Casio Watch with Bluetooth (nrf52832) Part 4

11/5/2023

The design was flawed from the start because I made some assumptions about the lcd display. Turns out, it is multiplexed, which requires an AC signal along with some finer voltage control rather than 1 and 0. The options are now to either find a ble SOC which also has LCD driving capabilities built in, incorporate an LCD driver, or lose some segments and add in a dac like here. The issue with the last solution is also I would need to find room for a dac or resistor network in an already crammed footprint. For a more detailed explanation, Joey Castillo had an excellent talk on the subject here. Though I knew about the sensor watch I did not know that the creator also documented some of the more challenging issues.

I got the BLE functionality working after a while because I didn't realize some parts of the board that should have traces did not. The cause of this is likely the transition I had to make from EasyEDA to KiCAD. Should have run the DRC there.

All in all, even if I don't go through with this project, I did learn a lot. I even have parts to make something very similar, the restriction to be a drop in replacement for an existing watch is rather strict. Adding in a driver chip should actually not be too difficult because all of the many traces running from the nrf52832 to the lcd would clear up a lot of room.

About the dev board, it turns out I did not brick it and instead my computer sometimes does not detect it. I am not sure what causes this, but I have not had the same issue since I replaced the cable.