Q on board to board connections

[Jonathon_Doran]

I have wanted to build a new macro keyboard for some time now, and always get distracted by the real world before I finish.  I am making another attempt this summer...

Rather than put all of the keys in one large matrix, I got the idea that I might rather group them together in groups of 6.  The reason for this is that there are 36 keys total and I could separate these to make it easier to find a particular key by touch (without looking).  The old G15 keyboard had macro keys in 3 groups of 6.  Another reason for doing this is that I could have the key matrixes on 2-layer daughter boards and possibly connect them to the 4 layer main board.  This would allow me to have the MCU board under the key matrix (making the case taller but less desk area).  It might also cut costs a bit.

I don't know if this is a dumb idea or not.  So I guess that is my first question.

Next, while I'm planning to run the MCU at 72MHz, the key matrix scanning will be slower.  I don't know what that frequency will be, but I can certainly control it.  I could run it from a timer if I need an exact value.  A few hundred kHz seems reasonable.  I need a 48MHz clock for the USB connection, and had no reason to run the MCU slower before now.

I envision the board-to-board connections would be under 10cm each.  If I made the key matrix one single PCB it would be on the order of 120x130mm.

I think 5 signal wires separated by 4 ground wires would be sufficient to run to each daughter board.   My SWD connector has 10 lines, so I might be able to use these without adding to the BOM.

I have no schematic with this scheme for now, but can draw one up if needed.  I was hoping to get a rough idea if this would work before making such a drastic change to the design.

Edit:  I forgot to mention the programmable LEDs.  That would mean two more wires, so I guess I would need a different connector.  I'm not sure how fast I have to run the LEDs.  I believe they latch on "reset".

Any thoughts/suggestions?

[cpfeiffer55]

Your packaging might be a perfect use of Z-Silver connectors between your boards. It could make the overall size more compact and less of a pain to interface. Here is the website - https://www.zaxisconnector.com/products/connector-strips/z-silver/

[Jonathon_Doran]

Not really.  The MCU board would need to be as large as the key matrix then.  Flexible connectors (if they work) would be able to move out away from the MCU to each daughter board.

[Jonathon_Doran]

Well the LEDs will have to go, so I'm back to 5 signal wires per board.

My main concern is knowing whether it is OK to split off the keys to daughter boards, and how fast I can run the signals over that distance without doing something complicated.  Can I scan at 1MHz?  100kHz?  10kHz?
Again, I imagine about 10cm.  I'm thinking of a 10-wire cable like is used for SWD, with grounds between signal lines.