ERC PWR_FLAGS - Page 1
Forum section: electronics / beginners

author: darkling
author_rank: Newbie
timestamp: June 26, 2025, 09:51:34 pm
content: I am very new using KiCad and designing schematics so I'm having a difficult time placing the PWR_FLAG. I have a placed two of them in my USB_B but where ever I put them I still get an error message "Error: Input Power pin not driven by any Output Power Pins" what am i doing wrong? I appreciate any help.

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2025, 10:30:54 pm
content: Your XC6209 symbol is apparently imported from somewhere and has wrong pin definitions.Fix it and you won't need any "PWR_FLAG"s.

author: ataradov
author_rank: Super Contributor
timestamp: June 26, 2025, 11:26:09 pm
content: You will still need one one on VBUS and GND, if the symbol is correct, since USB connector is not a power source.I don't understand what the issue is here, this configuration should work. What are the other errors/warnings?In any case, go into the symbol editor for all involved components and check the pin types.

author: darkling
author_rank: Newbie
timestamp: June 27, 2025, 12:05:10 am
content: Thank you! I fixed the pin definitions by using the datasheet.

author: Benta
author_rank: Super Contributor
timestamp: June 27, 2025, 09:25:43 am
content: Quote from: ataradov on June 26, 2025, 11:26:09 pmActually, USB_B has power outputs.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 27, 2025, 09:56:07 am
content: Quote from: darkling on June 26, 2025, 09:51:34 pmI should probably be not telling this to beginner but I never run ERC. Thanks to this my schematics are clear from mess made by many PWR_FLAG symbols and X-es at not used IC pins.You can be surprised why I write about many PWR_FLAGs but imagine what happens if each IC you will supply via ferrite bead + C filter. ERC is simple rules checker looking at each net separately - it is not clever enough to understand that if you have power at one L (or fuse) side you also have it at the other side.Many uC can source currents like 20mA at their outputs. Many ICs need much smaller than 20mA current. You can switch on/off parts of your circuit by just supplying something from uC output. KiCad will ask for PWR_FLAG in such situation also.

author: Benta
author_rank: Super Contributor
timestamp: June 27, 2025, 05:03:01 pm
content: Quote from: PGPG on June 27, 2025, 09:56:07 amSomewhat sweeping statement.I wouldn't say Ialwaysuse ERC, but mostly.It simply saves one from stupid mistakes, which still occur even though you've quintuply-checked the schematic. I use simulation a lot, and a zero error/warning count at least takes one issue out of the situation. For the final schematic artwork, I usually remove the PWR_FLAG symbols to keep it clean.Your ferrite-bead example is valid, but there are ways around that.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 29, 2025, 04:41:29 pm
content: Quote from: Benta on June 27, 2025, 05:03:01 pmI believe that each stupid mistake I will notice while routing PCB. When routing track I all the time know what signal goes through it and think if this connection is OK or not.When first time designing PCB with KiCad (V5) I didn't know that it is not enough to change power symbol name to have new power net (in program I used previously it was enough). So I had VCC (=3V3) symbols and V5 (=5V) symbols at schematic and I was thinking they both are separate nets. I don't know if ERC would notice such mistake (invisible power Pin names in symbols were the same and I think ERC not looks at symbol names). When routing PCB I noticed that KiCad someway connected 5V with 3V3 and wants me to route track shorting pads that I know are at 5V and at 3V3.Quote from: Benta on June 27, 2025, 05:03:01 pmIn years 2000-2010 I have been using PSpice to simulate supply filters. I wanted to know the effect of parasitic C and L components. But since 2010 I practically didn't used Spice. I just believe that for things I'd like to check, results I will get from Spice will be not adequate for real circuits and I prefere to check this in real.Reading KiCad forum I noticed (with surprise) that people run Spice a lot. I spend one day with KiCad (V7) to check that simulation works to know that if I will be needing it than I have a simulator (PSpice demo version (I have got about 1995) refused to work when I moved from 32bit to 64bit Windows) but since then I have never needed it.Quote from: Benta on June 27, 2025, 05:03:01 pmThe only solutions I know is to not have power input pins in IC symbols or not run ERC (I use this second).When there were ERC discussions at KiCad forum I suggested to allow users to specify in symbol definitions that this symbol transfers power through it. The simplest would be it to work for only 2 pin symbols. I believe it would reduce more than 10 times the number of questions (at KiCad forum): ERC tells me that I have Power Input pin not driven by any Power Output pin. What I should do?Each such question clearly tells us that there are people who tries to use KiCad without even reading Getting Started.When I planned to use KiCad I have printed all manuals available and read them all before even installing KiCad for the first time.

author: Benta
author_rank: Super Contributor
timestamp: June 29, 2025, 09:30:55 pm
content: Quote from: PGPG on June 29, 2025, 04:41:29 pmYour belief only works for PCB routing if the schematic is OK. I can route anything I like, but it doesn't mean it will work.

author: PGPG
author_rank: Frequent Contributor
timestamp: June 30, 2025, 10:06:22 am
content: Quote from: Benta on June 29, 2025, 09:30:55 pmI don't agree with you.I can't imagine what kind of mistakes you can have in mind that you can do and not notice it without ERC.If I use IC I just copy everything around it from its standard application. If I use different element value - ERC will not find it. If I forget to place inverters at signal lines when they are needed - ERC will not find it.I have never tried to use ERC, but I know how it works as many times I took place in ERC discussions at KiCad forum.If transistor will be wrongly connected then when routing, even it is not shown at footprint I know where are b,e,c and I believe I will notice if their use is not correct.If the digital IC input will be left unconnected I believe I will notice it when routing as IC footprint for me is not just set of pads but I know what this circuit is doing and always check if see a pad being left unconnected why it is so.I have to think how circuit works when routing as not all information about how things should be routed can be expressed in netlist. For example HTRC110 we use need one pin being decoupled with 100n to GND but also being shielded by GND track around it.Of course it is only "I believe". Except the one situation I described (with connecting +5V with +3V3) I had no chance to check if I will really find schematic mistake.My schematics are rather simple (uC + supply and some inputs/outputs/communication ports) so till now just never happened to make error in schematic.