Hello. New to KiCAD, but have done digital design before.
Wondering why I’m getting the frequent and dreaded “Pin connected to other pins, but not driven by any pin” in the particular schematic I’m creating. I have a 10K potentiometer (variable resister) with the two outer pins connected to the +5v and GND rails. Both rails are marked with PWR_FLAGs and connected to +5V and GND power connectors. The center wiper is connected to a device (LCD 4x20 V0 pin) whose pin is designated “Power input”. The pot’s wiper pin is designated “passive”.
Am I doing anything wrong? If not, any way to suppress the warning that still leaves a valid schematic?
ERC is a net based algorithm that knows nothing about the electrical properties of any device. All it does is check all pins on every net against its set of rules (Every net is looked at in complete isolation as if the rest of the schematic would not exist).
Which in this case means you must tell kicad that the wiper of the pot really is a source of power. You do this by either changing the symbol of the pot or possibly better by placing a power flag there.
Because that pin on the LCD is called “Power input”, the ERC is looking for a “Power output” pin to drive it. You’ve only provided it with a “Passive” pin. There are a couple ways to correct for this:
Change the pin type on either the LCD or Pot to make ERC happy (either put the new symbol in a local (to the project) library, or in your own custom libraries). Recognize that this might cause issues on future projects if you use those newly modified symbols.
Put a PWR_FLAG on the “output” (wiper) of the Pot to indicate this is a power “supply” for the pin of the LCD.
Ignore this specific ERC hit as the ERC isn’t telling you that it won’t work, rather it is saying that that net looks fishy and you should make sure it really is what you intended.
Remember, the ERC doesn’t know anything about the function of any of the symbols. It doesn’t know the function difference between 3-pin pot, a 3-pin transistor, or a 3-pin auto-transformer (or any other 3-pin device). It only knows what it has been told what the pin-types are. It compares against a matrix (see the options tab of the ERC window), and checks that each net that has an input has a single output of the same type (power or signal). For signal input nets I forget if a tri-state pin will be acceptable by ERC as a signal output. But, even if it is, if you never use that tri-state pin as an output your circuit won’t work and ERC will never know.