How a devboard works (and how to make your own)(kaipereira.com) |
How a devboard works (and how to make your own)(kaipereira.com) |
Without that critical piece of design work, you may as well call this "How to build a Raspberry Pi Nano from scratch". Which, to be fair, is also a good article to write.
But step 1 for really building a dev board is answering the question, "What do I need from this that I can't get from a $5 Amazon purchase?"
A month of enjoyment tinkering on a hobby just for the hell of it.
The website went down because of the traffic and large images, so I've temporarily switched hosting, and it should stay up now (DNS propagation might take a bit though), but I'm going to get those images smaller ASAP, thanks to everyone who posted web archives!
I'm also going to alter some of the reasoning for some of the stuff like decoupling capacitors, but the guide is still meant for complete beginners, and lots of the terminology/reasoning can be pretty overwhelming, and I still have a lot to learn about decoupling/other stuff!
I'll also add a part about what you actually need on your devboard, that's a great suggestion!
You can find a JOURNAL.md in EVERY SINGLE one of my hardware projects https://github.com/KaiPereira?tab=repositories so if you guys want to see more guides/tutorials, let me know :D
(And the author doesn't seem to understand decoupling capacitors, but most people don't understand decoupling capacitors, including most datasheet authors, so that doesn't surprise me.)
Also, KiCad's "solutions" for BOMs are hilariously, absurdly terrible. But that helps me earn a living, so I can't complain too much....
As for decoupling, I've written about how to do it in various places scattered across the internet. I should really collect it all in one place one of these days.
Me at eevblog: https://www.eevblog.com/forum/projects/location-and-value-of...
A discussion here a while back (not all of which I agree with): https://news.ycombinator.com/item?id=42830948
- you have to look at it in frequency domain as well;
- speed of light is too slow;
- often capacitors are inductors, even more so when mounted on PCB;
- capacitance is not what is written on the component.
I am teaching this to robotics and computer engineering MSc students. Quite nice intro book into the topic that I recommend to my students: https://www.oreilly.com/library/view/principles-of-power/978...
At no point in the entire design process within KiCad does this design ever get part numbers assigned for its resistors or capacitors (to pick one thing). That's not acceptable for professional work. A Bill of Materials ABSOLUTELY MUST DESCRIBE THE MATERIALS used in the assembly of the PCBA. Internal part numbers are perfectly acceptable: assigning a generic RES-0402-10k or WIDGETCO-PN987654321 or whatever would be OK by me. KiCAD does not really do this, as evidenced by how this design reaches "design complete" without being... complete.
EVERYTHING MUST HAVE AN ORDERABLE PART NUMBER OR INTERNAL PART NUMBER.
But the resistors are the easy parts. SMD thick film resistors are basically all interchangeable. Things like the ceramic capacitors (MLCCs) cannot be substituted for each other because they all bloody well behave differently so good luck getting the same behavior if you randomly pick a capacitor vendor on LCSC each time you build boards! That's how EEs get fired, and deserve it.
When I generate a build package in Altium, it has complete lists of every part in the design, its MPN, at least one supplier to purchase it, and the SPN for that MPN. (Sometimes internal parts don't have suppliers assigned, but then they're tracked elsewhere in the project or company organization.) There are also assigned alternates in many cases, though they are typically inadequate. Larger projects should use entirely internal part numbering, but that's a real headwind for smaller stuff. All of this comes from a single master part database, plus a project part database, and never gets touched by hand after a library entry's birth.
KiCad is so crap at doing this that last month a client literally paid me thousands of dollars to fix up a garbage KiCad design package that didn't have this information properly stored. There are several add-ins for KiCad that claim to do BOMs better... but it's not standard, so if you don't document what you did and how (which you do not have to do in real CAD software, because it is built in and the CAD vendor documentation plus the years of built-up lore will cover you!) it may as well not exist. Certainly it didn't exist in any sort of usable form for this project.
Any archived version?
Well, that bodes well for the rest of the article..
No, that's the problem, right there. KiCad doesn't have a parts library. KiCad has schematic elements you can manually tack parameters on to. And everyone does it slightly differently!
There's no way to have a set of parts you can select from that are guaranteed-approved (because we used them previously or whatever). There's no way to have the software just show you the entire E24+E96 series of standard 0603 resistors so you can just grab one and go and know that it's orderable. There's no way to be able to get that confusing series of diode footprints and orientations right once and store it forever so the software gets it right for you from now on. (Diodes are the worst. At least Panasonic got out of the diode business. And, yes, I was thinking of their product line when I said that.)
Yeah, sure, you can do anything by tacking parameters on schematic components. Sometimes you even have to do that. Just like programming in assembly: there's no job it can't do. But sometimes you really need a little bit more, right?
And that's notwithstanding the fact that many people who use KiCad, like our article author here, don't even bother to tag parts with an MPN parameter! That's a cultural issue, but it is one the software can and should be pushing back on.
Unfortunately there is no standard name for these parameters beyond footprint, value and some others. MPN, brand, rating etc. are not default. This hinders bigger adoption I think, and makes every project unique. This should be improved.
Only if you've put in the effort to make that database in the first place, which Altium makes pretty annoyingly difficult. I really don't like Altium's mess of options for parts libraries, the defaults for which all suck and a sensible setup requiring a lot of fiddling. I haven't dived too deep into KiCad's setup here recently, but it'd have to try hard to be worse than Altium. The difference is cultural, not tooling.
With those prerequisites I only have to make 2-3 footprints from scratch per project as needed, and less so as our library grows.