CPU core voltage
Encyclopedia
The CPU core voltage is the power supply
voltage
supplied to the CPU
(which is a digital circuit
), GPU
, or other device containing a processing core. The amount of power
a CPU uses, and thus the amount of heat it dissipates, is the product of this voltage and the current it draws.
In modern CPUs, which are made using CMOS
,
the current is almost proportional to the clock speed
, the CPU drawing almost no current between clock cycles. (See, however, subthreshold leakage
.)
In order to help conserve power and manage heat, many laptop
and desktop
processors have a power management
feature that allows software (usually the operating system
) to adjust the clock speed
and core voltage dynamically
.
The trend is towards lower core voltages, which conserve power. This presents the CMOS designer with a challenge, because in CMOS the voltages go only to ground and the supply voltage, the source, gate, and drain terminals of the FETs have only the supply voltage or zero voltage across them.
The MOSFET
formula:
says that the current 
supplied by the FET is proportional to the gate-source voltage reduced by a threshold voltage
which is dependent on the geometrical shape of the FET's channel and gate and their physical properties, especially capacitance
. In order to reduce
(necessary both in order to reduce the supply voltage and to increase current) one must increase capacitance. But, the load being driven is in fact another FET gate. The current needed to drive it is proportional to capacitance, which thus requires the designer to keep it low.
The trend towards lower supply voltage therefore works against the goal of high clock speed.
Only improvements in photolithography
and reduction in threshold voltage allow both to improve at once. On another note, the formula shown above is for long channel MOSFETs. With the area of the MOSFETs halving every 18-24 months (Moore's law
) the distance between the two terminals of the MOSFET switch called the channel length is becoming smaller and smaller. This changes the nature of the relationship between terminal voltages and current.
When a processor is overclocked
the processor increases the core voltage at the cost of system stability, power consumption and heat dissipation. This is known as overvolting
. Overvolting generally involves running a processor out of its specifications, which may damage it or shorten CPU life. A notable overclock with extreme overvolting was performed by Tom's Hardware, where an Intel Pentium 4 processor
was run at 5 GHz while cooled with liquid nitrogen
.
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
supplied to the CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
(which is a digital circuit
Digital circuit
Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
), GPU
Graphics processing unit
A graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
, or other device containing a processing core. The amount of power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
a CPU uses, and thus the amount of heat it dissipates, is the product of this voltage and the current it draws.
In modern CPUs, which are made using CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
,
the current is almost proportional to the clock speed
Clock rate
The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
, the CPU drawing almost no current between clock cycles. (See, however, subthreshold leakage
Subthreshold leakage
The Subthreshold conduction or the subthreshold leakage or the subthreshold drain current is the current that flows between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages below the threshold voltage. The...
.)
In order to help conserve power and manage heat, many laptop
Laptop
A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...
and desktop
Desktop computer
A desktop computer is a personal computer in a form intended for regular use at a single location, as opposed to a mobile laptop or portable computer. Early desktop computers are designed to lay flat on the desk, while modern towers stand upright...
processors have a power management
Power management
Power management is a feature of some electrical appliances, especially copiers, computers and computer peripherals such as monitors and printers, that turns off the power or switches the system to a low-power state when inactive. In computing this is known as PC power management and is built...
feature that allows software (usually the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
) to adjust the clock speed
Dynamic frequency scaling
Dynamic frequency scaling is a technique in computer architecture whereby the frequency of a microprocessor can be automatically adjusted "on the fly," either to conserve power or to reduce the amount of heat generated by the chip...
and core voltage dynamically
Dynamic voltage scaling
Dynamic voltage scaling is a power management technique in computer architecture, where the voltage used in a component is increased or decreased, depending upon circumstances. Dynamic voltage scaling to increase voltage is known as overvolting; dynamic voltage scaling to decrease voltage is known...
.
The trend is towards lower core voltages, which conserve power. This presents the CMOS designer with a challenge, because in CMOS the voltages go only to ground and the supply voltage, the source, gate, and drain terminals of the FETs have only the supply voltage or zero voltage across them.
The MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
formula:
says that the current supplied by the FET is proportional to the gate-source voltage reduced by a threshold voltageThreshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
which is dependent on the geometrical shape of the FET's channel and gate and their physical properties, especially capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
. In order to reduce
(necessary both in order to reduce the supply voltage and to increase current) one must increase capacitance. But, the load being driven is in fact another FET gate. The current needed to drive it is proportional to capacitance, which thus requires the designer to keep it low.The trend towards lower supply voltage therefore works against the goal of high clock speed.
Only improvements in photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
and reduction in threshold voltage allow both to improve at once. On another note, the formula shown above is for long channel MOSFETs. With the area of the MOSFETs halving every 18-24 months (Moore's law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
) the distance between the two terminals of the MOSFET switch called the channel length is becoming smaller and smaller. This changes the nature of the relationship between terminal voltages and current.
When a processor is overclocked
Overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...
the processor increases the core voltage at the cost of system stability, power consumption and heat dissipation. This is known as overvolting
Dynamic voltage scaling
Dynamic voltage scaling is a power management technique in computer architecture, where the voltage used in a component is increased or decreased, depending upon circumstances. Dynamic voltage scaling to increase voltage is known as overvolting; dynamic voltage scaling to decrease voltage is known...
. Overvolting generally involves running a processor out of its specifications, which may damage it or shorten CPU life. A notable overclock with extreme overvolting was performed by Tom's Hardware, where an Intel Pentium 4 processor
Pentium 4
Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...
was run at 5 GHz while cooled with liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
.