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.