The Arduino Uno secret pins

I recently found a fantastic chart (link below) and learned that the two pins adjacent to AREF and the ICSP header are pins 18 and 19 corresponding to A4 and A5 (or SDA and SCL pins if you are using I2C). I would have used those guys if I had known! Another discovery was that, contrary to the product description, the Arduino Uno has 20 digital I/O pins. All you gotta do is declare the pins A0 – A5 as digital input or output, and BOOM, done! But hold on…there’s the mysterious unlabeled pin next to IOREF… :o

Link to Arduino Uno pinout[/url] Uno R3 Pinout.png

IOREF +5volts

Unmarked pin I believe that pin is not connected.


It is frequently mentioned in the Forum that the analog pins can also be used for digital I/O

How can pins be A4 and A5 and also be SDA and SCL ?


It’s also documented here.
Imagine that.

How about Nick Gammon's photo with added labels ? Or the official schematics as can be expected with true open hardware ?

Plenty of ways to find these things out i'd say. And by the way, when did that guy OP is (kind of) linking to start with those graphics ? Seen something similar years ago, created (or maintained) by a user with the name PighiXXX.

PighiXX link - very useful


AWOL: It's also documented here. Imagine that.

Shhh! They're secret. :)