It has options to be powered by a Li-Ion battery (rechargeable via the USB port, with a MCP73831), as well as by a CR2032 coin cell. There’s an other option for the 1,8V EEPROM SDO line pull-up voltage adaptation.
All the options are selectable by fitting or not 0 Ohm resistors and adding the dedicated components.
In the Li-Ion version, it’s supposed to fit into a case like that (but with a hole for a button and a LED). It’s 19mm x 40mm.
The blue connector is the ICSP program port. I designed it to be “off-snappable”, as it’s connected only by three bits of PCB, where the signals pass.
NB: Some of my 3D models aren’t exact (like the battery socket, the 3D model is SMT instead of the through hole I have)
Here, an other PCB, Li-Ion only. Just for fun, I wanted to see how small the PCB (19mm x 38mm) can be routed with the components on the same face. On the bottom, there’s only a contact-style connector for ICSP:
The components names couldn’t fit on the top side, so I put them on the bottom.
The PCB could maybe get even smaller with a smaller 3,3V regulator, in a SOT23 package instead of the SOT223. The 32,768KHz crystal could also use a smaller package. But here, I hit the limits of the 2 layers, 6mil/6mil routing and I’m not sure some tracks won’t interfere each other.
The prototype on the first two images is the one I’ll be using to continue the software development.