Almost all integrated circuits (ICs) have at least two pins which connect to the
power rails of the circuit they are installed in. These are known as the IC's power
supply pins. However, the labeling of the pins varies by IC family and manufacturer.
Typical supply pin labeling
Positive supply voltage
Negative supply voltage
The simplest labels are V+ and V−. However, internal design and historical tradition have led to a variety of
other labels being used. Also, V+ and V− may be confused with the + and − voltage inputs to ICs like op amps.
Sometimes one of the power supply pins will be referred to as ground. In digital
logic, this is nearly always the negative pin; in analog integrated circuits, it is most likely to be a pin intermediate in
voltage between the most positive and most negative pins [citation\ needed].
Double subscript notation uses similar looking placeholders with
subscripts. In that notation the subscripted letters denote two points.
VCC (note: lower case is often used instead of subscript, e.g. "Vcc") is an electronics designation
that refers to voltage from a power supply connected to the "collector" terminal of a bipolar transistor. In an NPN
bipolar junction transistor, it would be +VCC, while in a
PNP transistor, it would be −VCC.
There is debate over the origins of the double letter subscript naming
convention. One proposal is that it originated as an abbreviation of
the supply voltage for a common collector amplifier with the other
power supply names mimicking this fashion. Double letters may
also have been used to clearly indicate that a power supply voltage is
being referred to.
In general, double letter subscripts refer to power supply voltages. For example, VEE refers to the
"emitter" voltage as supplied by the power supply pin. In NMOS logic, VSS
refers to the "source" voltage, and VDD is the "drain" voltage likewise.
Within a circuit, single letter subscripts refer to that voltage relative to ground; for example, VC is the
"collector" voltage relative to ground. Two different letters indicate the voltage between two terminals; for example:
VBE is the "base" to "emitter" voltage drop, while VCE is the "collector" to "emitter" voltage.
CMOS ICs have generally borrowed the NMOS convention of VDD for positive and
VSS for negative despite the fact that both positive and negative supply rails actually go to source terminals
(positive supply goes to PMOS sources, negative supply to NMOS sources). ICs using bipolar transistors have VCC
(positive) and VEE (negative) power supply pins.
In single supply systems (e.g., most modern digital and analog circuits) the negative power supply pin is also commonly
referred to as GND. In "split rail" supply systems (e.g., older analog circuits) positive, negative and ground power supply pins
More advanced chips will often have pins carrying voltage levels for more specialized functions in or out of the chip and
these are generally labeled with some abbreviation of their purpose. For example VBUS for the 5 volt supply needed for
a bus-powered USB device or Vref for the reference voltage for an