Hello,
When wiring a parallel to serial shift register (such as the 74HC165E) to the Arduino for adding 8-push buttons at the inputs, is it necessary to add a pulldown resistor at each IC input?
Thanks.
If those buttons are SPST then you are going to need to pull up or pull down on the inputs otherwise they will be floating and can pick up a charge and register on the 165 as high even though the button is not depressed. It matters not if you set the pull up on the Arduino pin that the 165 is connected to because the issue will be on the parallel inputs on the 165 itself. If the buttons happen to be SPDT you can go ahead and connect one side to +0V and the other to +5V and you will be good.
Source:
http://www.kpsec.freeuk.com/components/74series.htm
74HC and 74HCT family characteristics:
Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!
regardless put a pull up or down or de bounce it...
Doc
Thank you!