Turns out that was the problem. Using a Hewlett Packard HLMP-4000 LED in a circuit that was designed for 3.0V and with the LED operating around 25mA (even up to 30mA, but we won't tell anybody). Circu ...
I have simplified the C code down to its roots, and problem still remains. Taking pin A1 low causes A6 to also go low (or float?). If I REM out the output_low(PIN_A6) line, A6 will remain high, as int ...