Pulling multiple I/Os to Vcc through only 1 pull up resistor

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    Question: Is it fine to pull up (or down) multiple signals through only 1 resistor?



     In general, you certainly could provide common pull-ups for a few I/Os. However, it is not recommended to use only one pull-up for several (>10) unused I/Os. All devices have some amount of input leakage (aound 10 uA). This is per I/O. So if you want to tie 36 I/Os together, there would be 360 uA total leakage current that would have to be sourced through the lone pull-up resistor. You would have to use a pretty strong pull-up to guarantee that all the I/O were above VIH. Additionally, the switching current of an input will be more than the leakage current. Again, this current would have to be sourced through the pull-up resistor. A ballpark recommendation is for every 10 unused I/Os, we would recommend approximately a 3 K-ohm pull-up.