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Abrar , 09-30-2025, 11:42 AM

Hi @Robert Feranec ,
It's been a great pleasure watching your keyboard development series. And I started my own keyboard development project using **RP2040**. but I want to create a **75% **layout **modular** keyboard that has some unique features:

1. 82 switches and 4 rotary encoders on the main board,
2. hot-swappable num-pad module with side-mounted
magnetic pogo pin connector.
3. a castellated mounted RP2040 4-layer SOM module(to keep key-switch pcb board at 2-layer)

But I am facing some problems:

1. RP2040 GPIO pins are limited
2. Not enough space available to mount the SOM module on the switch board
3. Uncertainty about whether using the RP2040’s ADC channels in the key matrix could cause issues

I would really appreciate any suggestions.

Robert Feranec , 09-30-2025, 01:28 PM

this is working ok

Robert Feranec , 09-30-2025, 01:28 PM

Abrar , 10-03-2025, 10:17 AM

Thanks @Robert Feranec for your suggestion on the ADC channel.
I have an issue with space management with placing the RP2040 SOM module in the keyboard pcb. there are barely any space left on the keyboard pcb. now I can think of some ways.
* make the **RP2040 SOM** module **10.5mm*40mm** with and fit it under the **first row**(marked in the pcb)
* make the RP2040 SOM module bigger and fit it beside the **space-key bar**
* if I fit the SOM module beside the space-key bar, the usb differential traces will be longer. not sure if it will cause any data interruption
* should I make the SOM module with **castellated pins** or **normal pad under the back layer**

Robert Feranec , 10-03-2025, 10:42 AM

maybe where the right shift and enter are you may have more space? also, maybe the module doesn't have to be rectangular if you need to fit it there? castellated holes may be useful if you will have to solder it by hand.

Abrar , 10-21-2025, 08:00 AM

My main reason for creating the SOM was to keep the keyboard’s large PCB as a 2-layer design, in order to save the additional cost associated with a 4-layer PCB.

As you suggested, I designed an SOM that fits in the space between the right Shift and Enter keys. However, it seems to be much more complicated and expensive to fabricate and assemble than I initially expected. Because of this, I’m considering not including the SOM in the keyboard design.

At this point, I’m thinking about two options:

Make the keyboard a 4-layer PCB, which would solve all the issues but add some extra cost.

Keep it as a 2-layer PCB using the RP2040, though I’m concerned this might introduce signal interference or noise.

what do you think I should do @Robert Feranec ?

Robert Feranec , 10-21-2025, 01:46 PM

the large keyboard I built was expensive - it was not only about PCB, but also the switches were expensive, enclosure was expensive, etc ... So be ready, it will not be cheap. but yes maybe if you use RP2040 module it will be cheaper, but then you may need to compromise on size/shape

Abrar , 10-21-2025, 01:52 PM

If I skip the SOM module. And place everything in the keyboard PCB, and make it a 2 layer design, will it be ok? Or should I go with 4 layer design

Robert Feranec , 10-21-2025, 01:55 PM

I prefer to have reliable prototype and optimize later .. so I would rather go for 4 layer PCB in the intial version to be sure if something is not working ok that it is not layout causing the problem.

Sniper2 , 10-22-2025, 07:50 PM

you can route the RP2040 on 2L , did that and also kept a nice GND under

Sniper2 , 10-22-2025, 07:51 PM

tho it needed a few large resistors as jumpers over the USB

Sniper2 , 10-22-2025, 07:52 PM

then again that was for my form factor + it was years ago might be able to do it sort of clean now

Abrar , 10-27-2025, 05:33 PM

I get it, thanks. but right now I am little confused about the stabilizer hole footprint. I am not sure if this is ok or not. [it is the footprint of the key "backspace"]
what is your thought @Robert Feranec ?

Robert Feranec , 10-28-2025, 12:12 AM

maybe I dont understand what the picture shows - stabilizer should be a separate component, it is not included in switch footprint.

Abrar , 10-28-2025, 04:23 AM

I know it should be seperate components. But I wanted to combine them into a single footprint.
Will it cause any problem?

QDrives , 10-29-2025, 12:48 AM

If it is two components to purchase and/or mount, you should make two components for the BoM and two for assembly (footprints).

Abrar , 10-30-2025, 05:52 AM

ok. I get it. But the stabilizer will be source separately. thats why I wanted to combine that footprint

Abrar , 10-30-2025, 06:30 AM

@Robert Feranec Should I use this same component for the large-size keyboard, or do you recommend something better?

Robert Feranec , 10-30-2025, 10:12 AM

size of the keyboard doesnt influence the buffer

Abrar , 10-30-2025, 10:55 AM

so is it ok if I use it where more 80 leds are used?