Managing equivalent units (for example for 74xx logic) - Page 1
Forum section: electronics / general

author: berke
author_rank: Frequent Contributor
timestamp: July 29, 2022, 08:16:02 am
content: Hi all,I have a bunch of logic gates (74xx series NAND, NOR) and I didn't pay much attention when capturing the schematic, so I have too many ICs as I'm only using one IC per gate! I can edit the annotation and the units by hand, but is there a better, less tedious way?(Yes, I did search the forum with terms such as "merging kicad logic"...)I'm using KiCad 6.0.6.

author: langwadt
author_rank: Super Contributor
timestamp: July 29, 2022, 10:53:29 am
content: wouldn't you want to do it by hand anyway?they might not all have the same voltage, you wouldnt' want to route across the whole board or to the opposite side of a chip if swapping makes it much easier, etc.

author: berke
author_rank: Frequent Contributor
timestamp: July 29, 2022, 11:18:53 am
content: Quote from: langwadt on July 29, 2022, 10:53:29 amThey're all the same but valid point for routing. That's what I ended up doing anyway. For posterity here is the workflow :- Generate a BOM and look at the collated components section, say you have 8 7400's- Go over each gate- Press E to edit the IC number, use the first available- Select the unit (A,B,C then D)- When done move to the next unit- There will be gaps in the IC numbering, so reannotateYes during layout I may have to do pin/gate/IC swapping.

author: retiredfeline
author_rank: Frequent Contributor
timestamp: July 29, 2022, 01:02:17 pm
content: Quote from: berke on July 29, 2022, 11:18:53 amThere is no problem with having gaps in the numbering, but if they bother you, do as you please. I sometimes take advantage of gaps, for example, the left channel of an amplifier might have components 1xx, and the corresponding ones on the right 2xx.

author: julian1
author_rank: Frequent Contributor
timestamp: July 29, 2022, 11:22:53 pm
content: One tip. Drop all the symbols Units on a schematic, whenever a new part is added. That way you can visually see what logic is used/available for routing at any time.It is preferable to do this anyway to ensure nothing is forgotten, and that any unused input is tied off to avoid oscillation/floating pins. I think DRC sometimes fails to check this condition.Otherwise, maybe you can work out the high-level routing on paper first.

author: berke
author_rank: Frequent Contributor
timestamp: July 30, 2022, 06:43:51 pm
content: Quote from: julian1 on July 29, 2022, 11:22:53 pmGood idea. I think schematic DRC doesn't complain about unconnected pins (or maybe they are warning but there are hundreds of those and most are silly) but they get caught during the PCB DRC.QuotePaper has its place, but I think some GUI code could help with this problem. Maybe something like a "Copy IC number". You enter copy IC number mode. You click on a source gate. You click on target gate. It picks the next gate from that IC that hasn't been touched yet. But it's a minor use case and there are more important things to fix in the GUI.Is this the kind of thing that is easier with commercial software ?

author: julian1
author_rank: Frequent Contributor
timestamp: July 30, 2022, 09:27:10 pm
content: I think Altium has a generic/ pin remapping system for fpga parts, that can be used to ease routing. Perhaps there is a public interface whereby one could create a model of possible pin substitutions so that repeating 74 series gate logic could be optimized.But there is also value in simplicity. And it is easy to imagine that these kinds of features would be very hard to get right/ and require a ton of user feedback. So I could see why it is not really a priority for Kicad feature development.