Hi there,
here are three externals that give the ability to get/set states bits from/to gpio pins of an arm mini computer (in my case a cubieboard 2, but can be reworked to suit other needs), and to read data from a pCF8591 4 channels ADC in order to make interface between potentiometers (for example) and pd, using the i2c protocol.
I'm not pretending they are nicely written, as for these very first externals of me I had to wake up my poor 10 years old c programming skills. They use the userspace files, and I know it's possible to work at a lower level and get slicker results. (see https://www.olimex.com/forum/index.php?topic=436.0) But they are the best I can afford and they do the job with a reasonable cpu increase. None of them are internally clocked, so the user has to set a metro from outside and therefore sets the sampling period.
I choosed the ADC chip model because it was available in DIP, easily usable on a breadboard, no advertising here 
Nau