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I have been reading the datasheet of RV-3028-C7. Everytime it talks about a pin that might be unused in a certain scenario (should be logic low), it instructs to connect the pin to ground through a 10k resistor.

Page 103: https://www.microcrystal.com/fileadmin/Media/Products/RTC/App.Manual/RV-3028-C7_App-Manual.pdf

My question is why we don't connect it directly to ground? Why we connect it through a resistor?

Is this some kind of best practice or what? Should I do it with all other components where I have an unused pin and I want it to pull it to logic low?

Thanks.

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    \$\begingroup\$ I read the datasheet and can't see such suggestions. Please point to a datasheet you are reading. \$\endgroup\$
    – Justme
    Sep 25, 2022 at 13:11

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Your provided source material explains does not say it is required to use a resistor. All it says is that you must not leave the pin floating. It then suggests that, if desired, you can use a resistor and then it tells you why: To keep the functional test possible. enter image description here

That means it is saying that connecting through a pull resistor lets you override it and use the pin if required such as during testing.

If the input is a BJT base, connecting to ground through a resistor is necessary to limit current through PN base-emitter junction. Connecting the base directly to ground in this case would be producing a short via diode if there is a high-side PNP transistor inside the input. The same thing is true for pull-up resistors and low-side NPN transistors.

But if the input is a CMOS this isn't a problem because the gate is insulated and cannot pass DC.

But on top of this, there are many reasons why you might not want to connect an unused pin directly to a power or ground rail, and sometimes you can't. It depends on whether the pin is an input or output and whether it is analog or digital.

  • Directly connecting unused outputs to a ground or power rail is very bad. Outputs are driving a signal voltage so you don't want to short them out. You could connect them to a power rail through a resistor but there is often little reason to since they are not floating and doing so would just result in increased current and potentially excessive or even damaging power dissipation.

That generally works for digital inputs and outputs. But analog inputs and outputs can have other considerations. You might not want to connect the inputs or outputs to the power rail, even through a resistor. You might not even be able to safely do so. See for more details: https://www.ti.com/lit/an/sboa204a/sboa204a.pdf

But basically:

  • Opamp inputs don't always have a common mode range that swings the entire range of the power rails in which case you don't want to tie the input to a ground or power rail with or without a resistor.
  • Even if the opamp could safely tolerate being tied to a rail you might not want to since it would cause the output to saturate and result in increased current consumption and potentially overheat. It can also cause cross-channel effects.
  • So the safest thing to do is wire up the unused opamp as a buffer that feeds to nothing and then bias the input of the buffer to mid-supply.
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  • \$\begingroup\$ Thank you very much for this answer. It has accurately and thoroughly answered my question and I have also acquired very useful knowledge to improve my hardware design in the future. Thank you very much for this amazing answer. \$\endgroup\$
    – Mora
    Sep 27, 2022 at 9:54

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