1. Naming Power Rails Vdd or Vcc

    Today I was comparing some connections on a schematic to a reference design and I noticed the reference was using a net name ‘Vdd’ for a power rail that I had named ‘Vcc’. In most of my work, I use Vcc because I seem to have encountered it more in datasheets and references, or so it seems. For some designs, I’ve applied a Vcc prefix to all of my power rails, so when my net names are alphebetized, they all appear in a group, making it easier to select this group of nets in the PCB layout.

    Today I questioned what is the implied difference when a rail is named Vcc or Vdd. The historical difference is that Vcc was used in BJT circuits for connections to the transistor’s collector. Vdd came about when FET technology entered the world, and indicates a connection to a FET’s drain. On a similar note, “Vee” comes from BJT technology and indicates a connection to the collector while “Vss” comes from FET technology and indicates a connection to a FET’s source. (Reference: Vcc on Wikipedia)

    Really either convention will suffice, but as the hardware design field continues moving forward, it never hurts to know the history behind a naming convention.