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.

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.

• Awesome answer. Thanks! BTW, I didn't even think about using the analog pins to read multiple buttons that way. Thanks for the tip too. Jan 28, 2015 at 13:15
• Can this method be extended to detect multiple buttons pressed at the same time? Jan 28, 2015 at 13:51
• @LoganBlades Yes - within limits. If you think of button resistors as ~= injecting current into the lower resistor you'll see I have chosen values that are in an approximate binary ratio subject to std E12 values. ie I aimed at 10 20 40 80 160 and then took closest E12 value. So if you take the inverses it shows ABOUT how much they will pull a low value resistor up by - I say low value as that better approximates a current source. So contributions are in inverse R order of about 16 8 4 2 1 current units. So if you press B1 + B3 you get 20 current units, B3 + B5 = 3 etc. ... Jan 28, 2015 at 18:22
• ... The 3CU of B3 + B5 lies between the 4 CU of B3 and the 2 CU of B4 so you need enough resolution to distinguish between them. If you used the resistors to control a "proper" current source and binary weight the resistors you get 2N^N levels for N buttons keys. So 8 buttons gives 256 levels AND with a 10 bit ADC vyou notionally have 2 bits between each level. That would almost certainly be tighter than you could easily manage in practice - you'd need 0.1% resistors and a 0.1% accurate current source. 6 buttons is probably doable. Easily experimented with using eg an Excel spreadsheet. Jan 28, 2015 at 18:27

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