The UNO is not your typical keyboard controller, but the anniversary edition is actually only 34.2mm x 26.7mm. Hmmm.
KBD.news Published November 25, 2021
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The UNO has now sold over 10 million units, and people at Arduino celebrate this achievement with introducing a miniature redesign, to be produced in limited number (however, I couldn't find any info on the exact number of boards being made).
A real UNO was my entry to DIY electronics so I'm quite sentimental about this milestone.
While building a keyboard based on a classic UNO is absolutely feasible, I've seen only one such board if I remember right.
No wonder: the Atmega328P doesn't have native USB HID support, and the board itself is simply too bulky to fit most keyboard cases, especially splits.
Still, not so long ago, questions like "Can you use an Arduino Uno/Micro to build a keyboard?" were regular ones on r/mk.
And the exposed through-hole designs built on the 328P demonstrated that, while it's not the easiest way, it indeed can be used as the brain of your keyboard.
So while it still lacks native HID support, the anniversary UNO Mini with its way more keeb-friendly 34.2 x 26.7mm footprint is definitely something I expect to see at least in a few upcoming one-off builds.
Well, such a build would be still apparently a joke/fun build mostly because of the hefty price tag. For the same $45/40euro (plus shipping and VAT) you can still buy 8-10 Pro Micros from Aliexpress or 5 Keeboars from Adafruit.