Open Drain GPIO mode

[ianrrees]

On Matrix, there has been some discussion around an "Open Drain" GPIO mode, which could be used to drive shared interrupt or reset lines for example. This mode would have subtypes for Nmos and Pmos, and in each of those the pin could be in one of two states. In OpenDrain<Nmos>, the pin would either drive the output low, or leave the output disconnected.

It may be useful to enable the pull resistor when the pin is in the de-asserted state, which could be achieved using subtypes NFloating, NPullUp, PFloating, PPullDown (or according to wikipedia : ⎐, ⎒, ⎏, ⎑).

The implementation would work along the lines of https://ez.analog.com/dsp/blackfin-processors/w/documents/4978/blackfin-gpio-open-drain-functionality - basically instead of set_high() and set_low() acting on the OUT bit, they'll work on a combination of OUT and DIR.