Reverse engineering Logitech MX Master

Chip package type is WLCSP with diagonal pin pitch 0.4mm. Luckily boards don't appear to have more than 2 layers, and size of vias suggests that all pins are fanned out to the sides.

After careful examination under microscope I've identified following pinout of ribbon cable:

1. GND
2. ?
3. "Hidden" button
4. ?
5. SCK
6. SDO
7. CSB
8. SDI
9. ?
10. VDD_IO 2V - voltage is dropped to 1.6V for VDD with local regulator
11. ?
12. ?

There are no points on the little pcb available where wires could be soldered without removing connector. Luckily all connector pins have a test points on the next board that ribbon cable connects to.

To confirm pin assignment I tested them with oscilloscope. SCK and CSB pins checked out, but there is no activity on SDI, and SDO.

Thumb wheel board

This is daughter board that hosts thumb wheel, 2 thumb buttons, and 2 externally connected buttons with wires. It connects to the other button board via 12pin ribbon cable.

There are 2 ICs:

• SOT-23-6 with markings 228ATV that appears to be voltage regulator
• BGA with markings 157 T826 that likely is BMM150 magnetic sensor

It would be quite hard to solder any breakout wires to the small board, but another board with buttons has a lot of test points that hopefully can be used instead.

Next step would be to figure out ribbon cable pinout, and connect wires to button board.

Plan is to use raspterry pi pico for prototyping.