Couple Schematic/Symbols Questions - Page 1
Forum section: electronics / beginners

author: meshtron
author_rank: Regular Contributor
timestamp: November 14, 2023, 06:33:07 am
content: (I'm using Fusion 360 FWIW)I'm making some of my own symbols and footprints both to address some issues I've run into (non-optimal solder mask and stencil outlines) but also to be a bit more self-sufficient in my designs. Two things bothering me:1) My previous schematics have made (too) heavy use of just net labels. I think it's stylistically a bit better, and even better for moving from schematic to layout, to use connectors between components where I can. But, right out of the gate I have this situation (crossed lines pic) where the required pins trip all over each other between components. While this is maybe readable, would this be acceptable to those of you who are more experienced? I tried to find a way to get each connection to only cross one other line, but doesn't seem to be in the cards. Maybe I'm worrying about something that doesn't matter, but it looks a bit ugly to me.2) I made this symbol for my ATTiny and (heavily) based it off an existing library item and the symbol from TI. Now, I know symbols are designed to work with multiple footprints, but the pin layout on this symbol is a LONG ways off from the footprint. So, even if I can come up with a pleasant and clean-looking schematic, good chance I end up with lots of crossed wires during layout and/or choosing to swap pins (where possible) to help clean things up. Is that just "the workflow?" You make connections in schematic and then either revise or "deal with it" in layout? I could build a symbol (or a couple) that more closely matched the physical pin layout, and that seems like it would get me a lot closer - at schematic stage - to a viable and clean-ish layout.Anyway, kind of feel like I'm worrying about stuff that isn't all that important, but I'm going to make a concerted effort for the next (3rd) rev of my (first ever!) board to be more representative of a "commercial" product, but also workflow.As always, appreciate any thoughts and insights!

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 14, 2023, 07:37:55 am
content: Just come out of the sides of your ICs with short lines. Label them. Do not connect them. See example.Here's Dave's take on how to do this:

author: Terry Bites
author_rank: Super Contributor
timestamp: November 14, 2023, 10:47:13 am
content: That's life. You cant get away from crossovers without lot of faff. In time you'll get better at making things tidy and legible at the same time.Visualisation is critical skill, but again experience helps.Too many labels can make it hard to visualise the circuit.Keep them down to the easily remembered.Mostly I'd use them for busses, clock, reset, connectors and power nets.

author: Smokey
author_rank: Super Contributor
timestamp: November 14, 2023, 10:54:28 am
content: I never make schematic symbols with the pins numbered exactly the same way as the actual chip (like 1-8 left top to bottom, 9-18 right bottom to top). I hate library parts that do that. It makes terrible schematic flow.Check the part datasheet for the example circuit. They usually have a reasonable symbol pin layout so they can make the example circuit look more simple.Even with optimal symbols, I would MUCH rather use more ports than cross wires in a schematic. But that's a personal preference.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: November 14, 2023, 11:31:27 am
content: Quote from: meshtron on November 14, 2023, 06:33:07 amIf you are hand soldering, beware that footprints for ICs in the data sheets are intended for commercial manufacture and often have the pad match exactly the pins of the IC. I try and remember to increase the pad length slightly so I can place a soldering iron tip on the edge of the pad. 1mm is probably enough.SJ

author: meshtron
author_rank: Regular Contributor
timestamp: November 14, 2023, 03:48:10 pm
content: Thanks for the thoughts, folks. And yes - the bit posted by @JustMeHere is how I have been doing my schematics, but I saw or read somewhere that people find it tough to "follow" that sometimes. And, looking at my schematics, I definitely realize they're clear to me because I "birthed" them and might be less so to someone else.Sounds like I just need about 5 years more experience!

author: meshtron
author_rank: Regular Contributor
timestamp: November 14, 2023, 03:59:36 pm
content: Quote from: Solder_JunkieYeah thanks. I am actually pasting/reflowing myself (because I'm a glutton for punishment) so I can get away with pretty tight pads. But, I've still extended the TQFN-28 pads on the MCU a bit to try to get good solder fillets on the small part of the pad that comes up the outside of the chip AND have easier access to test/probe those pins if needed. Here's the original footprint (from the ATMEL Eagle library) on the left and my "scratch-built" one on the right. Fixed the excessive solder paste on the thermal pad too.I'm still trying to decide if I want to tackle putting a couple thermal vias on the pad. I think they *should* be there, but I've had so many issues getting this chip to solder consistently (because I got HASL boards and the stencil was over-soldering it) that I'm afraid to add any new complexity now that I've got it reliable.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 15, 2023, 07:34:44 am
content: Quote from: meshtron on November 14, 2023, 03:48:10 pmI used to try to connect everything, but I found that creating functional blocks was way more useful. There aren't so many crossing wires in the schematic. It is easier to see where caps and resistors should be in relation to their function. It makes the layout step much easier. In the example, I know the caps are for power and decoupling the ESP32 and the resistor is for biasing the OE (output enable) data line.