Modern Use
CMOS ICs have generally borrowed the NMOS convention of VDD for positive and VSS for negative even though both positive and negative supply rails connect to source terminals (the positive supply goes to PMOS sources, the negative supply to NMOS sources). ICs using bipolar transistors have VCC (positive) and VEE (negative) power supply pins.
In many single supply digital and analog circuits the negative power supply is also called "GND". In "split rail" supply systems there are multiple supply voltages. Examples of such systems include modern cell phones, with GND and voltages such as 1.2 V, 1.8 V, 2.4 V, 3.3 V and PCs, with GND and voltages such as -5 V, 3.3 V, 5 V, 12 V. Power-sensitive designs often have multiple power rails at a given voltage, using them to conserve energy by switching off supplies to components which are not in active use.
More advanced circuits will often have pins carrying voltage levels for more specialized functions and these are generally labeled with some abbreviation of their purpose. For example VUSB for the supply delivered to a USB device (nominally 5 V), VBAT for a battery, or Vref for the reference voltage for an analog-to-digital converter. Systems combining both digital and analog circuits often distinguish digital and analog grounds (GND and AGND), helping isolate digital noise from sensitive analog circuits. High-security cryptographic devices and other secure systems sometimes require separate power supplies for their unencrypted and encrypted (red/black) subsystems to prevent leakage of sensitive plaintext.
Read more about this topic: IC Power Supply Pin
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