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When you start overclocking a system by increasing its main frequencies, you'll also need to increase a few important voltages along the way, such as vcore, VTT, DRAM, PLL, and PCH.
In this article, we'll go over how these voltages work for Intel's Core i7, i5, and i3 CPUs. We'll also specify the defaults and recommended maximums for LGA 1156-based systems (Clarkdale and Lynnfield). However, most of this article should also be relevant to other systems as well.
If you want to get a look at what the voltage area of the BIOS looks like for our ASUS P7P55D-E Pro motherboard, take a look at the picture below.
Vcore (aka V-Core, CPU Voltage, Core Voltage)
Vcore determines how much voltage the CPU gets, and as you increase the CPU speed, you'll eventually have to start increasing vcore as well.
The default vcore depends on your processor's VID, which is different for every CPU and is set by Intel in the factory, but it's usually around 1.1 - 1.2 V. The absolute maximum vcore voltage recommended by Intel is 1.4 V, so I'd recommend staying under that for everyday use.
VTT (aka IMC, QPI/VTT, QPI/DRAM)
VTT goes by many names depending on what brand of motherboard you have, but it is the voltage for the Integrated Memory Controller (IMC) inside of the CPU. This controller speaks directly with the RAM, and as you increase the system's base clock rate ( BCLK), the IMC frequency also increases, so you'll have to up this voltage as you up BCLK.
The default VTT for Clarkdale and Lynnfield based processors is 1.1 V, and the absolute maximum (as specified by Intel) is 1.4 V.
DRAM / VDIMM
This voltage is directly fed to your RAM and will only need to be increased if you increase the speed of your memory or tighten its timings.
The default DRAM voltage for socket 1156 motherboards is 1.5 V, but the voltage you will actually use will depend on the specifications for your RAM. Intel's absolute maximum for this is 1.8 V, but they recommend that you keep it under 1.65 V.
Your DRAM voltage should never be more than 0.5 V above VTT, or it could cause system stability problems. Since the default VTT is 1.1 V, this means that your DRAM voltage shouldn't be higher than 1.1 + 0.5 = 1.6 V--unless you also increase VTT. So if you're using memory that has a default voltage of 1.65 V or you've overclocked to that amount, you should increase your VTT voltage to at least 1.65 - 0.5 = 1.15 V.
CPU Phase Locked Loop (PLL)
This option can be used to stabilize the CPU at high BCLKs. Some people need to change this, and some don't. You may need to start increasing this once you get to a BCLK of around 180 - 200.
The default PLL voltage is 1.8 V, and Intel's absolute maximum for this is 1.98 V, but you shouldn't need to get anywhere near that. Intel's new i7, i5, and i3 CPUs don't require much of an increase in PLL, so you can probably get by increasing it to at most 1.9 V.
Platform Controller Hub (PCH)
The PCH voltage feeds the p55 chipset on the motherboard, which is responsible for talking with all of the peripherals on your computer, such as USB devices, hard drives, and any add-on cards--except for the video cards--which now talk directly to the PCI-Express controller integrated into the CPU.
You probably won't need to mess with this voltage, but I just mentioned it because some people say that they needed to increase it from its default of 1.05 V to 1.1 V to achieve stability. I've never needed to mess with this though, so I'd only play with it as a last resort.
This article is part of our ultimate overclocking guide: How to Overclock: Intel Core i7, i5, and i3 CPU Overclocking Guide.
If you're following along in the guide, you can get back to where you just were by clicking the following link: Understanding the Basics.