Question:
Why are POWER PINS not becoming No Connect (NC) pins even after checking ‘Is No Connect’ property in Property Editor?

 

I have tried checking the 'Is No Connect' property in Property Editor and keeping the NC symbol on a power pin, but it still does not become an 'NC' pin.


Answer:

The No Connect checkbox in Property Editor does not work for the power pin.

 

To understand this, you should first understand that connectivity of the power pin is different from all other pin types in OrCAD Capture. Power pins get electrically connected to the signal of the same name by default, without any physical connection, that is, a power pin of the name VCC is connected to the signal of the name VCC in the design. Other pin types need to be connected through a wire to establish an electrical connection.

 

Now, the NC symbol or the ‘Is No Connect’ property does not change the electrical behavior of the pin. It is just a visual clue that a net is not to be connected to the pin. However, if you desire, a net can still be connected to that pin.

 

As we discussed before, power pins are automatically connected to the power signal with the same name. Making a power pin as NC requires changing the electrical behavior of the pin and disconnecting it from a signal, unlike other pins. So, marking a power pin as NC is done from the Assign power pin dialogue (select the design name and go to Tools > Assign Power Pin) where you can make it as NC or change its power name, both changing electrical connectivity of the pins. Also, this can only be done on the invisible power pins.

 

Currently, the visible power pins cannot be made as NC in Capture. For now, to prevent the connection of the visible of power pin to the same named signal, you need to connect a wire segment to the pin or make it invisible using the above method.