# Classification of VDD and VSS pins

I am creating a library component for KiCAD using a convenient online app at: http://kicad.rohrbacher.net/quicklib.php. The component I am making is the Microchip PIC24EP512GU814 in LQFP http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en554337.

The app asks me to assign a type to each pin. The types avilable are:

Input
Output
Bidir
Tri-state
Passive
Unspecified
Power Input
Power Output
Open Collector
Open Emitter


I must now assign one of these types to the VDD and VSS pins.

My best guess is that VDD should be assigned as Power Input. Would VSS also be considered as a power input?

Perhaps since VDD is analogous to VCC (and VSS to VEE), I could classify VDD as an Open Collector (and VSS as Open Emitter). However I'm not sure if VDD and VSS can be considered as "open" for an IC.

What should VDD and VSS be mapped to?

• Yes, VDD is the same as VCC. Jul 5, 2012 at 16:20
• @BrianCarlton That's right. Thankfully, before posting this question, I visited: electronics.stackexchange.com/questions/17382/…. Jul 5, 2012 at 16:24

I would recommend using Power Input for both VDD and VSS (or any power pins on an IC for that matter). If you choose to use these types of pin labeling, then it allows you to verify that all power supply pins are connected and that there is a source that can provide power.

If you set VDD or VSS as a Power Output then the software will see it as something that provides power for the circuit. This would also give you an error if you have multiple chips on the same power supply (e.g. two of these chips). The output of a linear regulator would be a good example of a Power Output.

Making both VDD and VSS the same is a bit counter intuitive compared to current flow, but these are being used as system terms, not electrical current/voltage terms.

• @stevenvh Yes :)
– W5VO
Jul 5, 2012 at 16:43
• You were absolutely right. I mistook pins for nets. (IIRC I once worked with an EDA package where you could also assign types to nets, hence my confusion. On second thought that doesn't make much sense. Whatever. Alzheimer Light, shall we say? :-)) Jul 5, 2012 at 16:47

The types are used to check that the connections you make make sense. If you connect two inputs together without any other pins on that net the DRC should give you a warning. So think what you would allow and not allow for Vdd and Vss.

I would assign Vdd as Power Input, since you can have several Power Inputs on the same net, with maximum 1 Power Output (from a voltage regulator, normally). For a moment I thought Vss could be passive, but that would allow you to connect an output to it, so that's not the right choice. Also Power Input for Vss, I think.