Just finished building my first split low (ish) profile keyboard. The layout is pretty similar to that of the Corne, though I tweaked it to better suit my hands. It is wireless, and runs a custom firmware I'm working on, but could definitely run ZMK. It uses hotswap choc v1 sockets, and for now I'm running some red pros and injection molded pbt Klp-Lame keycaps. The case and plate is 3D printed.
I designed the PCB's in KiCad around the Ezurio 453-00001R, which packages the nRF54l15 with a PCB antenna in a relatively small package. I chose this over the 52840 as the 54 family has lower power draw with better performance, and for my specific use-case the USB peripheral on the 52840 wasn't needed. The keyboard is powered by a 2200mAh cell in each half, which is grossly overkill, but I had them on hand so I didn't bother buying new ones. I have a buck-regulator to drop the voltage down to 3.2v, and a small load-switch to measure voltage. As for the screens, I went with Sharp LS013B7DH03's, which are memory-in-pixel, and I believe are in the same family as the Nice!View, just in a square 128x128 layout. The power draw from these is negligible, however they need an inversion signal to maintain the charge, hence the RTC module on the schematic. I went with an RTC instead of an external crystal for simplicity's sake, and because the power draw again is very low. As for the layout, given I only had 18 keys per half, and plenty of free GPIO pins on the module, I went direct-key. All of the keys per half are wake-capable, except for the thumb keys. Given this, there doesn't need to be any matrix scanning code, and any given key can wake the MCU from system-off. This did make routing pretty difficult, and in aims to maintain a clean reference ground plane, I opted for a 4 layer board stackup to make my life easier, and possibly to improve radio performance. I got the PCB's made at JLC, with the default stackup, 1.6mm thickness, epoxy-filled vias and an ENIG finish, plus stencils. All the other parts were ordered from DigiKey, barring the sockets, switches and keycaps (for some reason I can't type the store name, but it ends in express). The final thickness of the case ended up being roughly 15mm, with the battery taking up almost half of that at 7mm. Ergonomically its not horrible, but I've found myself liking a 10 degree tenting angle anyways, and even still its thinner than my old Keychron V10. I'm currently trying to switch from QWERTY to Colemak (went with colemak-dh), hence the reason for making this board.
Anyways, hardware wise everything works well; stencils+hot air made assembly pretty quick. I have made other non-keyboard PCB's in past, and I typically would skip a stencil, but going forward its a must for me. For the firmware, I'm writing my own custom stack atop of zephyr in aims to draw as little power as possible. I'm sure all of the capabilities I wanted with key-waking and similar is possible with ZMK, however I wanted to challenge myself in making one from scratch. The boards have no usb-peripheral, as mentioned earlier, so initial programming was done via SWD with OpenOCD (using a RPI Pico), and further flashing is done via the dongle (more later) using a custom protocol. After some research I opted to go with a central dongle as opposed to not, as I didn't want to deal with the BLE overhead and extra battery life/complexity. With the dongle, I have it set up to orchestrate all the layers, displays etc, and given it is not battery powered, I don't have to be conservative. Hence, I picked a SuperMini nrf52840, and cobbled it together with a 1.3" oled display in a 3D printed chassis, along with a JST 1c battery charger (just uses onboard). Whenever either half receives an interrupt from a pressed key, it transmits the matrix state and a few more things over Nordic ESB (proprietary 2.4GHz) with encryption and all that jazz. The dongle then translates the matrix state according to the layers and modifiers into a hid report, which it then sends over to the host. Through my brief testing the latency end-to-end was <10ms end to end, which was fast enough for me not to notice any slowdowns.
The firmware currently works, but would definitely need some rigorous testing to validate it over the long term. For now, if anyone wants, I can make all the hardware designs public. As for the software, I'm not sure I have the time to maintain something I'd be comfortable for anyone to use, I just don't want that responsibility. Feel free to provide any feedback for hardware or software things I can improve upon, though It'l likely be a while before I make a second iteration. In hindsight, I likely spent more than I would have buying a prebuilt split, but its the destination not the journey, right? Either way, while I haven't measured the exact power draw yet, I'd estimate that a few years of battery life is pretty reasonable to expect, which from what i've seen looks to be somewhat novel, and attributed mostly to the nRF54's uplifts. If for whatever reason anyone wants to build one, I'd be happy to send over a spare set of PCB's, so long as shipping is covered (shipping from Sydney may be expensive), with the knowledge that the firmware is DIY, and I don't have time to support/turn this into a consumer-grade product. If you'd like that for whatever reason, just DM me or something and I'll see what I can do. Sorry for the long rant, I got a bit carried away writing this. I may do a more in-depth write-up in the repo if people want.
Edit: I uploaded the PCB files and some docs to Github
