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Power Control Unit and Overclocking Functionality

Besides external interface controllers, another important part of the Sandy Bridge System Agent is Power Control Unit (PCU). Just like in Nehalem processors, this unit is a programmable micro-controller that collects information about temperatures and currents in different parts of the processor and can control its voltages and frequencies interactively. PCU implements power-saving functions as well as Turbo-mode, which developed even further in the new Sandy Bridge microarchitecture.

All functional units of the Sandy Bridge CPUs are split into three domains, each using its own independent clocking and powering algorithms. The first primary domain contains processor cores and L3 cache working at the same frequency and voltage.   The second domain is the graphics core working at its own frequency. The third domain is the System Agent itself.

This structure allowed Intel engineers to implement Enhanced Intel SpeedStep and Turbo Boost technologies simultaneously and independently for the processor cores and the GPU. They have already used the same approach in their mobile Arrandale processors, but that implementation was simple: it worked via driver. Sandy Bridge has a fully hardware solution that controls the frequencies of computational and graphics cores interdependently, considering their current power consumption. This way processor cores can be overclocked more effectively in Turbo mode when the graphics core is idle, and the other way around: the graphics core can be overclocked significantly when the processor cores are not fully utilized. You can get a great idea of how aggressive Turbo mode in Sandy Bridge is from the way the frequencies may increase over their nominal setting: by four increments for the CPU and up to 6-7- increments for the GPU.

However, these are far not all the innovative changes made to the Turbo Boost technology. The advantages of its new implementation imply that the PCU can control the frequencies in a more intellectual way, taking into account the actual temperatures of the processor units, and not only their power consumption. It means that when the CPU works in favorable thermal conditions, its power consumption can exceed the TDP maximum.

Processor load is usually very uneven during typical everyday work. Most of the time the CPU is in power-saving state and needs to work very fast only for short periods of time. The processors doesn’t get overheated within these short periods due to inertia from the cooler heat conductivity. The PCU unit in Sandy Bridge processors that controls the frequencies believes that nothing bad will happen if at this time the CPU is overclocked more dramatically than the TDP can theoretically allow. As soon as the processor temperature starts approaching the critical thresholds, the frequency will be dropped down to safe levels.

This automatically suggests that it more beneficial to use highly efficient cooling systems in Sandy Bridge based platforms for achieving maximum performance. But do not get overexcited: there is a hardware limitation that will restrict the maximum length of “beyond TDP” operation with 25 seconds.

As for regular overclocking, performed with traditional methods and tricks, we can expect quite a few serious changes here as well. And they will hardly be welcomed with too much enthusiasm. Striving for maximum integration, Intel had to move the base clock frequency generator being moved to the chipset in LGA1155 platforms. However, this measure isn’t the one to have a fatal effect on the common overclocking procedures. The main issue is that this clock frequency generator is the only one in the system and it is responsible for generating all the frequencies. And as we know, not all busses and controllers can actually tolerate overclocking that well. For example, when we increase the PCI Express frequency or USB or SATA controllers speed, instability may occur fairly quickly. This factor is going to be a serious obstacle during CPU overclocking by raising the clock generator frequency.

And the facts are as follows. Sandy Bridge processors have their clock generator frequency set at 100 MHz. The generator allows varying this frequency in a very broad range even with 0.1 MHz increment. However, any attempts to increase this frequency rapidly lead to system instability or failure. At this point we don’t know about any successful attempts to push the clock generator frequency beyond 105 MHz. In other words, the time tested overclocking method of raising clock generator frequency doesn’t work in Sandy Bridge processors and can only allow a 5% increase at best.

So the only way to overclock the upcoming LGA1155 processors is to increase their clock frequency multiplier. Intel is going to offer several Sandy Bridge based CPU modifications that will have an unlocked frequency multiplier and should theoretically overclock to 5.7 GHz (57 is the maximum multiplier allowed by this microarchitecture). However, these processors marked with a “K” in their model number will only be available in the upper price range and will cost a little more than their regular brothers.

The owners of ordinary CPUs will be offered artificially limited overclocking: these CPUs will also let you increase their clock multiplier, but only by four increments tops. Note that it is all only about overclocking, as the multiplier adjustment will not affect Turbo Boost technology, which will also contribute to this manual frequency increase. Moreover, Intel won’t limit the multipliers for the graphics core and memory frequency. In other words, it will be possible to overclock memory and graphics in any Sandy Bridge based system: a regular or an enthusiast one.

However, I doubt that it will be sufficient compensation for overclockers out there, so they will most likely be primarily interesting in CPUs with an unlocked multiplier, namely Core i7-2500K and Core i7-2600K. Especially, since the information available about their frequency potential seems to be very promising. For example, there is evidence that Core i7-2600K remains stable at up to 5.0 GHz frequency with only air cooling involved.


Screenshot courtesy of windwithme.

The above shown result was obtained with Prolimatech Mega Shadow Deluxe Edition cooler and the CPU core voltage increased to 1.45 V. Of course, this high voltage increase will hardly suit an everyday usage model, but we assume that at about 4.8 GHz frequency Sandy Bridge processors will be able to work 24/7.

 
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