AMD Phenom II X2 555
Besides Socket AM3 processors sold under Athlon II brand, we also took higher-end AMD CPUs for our today’s test session – Phenom II. Since the manufacturer offers solutions with different number of computational cores within this same family, we took one quad-core and one dual-core processor for our today’s investigative testing. For the dual-core model we went with Phenom II X2 555 - top Socket AM3 CPU with two computational cores and L3 cache memory. This processor is built on the same 45 nm semiconductor Deneb die as the quad-core Phenom II X4 CPUs. However, in this case two cores out of four are disabled. According to the specifications, Phenom II X2 555 works at 3.2 GHz frequency. Each of the two processor cores has its own L2 cache memory – 512 KB per core. Besides that, the CPU also has a shared 6 MB L3 cache.
Since AMD used the same 45 nm process for all their CPUs, it is not surprising that their electrical characteristics are similar. For example, our Phenom II X2 555 is designed for 1.4 V nominal core voltage, but differs from the above discussed Athlon II processors by slightly higher voltage of the integrated North Bridge that also contains L3 cache, which is set at 1.2 V.
The presence of L3 cache and higher clock frequency make the calculated TDP of the Phenom II X2 processors higher than that of Athlon II X2. The TDP for Phenom II X2 555, just like the TDP for other representatives of the same processor family, is set at 80 W. In reality, our Phenom II X2 555 processor consumed 74 W under maximum load and the total power consumption of the system equipped with it reached 123 W.
Deneb processor die is one of the best overclockable dies AMD has these days. Processors based on dies like that can often go as high as 4 GHz without any extreme cooling systems involved. Our Phenom II X2 555 was also no exception: we managed to push it as high as the notorious 4 GHz. However, we had to increase its Vcore by 0.15 V. but even if we hadn’t resorted to voltage increase, we would have still overclocked it quite nicely: it remained perfectly stable at 3.8 GHz with the nominal 1.4 V Vcore.
I have to say that Phenom II X2 555 belongs to the Black Edition series, which means that it has an unlocked clock frequency multiplier. Therefore, it is extremely easy to overclock without increasing the clock generator frequency or changing the frequency of the HyperTransport bus, memory or North Bridge integrated into the CPU. We definitely took advantage of this opportunity during our preliminary “estimate” test.
The table below shows the frequencies at which we took power consumption readings:
All other voltages that were not mentioned in the table above remained at their defaults. Moreover, I have to point out that this time we overclocked using unlocked clock frequency multiplier, which is a very convenient but not the most energy-efficient approach. The thing is that when you change the clock multiplier or set it to a certain fixed value, it disables Cool’n’Quiet technology that would drop the processor frequency in idle mode. Therefore, if you want to improve your system performance by overclocking and do not want to sacrifice the system energy-efficiency in idle mode, we wouldn’t recommend using the unlocked clock multiplier of AMD Black Edition processors.
The results of our total power consumption tests for the Phenom II X2 555 system taken under maximum load are given on the following graph:
Well, this is a pretty familiar picture that doesn’t surprise us at all. While we overclock our processor without increasing its core voltage, namely in the interval from 3.2 to 3.8 GHz, the total system power consumption increases linearly, by about 2-3 W per every 200 MHz. After that we see a sharp increase in power consumption caused by increased processor Vcore. As a result, the next 200 MHz on the CPU cost us quite a lot: 37 W.
The platform power consumption along the power lines is laid out below:
When the processor frequency as well as its core voltage rise, we can see the current increase along only one of the lines – the processor power line using a separate 8-pin cable. All other currents remain unchanged during overclocking. By the way, take a look how greatly processor power consumption may increase during maximum overclocking. Even the dual-core Phenom II X2 555 working at 4.0 GHz frequency needs 120 W of power, which is in fact 1.5 times higher than its calculated TDP. However, if you overclocked this processor without changing its core voltage, then the processor current won’t increase by more than 10%. Therefore, this overclocking should be considered absolutely safe: any mainboards, even the budget ones without enhanced processor voltage regulator circuitry will be able to cope with it just fine.