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DDR3 Frequencies and Overclocking

Increased efficiency is not the only advantage of the new memory controller. Sandy Bridge processors boast extended list of supported memory frequencies. Although Core memory controllers officially support only DDR3-1066 and DDR3-1333 SDRAM, they can clock the memory at higher frequencies just fine, and, in fact, this is no news to us. For example, Core i7 LGA1156 processors have settings that allow using DDR3-1600 SDRAM, while overclocker Core i7-875K and Core i5-655K CPUs support even higher memory frequencies than that. LGA1155 processors from the Sandy Bridge family support memory frequencies even beyond DDR3-1600. Any Core i3, Core i5 and Core i7 LGA1155 processors also unofficially support DDR3-1866, DDR3-2133 and DDR3-2400 SDRAM.

The memory frequency in Core processors is set as the base clock generator frequency (BCLK) times a corresponding multiplier. The multipliers supported by each processor type actually determine the acceptable frequency intervals. Sandy Bridge processors have a longer list of supported multipliers than their predecessors. That is why the BIOS option responsible for the memory frequency in LGA1155 systems looks as follows:

However, do not forget that new LGA1155 systems do not allow overclocking by raising the base clock generator frequency. The changes made to the clocking algorithms in LGA1155 systems allow only a miniscule adjustment of the BCLK frequency: in most cases the system will immediately lose its stability if the BCLK is increased by more than 5% above the nominal setting of 100 MHz. In other words, while in LGA1156 systems we could increase BCLK frequency to overclock not only the processor, but also the memory, this approach will not work in the new LGA1155 platforms. Now overclocking is merely the multiplier adjustment, so it is quite logical that the new Sandy Bridge processors acquired a wider range of settings for DDR3 SDRAM. This way Intel preserves the opportunity for users to take advantage of those overclocker memory modules. In other words, we had to overclock the CPU to ensure that our memory could run at higher frequencies, but today we can use high-speed DDR3 SDRAM kits without any CPU overclocking. We don’t even need to have a special overclocker processor modification with an unlocked clock frequency multiplier: any LGA1155 CPU allows you to enable a memory mode like DDR3-1866 or DDR3-2133.

At the same time you do not need any tricks to ensure that Sandy Bridge will work stably with the memory running at high frequencies. If the memory modules you are using can work at a certain frequency and with certain timings originally, then all you need to do is enforce these settings in the mainboard BIOS and that’s it. The memory controller works perfectly fine with any available settings at its default voltage. The only thing we would recommend in order to ensure better stability with DDR3-2400 SDRAM is to increase slightly the System Agent voltage (VccSA), but no more than to 1.2 V. as for the memory DIMM voltage, the recommendations here remain the same as for any other Nehalem processors. It is not recommended to increase the memory voltage beyond 1.65 V, because it may cause the processor memory controller failure.

 
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