Beginning

Published on 2020-05-03 in Fluff M0.

I was working on a version  Kubik M0 that would be compatible with FeatherWings, and I started to wonder whether I could do it with the QFN-32 version of the SAMD21 chip. I knew that the original Feather M0 switched to a bigger package, because there weren’t enough pins, but I wanted to cheat anyways — I didn’t want an external flash, and I would use the internal oscillator, which saves me a bunch of pins already. I would also use the SWD and SWC pins as regular pins — who needs debugging. So I looked at the original schematics, and started to create my own version.

I wanted to re-use the same pins as much as possible, and I mostly succeeded. Of course some of the pins are simply not there on the QFN-32 package, so I had to find substitutes for those. That includes most of the analog pins, the SPI pins, and a few of the digital pins — not very bad. In the end, I even added an extra A6 pin to the board, and had two pins unused (I choose PA27 and PA28, since they can pretty much only do GPIO and nothing else).

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Then I started working on the actual PCB layout. It turns out that it’s not very easy, because the pins are distributed pretty much randomly — they probably made sense on the bigger package, but with the smaller one I had to use a lot of vias. My first attempt was pretty bad, I had more vias than pins. So I deleted it and started from scratch, planning a little better this time. I came up with this:

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The big black circles are holes — I figured out that since the PCB is empty anyways, I can at least make it a little bit lighter. I could have used that space for a prototyping area, but it’s kinda bad shape for that.

I’m going to order this with my next batch of PCBs and then make a custom CircuitPython firmware for it.