user6883: I am thinking about building a circuit that is literally going to take 11 of the 14 digital pins of an UNO to communicate with an SD card and an external EEPROM chip (EEPROM programmer).
Using 4 shift registers and the SD card, I will have 1 digital pin left.  Well, I'm also reserving pins 0/1 for serial communication.
Anyway, I need to read some external buttons and I have the 6 analog pins that I haven't touched.  
I will only need about 4-5 push buttons in my design so they should fit nicely on A0+.
So, my question is, will I run into any issues doing this?  Or, are the analog inputs better left for true analog reading?
Thanks.

user3879: If you have no use for analog pins there is no problem. A5/A6 are used for I²C (on the Uno).

user6959: The Analog pins are essentially identical in functionality to digital pins when used as digital I/O.
If desired you could "be clever" and use an analog input in analog mode to read multiple keys using one pin. eg using 10k, 22k, 39k, 82k, 150k in series with 5 buttons from V+ to pin and a say 4k7 to ground would result in 5 voltages which were easily distinguished using an analog read. This is a very very old technique indeed and allows far more than 5 keys on one input pin. THere are limitations which can be discussed if required. 
R1 ... R5 correspond to buttons 1...5
Vout = V+ x Rb / (Rb + Rbutton)
Example only  
Rb = 39k
R1 = 10k
R2 = 22k
R3 = 39k
R4 = 82k
R5 = 150k
Gives   

The % of V+ in column 2 when a button is pressed.  
The voltage in column 3 with V+ = 5V 

Delta-V is the difference between that key and the one below - very ample clearance. 



6883: <p>I am thinking about building a circuit that is literally going to take 11 of the 14 digital pins of an <code>UNO</code> to communicate with an SD card and an external EEPROM chip (EEPROM programmer).</p>

<p>Using 4 shift registers and the SD card, I will have 1 digital pin left.  Well, I'm also reserving pins 0/1 for serial communication.</p>

<p>Anyway, I need to read some external buttons and I have the 6 analog pins that I haven't touched.  </p>

<p>I will only need about 4-5 push buttons in my design so they should fit nicely on A0+.</p>

<p>So, my question is, will I run into any issues doing this?  Or, are the analog inputs better left for true analog reading?</p>

<p>Thanks.</p>
 3879: <p>If you have no use for analog pins there is no problem. A5/A6 are used for I²C (on the Uno).</p>
 6959: <p>The Analog pins are essentially identical in functionality to digital pins when used as digital I/O.</p>

<p>If desired you could "be clever" and use an analog input in analog mode to read multiple keys using one pin. eg using 10k, 22k, 39k, 82k, 150k in series with 5 buttons from V+ to pin and a say 4k7 to ground would result in 5 voltages which were easily distinguished using an analog read. This is a very very old technique indeed and allows far more than 5 keys on one input pin. THere are limitations which can be discussed if required. </p>

<p>R1 ... R5 correspond to buttons 1...5</p>

<p>Vout = V+ x Rb / (Rb + Rbutton)</p>

<p>Example only  </p>

<p>Rb = 39k<br>
R1 = 10k<br>
R2 = 22k<br>
R3 = 39k<br>
R4 = 82k<br>
R5 = 150k</p>

<p>Gives   </p>

<ul>
<li><p>The % of V+ in column 2 when a button is pressed.  </p></li>
<li><p>The voltage in column 3 with V+ = 5V </p></li>
</ul>

<p>Delta-V is the difference between that key and the one below - very ample clearance. </p>

<p><img src="https://i.stack.imgur.com/F9KCN.jpg" alt="enter image description here"></p>

<p><img src="https://i.stack.imgur.com/Cq9kc.jpg" alt="enter image description here"></p>
