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[nextpage title=”Introduction”]

One of the oldest tricks to increase the probability of a successful overclocking is to increase the voltage from the component you want to overclock. Nowadays, even entry-level motherboards present some voltage adjustments, with high-end models coming with a myriad of them. The problem is that even hardcore enthusiasts have a hard time understanding what each option really means. In this tutorial, we will explain the exact meaning of each one of them in clear language.

The motherboard manufacturers are the ones to blame for this whole confusion. Even though CPU and chipset manufacturers have official names for all voltages their components use, each motherboard manufacturer for some strange reason calls the same thing with different names. Usually, the manual does not explain the meaning of each function – manuals usually simply repeat the name of the function as an “explanation.”(Duh!) The same is true if you ask for help inside the motherboard setup.

Voltage options are changed inside the motherboard setup, which is entered by pressing Del (or F2 on some motherboards) after turning on the computer. But we think you knew this already, since you are interested in a very specific subject.

To understand voltages, you need to understand a little more about how each CPU manufacturer deals with voltages in their product line.

[nextpage title=”AMD Processors”]

Processors from AMD make use of the following voltages (the names below are their “official” names, as set by AMD):

  • VDD: This is the main CPU voltage, which can also be unofficially referred to as Vcore. Usually, when we say “CPU voltage” we are talking about this voltage. The option that changes this voltage will show up on the motherboard setup as “CPU Vcore,” “CPU Offset Voltage,” “CPU Voltage at Next Boot,” “CPU Vcore 7-Shift,” “Processor Voltage” or “APU-Core Over Voltage.”
  • VDDNB: This is the voltage used by the CPU integrated memory controller, by the CPU HyperTransport controller, and by the CPU L3 memory cache (if available). These components are collectively called “NB” or “North Bridge” by AMD. The problem is that one of the chips from the motherboard chipset can also be called “NB” or “North Bridge,” and most users will get lost trying to figure out what is really being configured when an option has “NB” on it; thus, we will have to explore this subject in more detail. On AMD CPUs up to socket AM2, the VDD and VDDNB voltage are the same. Starting with Socket AM2+ CPUs, AMD started to use separate voltages for the CPU and for the memory controller. (AMD calls this “split plane” or “Dual Dynamic Power Management.”)
  • VDDA: This is the voltage used by the clock multiplier circuit inside the CPU, also known as PLL (Phase-Locked Loop). This voltage can be changed through options like “CPU VDDA Voltage” or “CPU PLL Voltage.” Usually, only high-end motherboards have this option.
  • VDP: On AMD “APUs” (CPUs with integrated graphics controller), the motherboard may have an option for you to set the voltage of the graphics controller, called “VDP Voltage,” “IGD Voltage” or “IGP Voltage.”
  • VDDIO: This is the voltage used by the signals on the memory bus. JEDEC (the organization that standardizes memories) calls this SSTL (Stub Series Termination Logic) voltage. This is the famous “memory voltage” configuration that can be found under several different names such as “DIMM Voltage,” “DRAM Voltage,” “Memory Over Voltage,” “VDIMM Select,” “Memory Voltage,” “DDR PHY,” etc. The default value for this voltage is 1.8 V with DDR2 memories (SSTL_1.8) or 1.5 V with DDR3 memories (SSTL_1.5).
  • VTT: This is the voltage that is used to feed the termination logic inside the memory chips. By default, it is set as half of VDDIO. Pay attention because Intel CPUs have a voltage called VTT that has a different meaning/usage.
  • MEMVREF: This is the memory reference voltage, which “configures” both the CPU and the memory module with the voltage level that separates what is to be considered a “0” or a “1,” i.e., voltages found on the memory bus below MEMVREF are to be considered a “0,” and voltages above this level are to be considered a “1.” By default, this voltage level is half of VDDIO (a.k.a. 0.500x), but some motherboards allow you to change this ratio, usually through two options: “DRAM Ctrl Ref Voltage” (for the control lines from the memory bus; JEDEC’s official name for this voltage is VREFCA), and “DRAM Ctrl Data Ref Voltage” (for the data lines from the memory bus; JEDEC’s official name for this voltage is VREFDQ). These options are configured as a multiplier. For example, “0.395x” means that the reference voltage will be 0.395 times VDDIO.
  • VLDT: This is the voltage used by the HyperTransport links from the CPU. This voltage is referred to as “HT Voltage,” “HT Over Voltage,” “NB/HT Voltage” and similar names. The default value for this voltage is 1.2 V.
  • PCI Express Voltage: On AMD “APUs” (CPUs with integrated graphics controller), the processor has an embedded PCI Express controller, which is used for connecting the CPU to an external video card. Some motherboards have an option for you to set the voltage for the PCI Express lanes controlled by the CPU, through an option called “APU PCI-E Over Voltage” or similar. Keep in mind that the chipset also controls more PCI Express lanes, and the motherboard may have a separate voltage adjustment for these lanes.

The challenge on motherboards targeted to AMD processors is to figure out what “NB” means inside the voltage configuration options. As explained, “NB” can mean the North Bridge (memory controller, HyperTransport controller, and L3 cache, if present) inside the CPU or the North Bridge chip from the chipset. Here are some hints to find out which one is applicable.

If “NB” is written together with “CPU,” “APU,” or “Processor,” then the option is to configure the VDDNB voltage line from the CPU. For example: “CPU/NB Voltage,” “CPU NB Over Voltage,” “CPU/NB Offset Voltage,” “Processor-NB Voltage,” and “APU-NB Over Voltage.”

If there is only one voltage option using the name “NB,” then it is probably used to configure the VDDNB voltage line.

If there are more voltage options showing up as “NB,” and the motherboard also has a “CPU/NB Voltage” option, these other options are for the chipset and not for the CPU. For a real example, consider a motherboard that has these three options: “CPU/NB Voltage,” “NB Voltage” and “NB 1.8 V Voltage.” The first option refers to the CPU VDDNB line (memory controller, HyperTransport interface, and L3 cache), while the other two refer to the motherboard chipset.

The default voltages vary depending on the CPU. One of the first things a serious overclocker should do before trying to change voltage options is to discover what the default values are for the CPU. This can be found in a document from AMD called “Power and Thermal Data Sheet,” which has a version for each CPU family.

[nextpage title=”AMD Processors – Chipset Options”]

Chipset-related options include all voltages that are not the ones described on the previous page. They include:

  • NB Voltage: If you made sure that the “NB Voltage” option on your motherboard does not relate to th
    e CPU VDDNB voltage (see previous page), then this option refers to the voltage from the North Bridge chip from the chipset.
  • NB 1.8 V Voltage: Chipsets from AMD use two separate voltages: one with 1.2 V (which is configured through the option above and called VDD_CORE) and another with 1.8 V, which is configured through this option, and is usually the voltage used by the chipset clock multiplier circuit (PLL, Phase-Locked Loop).
  • FCH Voltage: Chipsets targeted to “APUs” (CPUs with integrated graphics controller) are called FCHs (Fusion Controller Hubs). Therefore, this option controls the chipset voltage and is equivalent to the “NB Voltage” option.
  • Graphics engine voltage: This option, available on some motherboards with on-board video, allows you to increase the voltage from the chipset integrated video controller, which is useful if you are overclocking the motherboard graphics engine. This option is also known as “mGPU Voltage,” “IGD Voltage,” and “IGP Voltage.”
  • SidePort voltage: This is the voltage that feeds an on-board video memory chip used by the on-board graphics engine from the motherboard, on motherboards that have this feature.
  • SB voltage: This is the voltage to be used by the South Bridge chip from the chipset.
  • PCI Express voltage: This is the voltage to be used on the PCI Express lanes that are connected to the chipset. You may want to raise it if you overclock these lanes. It can be found through options such as “PCIE VDDA Voltage,” “VDD PCIE Voltage,” and “PCI-E Over Voltage.”

[nextpage title=”Intel Processors”]

Intel processors use the following voltages (the names below are the official ones):

  • VCC: This is the main CPU voltage, which can also be unofficially referred as Vcore. Usually, when we say “CPU voltage” we are talking about this voltage. The option that changes this voltage will show up on the motherboard setup as “CPU Voltage,” “CPU Core,” etc.
  • VTT: This is the voltage rail that feeds the integrated memory controller (on CPUs that have this component), the QPI bus (on CPUs that have this component), the FSB termination (on CPUs that are based on this architecture), the L3 memory cache (on CPUs that have this feature), the thermal control bus (PECI, Platform Environmental Control Interface, on CPUs that have this feature, except from second-generation Core i processors on, where this bus is fed by the VCCIO voltage) and other circuits, depending on the CPU. It is important to understand that on AMD CPUs, “VTT” is the name of a different voltage; the VTT on Intel CPUs is the equivalent of the VDDNB from AMD CPUs. This voltage can be changed through options such as “CPU VTT,” “CPU FSB,” “IMC Voltage,” and “QPI/VTT Voltage.”
  • VCCSA: Starting with the second-generation Core i processors (“Sandy Bridge”), the VTT voltage was renamed to VCCSA, and is called “system agent.” It feeds the integrated PCI Express controller, memory controller, and display engine (i.e., the “2D” part of the graphics engine).
  • VCCIO: Available starting with the second-generation Core i CPUs (“Sandy Bridge”), this voltage is used for feeding all input/output (I/O) pins of the CPU, except memory-related pins. On CPUs that have this voltage, it is also used to feed the thermal control bus (PECI, Platform Environmental Control Interface).
  • VCCPLL: Voltage used by the CPU clock multiplier (PLL, Phase-Locked Loop). This voltage can be changed through an option called “CPU PLL Voltage.”
  • VAXG: Voltage used by the video controller embedded inside the CPU, on CPUs that have this component. This option can be called by names such as “Graphics Core,” “GFX Voltage,” “IGP Voltage,” “IGD Voltage,” and “VAXG Voltage.”
  • CPU clock voltage: Some motherboards allow you to increase the voltage of the CPU base clock, through options called “CPU Clock Driving Control” or “CPU Amplitude Control.”

Now let’s take a look at the memory options.

[nextpage title=”Intel Processors – Memory Options”]

While all CPUs from AMD have an embedded memory controller, this is not true with models from Intel, where only the newer models (Core i3, Core i5 and Core i7) have this feature. Therefore, voltages present on the memory bus can be produced either by the CPU or by the North Bridge chip from the chipset (MCH, Memory Controller Hub), depending on the platform you have. This is why we are posting memory-related voltages on a separate page.

The memory bus requires three different voltages:

  • VDDQ: This is the voltage used by the signals on the memory bus. JEDEC (the organization that standardizes memories) calls this SSTL (Stub Series Termination Logic) voltage. This is the famous “memory voltage” configuration, which can be found under several different names such as “DIMM Voltage,” “DIMM Voltage Control,” “DRAM Voltage,” “DRAM Bus Voltage,” “Memory Over-Voltage,” “VDIMM Select,” “Memory Voltage,” etc. The default value for this voltage is 1.8 V with DDR2 memories (SSTL_1.8) or 1.5 V with DDR3 memories (SSTL_1.5).
  • Termination voltage: This is the voltage that is used to feed the termination logic inside the memory chips. By default, it is set at half of the VDDQ/SSTL voltage (“memory voltage”). This option, if available, will be listed as “Termination Voltage” or “DRAM Termination.” Pay attention, because this voltage with AMD CPUs is called VTT, but with Intel CPUs VTT is the processor’s secondary voltage (see previous page).
  • Reference voltage: The memory reference voltage “configures” both the memory controller and the memory module with the voltage level that separates what is to be considered a “0” or a “1,” i.e., voltages found on the memory bus below the reference voltage are to be considered a “0,” and voltages above this level are to be considered a “1.” By default, this voltage level is half of the SSTL voltage (a.k.a. 0.500x), but some motherboards allow you to change this ratio, usually through options like “DDR_VREF_CA_A,” “DRAM Ctrl Ref Voltage,” and similar. “CA,” “Ctrl” and “Address” refer to the control lines from the memory bus (JEDEC’s official name for this voltage is VREFCA), while “DA” and “Data” refer to the data lines from the memory bus (JEDEC’s official name for this voltage is VREFDQ). These options are configured as a multiplier. For example, “0.395x” means that the reference voltage will be 0.395 times the SSTL voltage. Usually, motherboards targeted to Intel processors allow you to control these voltages for each memory channel. So “DDR_VREF_CA_A” means the control reference voltage for channel A, while “DDR_VREF_CA_B” configures the control reference voltage for channel B.

[nextpage title=”Intel Processors – Chipset Options”]

Chipset-related options include all voltages that are not the ones described in the previous page. They include:

  • North Bridge voltage: This is the voltage that feeds the North Bridge chip from the motherboard chipset. It is important to note that Intel calls the North Bridge chip the MCH (Memory Controller Hub, on motherboards targeted to CPUs without an embedded memory controller), IOH (I/O Hub, on motherboards targeted to CPUs that have an embedded memory controller using a two-chip chipset solution) or PCH (Platform Contoller Hub, on motherboards targeted to CPUs that have an embedded memory controller using a s
    ingle-chip chipset solution), so the name of this option can vary. PCH chipsets have two separate voltages, VccVcore (usually referred on the motherboard setup as “PCH 1.05 V” or “PCH Voltage,” which is the chip’s main voltage) and VccVRM (usually referred on the motherboard setup as “PCH 1.8 V” or “PCH PLL Voltage,” which feeds the clock multipliers inside the chip).
  • South Bridge voltage: This is the voltage that feeds the South Bridge chip from the motherboard chipset. It is important to note that Intel calls the South Bridge chip the ICH (I/O Controller Hub). Therefore, the name of the option can vary – “SB Voltage” and “ICH Voltage” are common names.
  • PCI Express voltage: If you want to change the PCI Express voltage, you will have to study and see where each PCI Express slot or lane is connected on your system. For example, some Intel CPUs can control one x16 or two x8 PCI Express connections for video cards, with the lower-speed slots being controlled by the chipset (PCH). On some other configurations, the PCI Express x16 slots are provided by the North Bridge chip (MCH or IOH) while the lower-speed PCI Express slots are controlled by the South Bridge chip (ICH). The voltage used by PCI Express lanes is usually hard-wired to the chip’s voltage rail and, therefore, are automatically changed when you change the CPU, the North Bridge (PCH/MCH), or the South Bridge voltage, depending on where the lanes are connected to. Some chipsets (most notably Intel X58) have a separate voltage supply for the PCI Express lanes, and on motherboards based on such chipsets, you may find separate configurations for adjusting the PCI Express voltage. For example, “IOHPCIE Voltage” would adjust the voltage from PCI Express lanes controlled by the motherboard North Bridge chip (IOH), while “ICHPCIE Voltage” would adjust the voltage from the PCI Express lanes controlled by the motherboard South Bridge chip (ICH).
  • PCI Express clock voltage: Some motherboards allow you to increase the voltage from the clock signal used by the PCI Express lanes. This option is called “PCI-E Clock Driving Control” or “PCI Express Amplitude Control.”