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I was designing new schematics and decided to inspect another library downloaded from kicad official sources on Github. The one thing that cought my eye instantly was that Electrical type for both GND and VCC was POWER INPUT.
Is it meant to be that way or just a workaround for an existing problem, eg. failing DRC?
VCC and GND are meant to be power inputs. ERC on the schematic will check that all power inputs are driven i.e. have a power output somewhere on the net. It'll also make sure you don't connect two power outputs together. An example of a power output would be the output terminal of a regulator.
The idea starts to break down if you put passives (inductors, chokes, resistors) that break up the net, but it can be helpful when used properly.
As a newbie I found this difficult to understand and it remains very non-intuitive (at least from a programmer's perspective) to have the ground (logical circuit output) "logically" defined as an input.
The answer from @mbrig put me on the right track, but I still had errors elsewhere until I understood the requirements to separately flag each power network with the PWR_FLAG symbol, explained here: KiCad pin connected to some others pins but no pin to drive it.
Create this kind of group somewhere else on your EE schema diagram, continue to use the more intuitive +#V/VCC and GND symbols in your actual main circuit, and the errors go away.
One common misconception in electronics is that a wire not connected to anything will have a voltage of "zero volts".
Most people seem to understand that in a typical circuit with lots of stuff connected to "+5V", there should be one thing (typically a connector accepting power from some outside power source, or a battery connector, or etc.) that actively drives that wire to +5V, and lots of loads (such as an ATmega processor or a LED driver or a 74x00 series chip or etc.) connected to +5V that consume power.
When only loads are connected to a +5V wire, KiCAD doesn't see how they are getting power, so the KiCad ERC correctly warns you that as far as it can tell nothing will get power. (When I have a battery and a flashlight bulb, and I only connect a GND wire between them, leaving the other side disconnected, the bulb stays dark).
For some reason, people don't always pick up on the fact that GND acts exactly the same way.
There should be one thing (typically a connector accepting power from some outside power source, or a battery connector, or etc.) that actively drives the GND wire or the GND plane to 0V, and lots of loads (such as an ATmega processor or a LED driver or a 74x00 series chip or etc.) connected to GND that consume power.
When only loads are connected to GND wire, KiCAD doesn't see how they are getting power, so the KiCad ERC correctly warns you that as far as it can tell nothing will get power. (When I have a battery and a flashlight bulb, and I only connect a +5V wire between them, leaving the other side disconnected, the bulb stays dark).
For the vast majority of integrated circuits, both +VCC and GND are power inputs. Something else needs to actively drive both of those pins to the right voltage, or the chip won't work.
This is related to the common misconception that electrical charges carry electrical energy -- the incorrect idea that batteries somehow "fill up" electrical charges with energy, the incorrect idea that "energetic charges" carry that energy through the +5V line to the chip which drains the charge of energy, and then the incorrect idea that "non-energetic charges" flow back through the GND wire to the battery to be charged up again.
In fact, electrical energy flows from the battery to the IC in one direction (from the battery to the chip) near the +5V and the GND wires at a large fraction of the speed of light. Simultaneously, electrical charges flow in opposite directions (one from the battery to the chip, the other from the chip back to the battery) inside the +5V and the GND wires at perhaps a few millimeters per minute. What's important for most circuits (and supported by KiCad's ERC/DRC) is the the flow of electrical energy.
Technically, ground can be at a higher or lower voltage then the output ( a positive or negative voltage supply for example). From this perspective ground pins should be bi-directional power pins. Current could be flowing either into or out of the ground pin, depending on the voltage of the output pins.
