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Energy-Efficient AMD Athlon II Processors

AMD is still using pretty old 45 nm manufacturing process. Besides, their processor architecture can’t be called particularly energy-efficient. As a result, the power consumption of their major products is fairly high, according to today’s standards, and is even higher than the power consumption of Intel CPUs with similar performance. However, AMD found a way out of this situation by offering special more energy-efficient CPU models for those applications where low power consumption are heat dissipation are critical. The microarchitecture of these processors is not any different, and they are manufactured using the same production process, but they are built with carefully selected semiconductor dies capable of running at significantly lower voltages.

As a result, we can sort contemporary desktop AMD processors into a significant number of different groups based on their calculated TDP:

Obviously, the CPUs from the bottom part of the table suit better for quiet energy-efficient systems. Also, you should keep in mind that the chipset contributes a lot to the overall platform heat dissipation, therefore AMD’s energy-efficient processors with 45 W TDP seem to be the best choice for small Mini-ITX systems. The platform with these processors inside will have almost the same overall heat dissipation as dual-core LGA1156 Core i3 or Core i5 CPUs, which are pretty popular among Mini-ITX system builders today. Although these CPUs from Intel have 73 W TDP, the overall heat dissipation of systems built around them turns out pretty low due to the simple companion chip that consists of only one chip serving as a South Bridge. The calculations below explain it all:

The 10-watt difference can be eliminated for the following reasons. The calculated TDP as presented by AMD is very close to the practical readings that you can get off the CPU and the system. And if for some processors the TDP is claimed to be 95 W, it means that it in fact consumes (and dissipates) right about that amount of power in reality under maximum load. As for Intel, they use a slightly different way of calculating their processors’ TDP and allow slightly larger margins. As a result, the mass production CPUs from Intel consume (and dissipate) about 10% less power in real applications than stated in the TDP description.

AMD’s energy-efficient processor family with 45 W TDP includes a few models, which have a letter “e” at the end of the numeric index that stands for the model number. For our tests we selected three top models from the dual-, triple- and quad-core representatives of this family: Athlon II X2 250e, Athlon II X3 420e and Athlon II X4 615e. Their detailed specifications are given in the table below:

The specs seems to be pretty ordinary, which is actually not surprising at all, because they are based on the same semiconductor dies as the regular processors. Their power consumption and heat dissipation are in fact lowered using very straight-forward approach: by lowering the core voltage and the clock frequency. While standard Athlon II X4 and Athlon II X3 processors with 95 W TP have 1.425 V nominal Vcore, the core voltage of the energy-efficient models is limited by 1.25 V. Energy-efficient dual-core Athlon II X2 do not need their Vcore to be lowered that much, so they can work at up to 1.4 V core voltage. As for the clock frequencies, the 45 W Athlon II X4 and Athlon II X3 models are 600 MHz behind their 95 W counterparts, while the 45 W Athlon II X2 is 200 MHz slower than the regular 65 W analogue.

When we took a closer look at our specific units, we discovered that their actual Vcore is even lower than what we have just named:

Athlon II X2 250e

Athlon II X3 420e

Athlon II X4 615e

Obviously, the core voltage for a CPU is selected according to the features of the semiconductor die it is based on. We could prove it by measuring the maximum power consumption of our test samples under maximum load (created by LinX 0.6.4 utility), which they receive via their proprietary 12 V power line.

As we see, AMD falls very precisely into the claimed thermal envelope and 45 W CPUs consume exactly what they should, according to the spec. Since the models with different number of active cores have almost the same power consumption, their heat dissipation is also almost the same: when we measured the CPU temperature of our tested processors using the thermal diode under the CPU socket, all the reading were also extremely close.

In other words, it turns out that all AMD’s energy-efficient processors are similar to one another in power consumption and heat dissipation, however, they offer dramatically different performance, as they have different number of active cores and work at different clock frequencies.

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