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Testing Participants: Preliminary Power Consumption Estimate for Tested CPUs

AMD Athlon II X2 255

AMD Athlon II X2 255 is the top processor model in Regor family that is based on proprietary dual-core semiconductor dies manufactured with 45 nm process. These processors are designed for Socket AM3 platform and are among the most affordable solutions for this platform. The nominal frequency of AMD Athlon II X2 255 is 3.1 GHz, and there is a 1 MB L2 cache for each of the two processor cores. You can see all CPU specifications on the following CPU-Z screenshot:

The nominal Vcore for our processor is 1.4 V, while the voltage of the North Bridge built into the processor is 1.175 V. The calculated TDP under load for this particular processor model is 65 W, according to the official specification. Our testbed equipped with this processor working in its nominal mode (without any overclocking) consumed the total of 111 W. Note that the power consumption measured along the 12 V processor power line was around 63 W, which is fairly close to the claimed calculated TDP.

As for overclocked mode without any Vcore increase, the CPU remained perfectly stable up until its clock speed hit 3.6 GHz. Further frequency increase was only possible if we increased the processor core voltage. As soon as we raised it to 1.5 V, we got our AMD Athlon II X2 255 working stably at 3.8 GHz.

I have to say that during our experiments we overclocked Athlon II X2 255 processor by raising the clock generator frequency, which means that not only the processor frequency increased, but so did the frequency of the North Bridge built into the CPU. However, it didn’t cause any problems for us: Athlon II processors do not have any L3 cache that is why they are less sensitive to overclocking than their elder brothers from the Phenom II series.

To reveal the type of dependence between the power consumption and clock frequency of our Athlon II X2 255, we took the corresponding readings in several key knots as stated in the table below:

All other voltages not mentioned in this table remained at their defaults.

The total power consumption of the test platform taken in the above mentioned system knots is shown on the graph. All readings were taken under maximum operational load created by LinX 0.6.4 utility.

You can easily notice that the power consumption increases mostly when the CPU Vcore is increased. Until this moment the power consumption graph remained pretty flat: 16-percent clock frequency increase from 3.1 to 3.6 GHz caused an only 8-percent increase in the total system power consumption. However, when the clock frequency increases from 3.6 to 3.8 GHz after raising the processor Vcore by 0.1 V, it leads to an additional 17-percent boost in power consumption.

The graph showing currents along the major mainboard power lines is an even better illustration for these numbers:

As we see, the biggest load falls upon the 12 V processor power line that delivers energy to our CPU. So, while overclocking barely affects the currents going through the 24-pin mainboard power connector, the power consumption along the CPU’s individual 12 V power line changes from 62 W to 91 W. Moreover, when we go from 3.6 GHz to 3.8 GHz frequency (namely, when further overclocking requires processor Vcore increase), CPU power consumption gets more than 20 W higher.

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