In electronics, especially when dealing with microcontrollers and integrated circuits (ICs), you will come across various pin designations such as VCC, VEE, VDD, and VSS. Each of these terms refers to different types of power supply pins:
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VCC - This originally stood for "Voltage at the Common Collector." The terminology comes from bipolar transistor logic, where the collection power supply is usually connected to a positive voltage. Today, VCC refers to the positive voltage supply for any IC, typically in TTL (Transistor-Transistor Logic) based circuits.
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VEE - This stood for "Voltage at the Emitter Emitter," corresponding to the negative power supply connection in systems using bipolar transistors, particularly in ECL (Emitter-Coupled Logic) circuits. In other types of ICs, it might refer to the most negative supply voltage in circuits that use both positive and negative power.
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VDD - This traditionally stood for "Voltage at the Drain Drain," with this nomenclature coming from MOS transistor logic where the drain terminal is connected to the positive supply voltage. It is now a common term for the positive supply voltage pin on CMOS (Complementary Metal–Oxide–Semiconductor) ICs.
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VSS - This stands for "Voltage Source Source." Much like VDD, this term comes from MOS logic, denoting the source terminal. VSS refers to the ground pin (0V reference) in a circuit and would be the most negative part of the power supply, serving as the return path for the current to the power supply.
It's worth noting that while the VCC and VDD both refer to the positive supply voltage, VCC is often used in the context of TTL logic levels, whereas VDD is used with CMOS logic levels. Both VEE and VSS serve as points of low potential (ground), but VEE is typically associated with a negative voltage in systems that use a dual power supply, whereas VSS is always associated with the system ground.