The trackball is currently missing some critical functionality: Clicking and scrolling.

Since I'm already using HID-remapper to change the trackball direction, I might as well use the RP2020-Zero's GPIO pins to add the missing functionality.

I wired up a 5-way directional switch and concealed USB-A port to the RP2020-Zero and set up the switch to function as the left, middle, and right clicks, as well as scroll toggle. That still leaves two directions unassigned. When scroll toggle is turned on, moving the trackball functions as scrolling, rather than pointing.

The wiring is still a mess, and I need to rework the housing, but for a prototype it works well.

I accidentally destroyed the PCB's of five mice while working on this, salvaging the sensor chips out of them. I was able to power up one of the sensor chips with a bunch of dupont wires and a breadboard. It might be possible to stick them in a smaller 3d printed housing and save some space, possibly even combine the halves back into a single unit. That remains to be seen. I rather like the split half idea, as it goes well with my keyboard.