I need to use a multiplexer to connect a few switches. I think I'll be using the 74XX151 if that matters (by the way, what's the difference between all of them? I found an eagle file for the 74LS151N but I guess they're all pin-compatible).
I have never used one before so I really don't have a clue, do I need to use pull-up resistors to pull the pins high or can I use the internal ones in the atmega?
I tried searching for an answer but couldn't really find anything.
The 151 is a TTL device so would actually naturally pull up to a 1 but this is quite a high impedance so that for maximum robustness you should wire pull up resistors of about 4K7 from the inputs to +5V with the switches pulling down to ground.
There is no need to have a pull up resistor on the connections between the Arduino and the 151 as these are normal logic outputs / inputs.
Yes, use pull-up resistors as Mike has suggested. Don't forget to connect the Grounds betweens the switches, the 74LS151 and the Arduino.
Great! I had already planned for pull-ups so there's plenty of room for them on the PCB.
And all grounds are sorted. Thanks!