... because of the absence of active internal pull-ups here. I think Erik already mentioned this.

Writing "0" to the corresponding SRF bits makes the port lines emitting "0" and by this discharging the caps. Afterwards, write "1" to the corresponding SFR bits to turn-off the discharging open-drain outputs and start a timer. Now, the potentials at the port lines begin to rise. Everything to do is to poll the states of P0 port lines, and, when skipping from "0" to "1", to read the timer to get the charging times.

Kai