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Intel Core i7 and Core i5 LGA 1155 Sandy Bridge
by Damien Triolet, Franck Delattre, Guillaume Louel et Marc Prieur
Published on March 21, 2011

Controlled overclocking
The lucky people who were first to see Sandy Bridge in action quickly discovered the overclocking issues with the platform. This is because of the bus, originally fixed at 100 MHz, which struggles to get beyond 108 MHz without making the machine crash. At first everyone naturally thought this limitation had been voluntarily imposed by Intel, seeing as they aren’t averse to overclocking, though only on models sold expressly for this purpose (the ‘extreme’ XE verions and the K series with an unlocked multiplier). Overclocking by increasing the bus clock holds across all the models and is therefore entirely out of Intel’s control.

The reason for the limitation was rapidly discovered: on Sandy Bridge, the clocks of the various buses are synchronised to the processor bus clock: PCI Express, PCI, SATA, USB are all clocked in direct proportion to the 100 MHz bus clock. This contrasts with previous platforms for which these clocks were generated asynchronously, that’s to say independently of the bus.

Desynchronisation of clocks, the system up till now, brings overall performance down beause the rationale behind it requires numerous cycles of waiting for accesses to the bus. Using a single clock gives a gain in performance but also means interdependency between clocks.

Several questions result. Intel says that the platform wasn’t supposed to be validated with synchronous clocks and that this is an unfortunate mistake rather than a deliberate choice. Intel assures us that forthcoming platforms (notably the high end versions on LGA 2011) will return to the asynchronous clock system. We’ll see! Voluntary or not, this overclocking limit could be expensive for Intel. We’ve seen plenty of users turn their noses up at CPUs in the past because of their lack of propensity to overclocking.

Intel has therefore made some concessions to help us swallow the medicine.

The most powerful models, (Core i7-2600 and Core i5-2500) thus exist in K versions, the multiplier for which can be freely fixed up to 57 x (or a maximum theoretical clock of 5.7 GHz) and which are priced only slightly higher than the standard versions (+ $23 for the 2600K, and + $11 for the 2500K). Note that to modify the multiplier on the K models, they must be installed on a P67 chipset and not an H67.

You can, all the same, dream a bit with the standard non-K models: choice of memory clock (DDR3 1067, 1333 but also 1600, 1867 and 2133), the GPU and setting the power threshold to regulate the turbo mode. You can increase the turbo multiplier by four notches, which takes you up an additional 500 MHz with four cores being used as you can see in the following table.

Overclocking isn’t completely dead then… just completely locked down by Intel, even if Intel are denying the deliberate nature of this officially. It will certainly be interesting to check out Intel’s choices when it comes to the forthcoming socket 2011 platform: will it bring back asynchronous clocks as announced? And if so, will it go back on the concessions made on the non-K versions, as well as on the attractive pricing of the K versions on LGA 1155?

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Overclocking in practice  

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