ALeggeUp's 1x1" Cortex-M0 Minimal Dev Board Build Log


#1

I have a couple projects that I’m mulling over that will need a microcontroller. I have a Nucleo board with an M3 on it but I think for some smaller projects that the M0 will work fine. Instead of getting another Nucleo board I figured it would be worth creating my own minimal dev board for the practice and because I would eventually need to figure out how to get it all working together later anyway.

Once I figured out what parts I would need and started playing around with the layout I thought it might be possible to fit everything onto a 1x1" square and then it became a self-challenge.

I was thrilled when I finally got OSH Park to tell me:
0.93x0.98 inches (23.52x24.79mm). $4.50 for three.

… so satisfying.

The only confusing part was how the measurement didn’t quite line up between KiCad and OSH Park, not sure if maybe it is accounting for kerf or if it’s that the board outline line is greater than 0? Either way, exceeded my goal, can’t complain with that!


#2

I think the measurement mis-match is usually because we base it on the center of the “edge cuts” line and they assume the outer diameter of that line–because yes, every line needs to have some width associated with it. However this is normally what i see when I put in a board that is 1x1 in KiCad and they say its 1.05x1.05, which doesn’t seem to be the case here. If you put your board on GitHub, happy to take a look.


#3

Of course I did, I’m a loyal apprentice that follows your lead :slight_smile:


#4

I tried changing the thickness of the edge cuts line to 0.0001" and that seemed to do the trick, KiCad and OSH Park now agree on the size. The only down side is when the line is that thin in KiCad it becomes hard to select for edit, so it will probably be a thing I do last, or I’ll probably just create a pre-submit script that changes “(layer Edge.Cuts) (width 0.15)” to “(layer Edge.Cuts) (width 0.00254)”.

I’m submitting the kicad_pcb file directly, not sure if the same thing happens with gerbers.

Just changing the thickness of the line saved me $0.05USD for the exact same board, so if my math is right OSH Park has picked our pockets for over a million dollars! (disclaimer: math is not right)

I kid of course, it’s just nice to have it match up so I don’t have to move the lines around to get the size I expect.


#5

I was under the impression the minimum width of a cut-out is 100mil (0.1") from their Cutout and Slots page. It will be interesting to see how your 0.0001" will turn out.
Reading this thread, I was also wondering if OSH Park include the 0.1" wastage in the board size. Since it is material used even though we don’t see it.


#6

Right, I don’t think the board outline is the same thing because they can use whatever drill size they want between boards to get to the outline, I think that’s more in the panelization, but you might be right, hopefully I don’t affect my panel neighbours.

Maybe @fustini has some insight into this?


#7

I found this doc on the OSH Park site: http://docs.oshpark.com/submitting-orders/board-outline/. The pertinent data is at the bottom, it reads “We recommend a small but nonzero line width; the fab will route so the edge of your board is at the center of your line, no matter how wide it is.” Then there’s a little image that seems to indicate that they use a 100 mil diameter drill for the cutout.


#8

Beautiful, thanks! That is what I was hoping, so I should get the exact same board either way, it’s just the way OSH Park calculates the outer dimension.


#9

Thanks @jonthomasson.

@ALeggeUp We make sure people don’t affect their neighbors, and the cost of that “wastage” is ours.


#10

So if I’m paying for the thickness of my line then I’m just paying for something you’ve already accounted for in your costs, at least that’s how I can justify to myself that I’m not stealing OSH Park nickels.

Thanks for confirming!


#11

This is really cool! There are so many projects I can think of that could benefit from a board like this. And it would be a great one to submit to the Hackaday square inch project contest if they ever do it again: https://hackaday.io/project/7813-the-square-inch-project

Is this something that’s going into the powerwheels project?


#12

Thanks! Yes, powerwheels project is definitely one where I think I’ll need it, to control the RGB LEDs at least, maybe some other smart car type features.

If it works the way I hope I’ll probably get a bunch of them to be able to quickly and cheaply hack a micro into something.

Good idea on Hackaday, I’ve been considering cross posting build logs on there, do you know if you usually have to wait until a contest starts, or can you enter a contest with something that’s already there?


#13

Yep, and the price can’t be beat for 3 boards for less than $5. This is an area that I’ve wanted to delve into but haven’t made the leap yet. I was thinking of getting a Teensy with an M0 or M4 in it to play around with, but now I’m tempted to build one of these.

I’m pretty sure Hackaday let’s you post projects that are already started to contests. At least that’s how it works with the Hackaday Prize. They may have it setup differently for different contests though. Anyway, keep up the good work!


#14

That would be very cool, I’m glad I put it out there if anyone finds it useful.

It’s still untested although I used many reference designs to try to make sure I got everything and programming won’t be quite as straight forward, but shouldn’t be too bad as long as you have a USB to serial of some sort

Confidence level is pretty high, but never 100%