2025-08-10 ~ 08-12

The correct boards for the unit and the rack have arrived. The rack units fit in the case nicely.

Completed assembly by hand-picking components and a hot-air gun, and the first bring-up went rather smoothly. The IMU was placing readings into its FIFO; the PWM audio was correctly playing through the speaker.

A ball unit connects to a rack unit through a 3.5-mm TRS cable, which carries a power pair and an analogue voltage output serving as an identifier. Here a problem arises: it is usual for the TRS connector to momentarily short the pins during hot-plugging, thus shorting the power lines. The power comes from a Li-Po battery, so the battery management system (BMS) detects the short and cuts off the supply, causing a momentary brown-out. Furthermore, the BMS I'm working with is strict in its recovery in that it restores the supply only when it is completely removed and reconnected, not when the shorting condition is lifted. Presumably, the BMS has a current threshold lower than the mA range. This keeps the entire installation powered off and renders the interaction infeasible.

The above assumption was confirmed by replacing the supply with USB power out of the debug probe, which correctly recovers. Note that safety should not be a concern here, as almost all USB power ports have protection mechanisms.

A simple fix is to add a polyfuse at the rack unit, cutting power to the ball (and maintaining the main power) during shorts. A better solution is to use a circuit to probe the shorting condition, but it appears overcomplicated for this simple installation.

This will add another week of turnaround. Meanwhile, there is still work to do on the balls.