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What are the different kinds of pins on a microcontroller for? And, how do you fit a microcontroller onto a crowded PCB? (lemmy.world)
submitted 1 year ago by to c/mechanicalkeyboards@lemmy.ml
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I'm making a custom keyboard with a Raspberry Pi Pico as the MCU, and i don't understand what all the different pins are for and how they should be wired up to my key matrix and trackpoint.

My current understanding is that GPIO are normal pins that can be wired to the rows and columns, but what of VBUS, VSYS, 3V3, and all the others that aren't just GPIO or GND? And how should the ground pins be used here?

The schematic currently doesn't include the trackpoint because i'm planning to hand wire it and the MCU to the PCB, so i can put them under the PCB in the case and i don't have to try to fit them onto the board. Is there a better way to connect these that won't require adding a bunch of space to the board to fit the MCU?

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[–] 3 points 1 year ago* (4 children)

You can put the MCU and other circuitry on the underside, but perhaps use at least a 3-layer PCB so you can run an internal ground plane under the power components.

Is this keyboard a one-off or are you doing production runs? Try to keep all the SMD components on the same side if you can.

If you're using the pico rather than a bare RP2040, you'll have a much harder time putting anything on the underside though.

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  • [–] [S] 1 point 1 year ago* (last edited 1 year ago) (3 children)

    It's a one off, just for me to use. The case i just finished designing has room for the Pico, trackpoint, and wires to those under where the PCB will go.

    How does the ground plane usually work on keyboards with only 2 layers? Would it be enough to just connect everything to a ground pin instead of paying for a third layer?

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  • [–] 1 point 1 year ago (2 children)

    If you can reasonably have the entire bottom layer be connected to ground without the top layer being too crowded, 2 layers could work.

    Wires to the underside components? Can you solder them to thru-holes on the underside of the PCB instead?

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  • [–] [S] 2 points 1 year ago* (last edited 1 year ago) (1 child)

    Not without moving some switches to fit the trackpoint and diodes to fit the Pico. But how hard can hand wiring that one part be⸮

    I'm still not clear on how the ground should work. Is it enough to just wire every row to a ground pin as well as a normal input pin?

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  • [–] 2 points 1 year ago

    I was halfway through a message about return path impedance, but remembered the pico is a carrier board with its ground built in it's USB connector, and the switches are hardly going to care.

    My bad, you'll be fine 👍

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