Question about power flags with microcontroller and charge pump

[alostmute]

Hello,
Relatively new to PCB design. I have never used power flags before. My microcontroller is connected to the board through pin headers. I have the 5V supply going to a charge pump which will output +/-10V supply to parts of the circuit. Will I need power flags on the 5V VCC line as well as the output from the charge pump of +10 and -10? Is every volage supply to be flagged this way? So if I add another 3.3V supply to be used in various parts of the circuit I need to flag that was well?

Thanks!

[paulvdh]

I think that the PWR_FLAG symbol is relatively unique to KiCad.
The Idea is that the power pins of IC’s are defined with the “Power Input” pin type, and the ERC complains if pins with this pin type are not connected to a “Power Output” pin.

The Output pins of voltage regulators are (should be) defined as the power output pin type, so if you have a voltage regulator on your PCB, that powers some IC’s then you (probably) do not have to add the “PWR_FLAG” symbol for such a net.

If However, you have for example an off-board power supply, and power is delivered via a connector, then KiCad does not know that, and you have to use the PWR_FLAG symbols to make the ERC happy.

Unfortunately the mechanism behind it is a bit simplistic. ERC is a relatively simple check, it is not a SPICE simulator, and if you have for example filter inductor or chokes or a fuse in your power supply then ERC does not recognize that and also complains, and can be silenced by adding more PWR_FLAG symbols.

The best way to handle it, is to first do some more reading, so you understand how the ERC and the PWR_FLAG works, then run ERC, and only add PWR_FLAG symbols where needed and after you understand why the ERC complained in the first place.

Some people do not like the PWR_FLAG symbols much and you can place them outside of the drawing area, and connect them for example with a local label. I prefer to put the PWR_FLAG symbols near the places where a power supply voltage is generated, or near the connector where it enters the PCB.

More details here:

ErrType(3): Pin connected to some others pins but no pin to drive it

Additional reading about pin types in general: Electrical type of schematic symbol pins (KiCad 4 and KiCad 5) A good explanation about power symbols can be found in this stackexchange answer by @Seth_h KiCad does check if every component is connected to supply lines (*1) as a missing supply is a common mistake to make. Getting the error “ErrType(3): Pin connected to some others pins but no pin to drive it” means that KiCad thinks that some supply pin is not properly connected. Either because …

[alostmute]

Running ERC does not currently generate any errors or warnings, this is with the power flag on the pin header that will connect to my microcontroller and the outputs of the charge pump. You said the ERC check is very simplistic so I will follow your suggestion and do some more readings before finalizing anything.
Thank you for the response and for linking further readings!

[Rene_Poschl]

I think there might be a mismatch in terminology here.

I would assume what you call power flag is what we would call a power symbol. This is a symbol that assigns a netname and allows to make a global connection (It is a global label). An example for such a symbol is the GND or VCC symbol from the official power library.

There is also one symbol called a PWR_FLAG. This symbol is to be used to tell ERC where the power comes from. It does not assign a netname and it does not make a connection.

[davidsrsb]

PCB DRC matters because it is a strong warning that you have errors that are likely to cause the PCB have manufacturing problems

Schematic ERC is just a hint that you may have design errors. As it is very hard to assign correct types to every pin, that is all it can be, a clue to make you stop and think.