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schematic

simulate this circuit – Schematic created using CircuitLab

This question might not have a close answer since I think it heavily depends on the considered pin and IC. However, consider, for the purpose of this question, a 555 timer. Suppose in my design I do not use the discharge pin (pin 7). What should I do with it? Should I use a pull up resistor? Should I leave it float? Should I connect it to GND through a resistor?

More in general, is there a standard practice for what to do when not using some pins of an IC?

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    \$\begingroup\$ (and no, there's not a standard procedure. Data sheets will tell you what you should do.) \$\endgroup\$ Mar 8, 2019 at 21:16

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I'm going to kinda-sorta disagree with @Bimpelrekkie -- in general, "ordinary" output pins (certainly logic outputs) can and should be left unconnected, input pins should be connected to a suitable voltage (VCC for TTL logic, VCC or ground for CMOS, somewhere in the acceptable input range for op-amps, comparators and other analog devices), and pins marked "NC" are safest left unconnected (sometimes the manufacturer uses these for in-package test, or for undocumented features).

But always, if the manufacturer says differently in their data sheet, do what they say.

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As Marcus comments: just follow the datasheet or the schematic drawn up by someone else.

There is no general rule concerning pins you do not want to use as it depends on the function of the pin.

In the 555's datasheet we see (on page one) that the discharge pin is connected to the collector of an NPN to ground. There is no harm to leave it unconnected. Connecting the discharge pin to ground is also OK, that would simply short Collector and Emitter and would make no difference to the rest of the 555's circuit.

Generally input pins need to have a properly defined voltage so the TRIG pin should never be floating as that can result in unexpected behavior.

The "CONT" is also an input but if it is left floating then there is an internal resistor divider which sets the voltage of the CONT pin (at 2/3 of Vcc). So the CONT pin can be left floating if needed. Often a capacitor to ground is connected to it to make that internal voltage more stable. You could also connect the CONT pin to some other voltage or a variable resistor to influence the voltage on the CONT pin. Some 555 circuits use this in some way.

So: it is perfectly fine to leave a pin unconnected if the circuit needs that.

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