simply edit the text file database in the utility to add the data for this processor, then calculate the new TDP value, which turns put to be 14.1W. It's only a 6W difference, but it's also a 30% drop. This is roughly confirmed by our Seasonic Power Angel power monitor which showed system AC power draw under full load to be 56W at default Vcore and 48W when running the CPU at 1.1V. Factor in the ~25% AC-to-DC conversion power loss and we have a 6W difference in DC power as calculated.

Fanless with Stock HS?

The reduced Vcore testing showed amazing temperature reductions with absolutely no performance penalty with both stock heatsinks, neither of which is anywhere near "high performance". Both stock fans are pretty quiet at 5V, but the obvious next question is, "Can either of these heatsinks be run without any fan at all?" My experience with the similarly cool PIII Tualatin processors told me that they would probably run too hot, but since these Pentium M's are so well protected thermally, I decided to try anyway.

The fans were removed from both heatsinks, and the CPU temp warnings in Motherboard Monitor turned off before running load tests. The test platform sits on an open bench, so there was no secondary airflow from either case fans or a PSU fan.

On the DFI board with the tiny little heatsink, the CPU temperature slowly increased until it was at 75°C, whereupon the system crashed. Motherboard temperature was barely 40°C at this time. The CPU heatsink was too hot to touch for more than an instant. The fanless AOpen heatsink ended up in a BSOD at 76°C, although it took longer to reach that temperature. CPU throttling was monitored during this testing using Throttlewatch. No throttling took place, even at 75-76°C. System temps as reported by MBM5, never got any warmer than mid/high 30°C on either board during this fanless load testing. So, no fanless here, at least not with the stock heatsink.

Fanless with Stock Heatsinks, CPU at 1.1V
*System crashed at this temp. Test terminated.

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