I've found that many of the GPIO pins are not usable on a stock installation of Raspbian with gpio-admin.
For example, I wired up a relay to GPIO 21 only to find after some debugging that it didn't work as desired. Its neighbors GPIO 17 and GPIO 22 both work as expected. GPIO 21 is labeled as PCMD_OUT, so I'm guessing it's being used for another purpose.
So I'd like a few things explained in a straightforward way:
- Which pins are usable to read/write "out of the box?"
- Which pins can be used with some modifications (and how)?
- Which pins cannot be used at all?