RP2040 Microcontroller Expansion Card

The idea of the card is to provide a microcontroller with a few gpio pins available, as well as a few covers to put on top, like LED strips and buttons, as well as a way to easily make more.

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Just a quick update, I basically finished the card’s schematic. The only thing I could see changing is the order of the pins

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This sounds fantastic! I like the idea of swappable covers for the card as well. If I can suggest, my priority list for covers would include i2c and/or spi, then others. From that you can always hook up Stemma QT and/or QWIIC devices, and use all the little devices (including IO expansion) from Adafruit / SparkFun / etc. Overall, my priority list would be something like:

  • I2C
  • TTL Serial
  • SPI
  • NeoPixel(s) indicator on outside shell
  • Button OR capacitive touch pad
  • more I/O pins
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Alright, so I worked on this some more, and found that I could not fit the required components in an expansion card, especially with the cover mechanism.
However, I did some more research and discovered the tomu.im, which definitely fits the space constraints, and contains an open source design. I am thinking about making boards based on that family. Each one would have an RGB LED, one or two touch pads, and some kind of pinout. What do people think of this idea?

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Note that we publish two different PCB outlines. The retrofit outline is much smaller as it is designed to fit inside existing Expansion Card enclosures: https://github.com/FrameworkComputer/ExpansionCards/tree/main/Electrical/KiCad_templates/Expansion_Card_Retrofit

The normal outline is much larger, as it is designed for a “lidless” enclosure that is 3D printable: https://github.com/FrameworkComputer/ExpansionCards/tree/main/Electrical/KiCad_templates/Expansion_Card

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Thanks for the reminder, I was already using the normal outline. The main reason I am having trouble with the RP2040 is because of its large size (for a microcontroller), 7 mm x 7 mm, as well as how many capacitors it requires. I am sure it is possible, but it looks significantly more difficult then other chips, without enough payoff.
I really appreciate how involved with the community you are.

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That’s kind of a surprise, I was actually considering experimenting with just building an enclosure for an adafruit QT PY RP2040 with a male/male adapter for connection.

Is that too big?

No, I think that would work, the only two issues I see is the wasted LED and button, and interfacing with the pins.

With all these microcontroller expansion cards being worked on where would people like the outputs to be placed?

I’d assume the main issue is going to be having the gpio accessible while the laptop is on a surface.
I’m planning on testing a few configs once my qt py arrives in the mail, but my current hypothesis for this build is probably some female pin headers on either side of the i2c connector.

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I was actually doodling with this tonight, except I was using a Seeedstudio XIAO with a usb C M/M adapter.
I’m picturing the card sticking out about 10mm (due to the usb adapter) to fit some dupont breadboard IO connectors in there.
Excuse the messy handwriting, I didn’t really plan on posting it to everyone when I was tinkering.




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As the Seeedstudio XIAO is open source with schematics available you could make your own PCB and get all the parts and make a proper expansion card, I can help with the design if needed.

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The Microcontroller Expansion Card reference design is also similar to the XIAO and uses the same MCU.

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I’ve been trying to find a good usb c male connector that I can swap the female connector on the QT for with a rework gun or hot plate. Unfortunately so far, all i’ve seen are ones that are a bit too short, or are designed to have the pins on either side of the pcb. I suppose I should spend more time digging through digikey or somewhere like that

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You need the molex 105444

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Wonder if it would be possible to solder these together (or a similar model designed for being hand soldered) to make a shorter male to male stub/adapter/interposer?

Found this little guy today.

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Okay now that is really cool!!
Never seen that one before. Good find!

Alright, I may take another crack at this. Are people alright with it sticking out a few millimeters?

@Yisroel_Newmark That’s probably fine. I wouldn’t say sticking out a few millimeters defeats the purpose of putting it in an expansion card.