Just another version of the micro word clock. Like the old version, this uses a matrix that is more widely available than the original matrix, including on adafruit. Unlike the older version, this version is very tiny (no larger than the display) and it incorporates a battery fallback for the RTC, as well as a USB connector for power. This was a fun layout as I had to balance my hype for assembling teeny tiny parts (generally low) vs size.
PCB Layout Trickery
there wasn’t a lot of real estate on this board once the display was placed, since it is through hole. The pins basically cage in the rest of the components. In order to get a clean layout (no vias on any of the display pins), I did a lot of pin swapping. Based on having to tweak the code last time for a new matrix, I knew that I had total flexibility here.
I also had to add a programming header somewhere but I really had no room on the front or back for even something like a tag connect. My solution was to add a connector via a castellated connector on the edge of the board. Then I used pogo pins and a little jig to make a bed of nails so the boards could be reprogrammed once assembled.
I also used a magnetic USB connector, which is ancient Greek for “an accident waiting to happen”. These cables come with a microUSB to tiny PCB adapter, and the cable itself snaps on and catches the adapter. the “hot” side has absolutely no protection that I can see, and actively accumulates magnetic (conductive) particles. The power is also routed through an open joint on the end of the connector (kind of a slip ring system). Since it basically adds two unreliable, moving slip joints to the power path, its very important for the coin cell to do its job to prevent the RTC from losing time.
I think the next stop for this project is to teach a few people how to use kicad. its a cool little project that’s hard to mess up, but that makes a pretty neat little widget without too much trick soldering.