My current AG:MF03 flashlight design incorporates a 20 LED display by charlieplexing them to 5 I/O pins from the microcontroller. I won’t go into the details of how charlieplexing works, but you can see more here. So anyway, when I programmed the chip to sequence through each LED individually, I noticed that multiple LEDs were lighting up at the same time in some cases, and none were lighting up in other cases. So for example, if I was trying to just light up LED2 by setting all I/O pins to the high-impedance state, except for the two connected to LED2, then maybe LED2, LED4, and LED5 would all light up. Alternatively, doing the same with LED3, none of the LEDs lit up.

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