It can be useful to say what direction or function a pin serves, particularly if you make use of some of the rules checking functionality in Eagle to validate your schematic and board files.
Eagle defines a number of possible pin directions, which we can see by right-clicking on any pin and selecting the properties window, and then expanding the 'Direction' drop down list, as shown below:
- nc - For 'Not Connected', which is used to indicate that this pin should not be connected to anything on your schematic. Assigning a pin as 'NC' will allow Eagle to warn us if you mistakenly connect something this this pin.
- in - For input pins
out - For output pins
io - Pin is bi-directional (input and output) * This is the default pin direction!
pwr - For VCC, VDD, VSS, GND, VBUS, VIN, VBAT, and similar 'power' pins
pas - For pins on passive parts like resistors and capacitors (no often used creating custom parts)
- Set VSS, GND@1 and GND@2 to 'pwr'
- Set IO to 'out', since this is actually the analog output on our device
This is purely a cosmetic step, but if you wish you can also make the pins larger or smaller. My own preference is to set the pins to 'short' (the default, shown in the image above, is 'middle').
If you want to, you can change the length in the properties window. Whatever size you decide to work with, though, you should try to be consistent about keeping the same length throughout your library.
You can see the results of setting the pin length to short in the image below: