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I am a newbie in electronics and I am designing a printed circuit board.

The IC I am using has some pins that I don't use in my circuit. Is it okay to ground those nets or leave them as they are?

I am asking this because I have heard that sometimes these nets can act as floating pins and could cause noise and errors. I am not sure whether this is true or not so please clarify my doubt and suggest me the right way.

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    \$\begingroup\$ Do you have a schematic? \$\endgroup\$ – Bart Jan 10 at 11:19
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    \$\begingroup\$ Check the datasheet. It will tell you whether to float, ground, or pull up, or [insert other constraints]. \$\endgroup\$ – PNDA Jan 12 at 9:04
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If you check the HW Design Manual then you'll see that it suggests the unused pins should be left floating. Please refer to Table 4 on p.15 and p.16.

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You will need to check the datasheets for each part with unused pins. In many cases grounding them is a BAD idea, though in many other cases it may be CRUCIAL. And in some cases you SHOULD leave the pins floating. For example, do not connect outputs directly to ground, as this could cause a short. Do not leave inputs floating, unless they have internal pull-ups/pull-downs. The datasheets should tell you what to do with unused pins, but even if they don't you should be able to determine what to do on a pin-by-pin basis by analysis. There is no one-size-fits-all solution for this problem, so you will have to sort through the pins individually to figure out how to connect (or not connect) them.

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One thing you CAN do when laying out the PCB is to ground all pins via a resistor on each pin.

Then you can fit 0 ohms for a solid ground, 1K or 10K to stop the pin floating (without damage if something drives it, plus you can pull it high during test), or infinity (no resistor) if you must leave it floating.

Final decision can be made for each pin during testing.

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    \$\begingroup\$ You can, but it's still not necessarily the right thing to do. Power consumption, for instance - many logic device families draw more power on a grounded input than on an input at 5V (or whatever your supply level is). And if those unused inputs are connections to a gate you're using - set/reset inputs to a flip-flop, for instance - then you definitely need to know what to say them to, otherwise the gate won't do what you expect. Ditto enables on microcontroller peripherals or anything like that. \$\endgroup\$ – Graham Jan 10 at 23:59
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    \$\begingroup\$ @Graham This answer is about designing the PCB with the flexibility to decide what to do with each pin when the board is populated. I'm not sure what part of that you're objecting to. \$\endgroup\$ – kwc Jan 11 at 1:45
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    \$\begingroup\$ This is a bad design approach. I second @Graham,s comment and, in addition, placing additional “unneeded” should be avoided for economic reasons. Why put 0 ohms resistors when a short will solve the problem? What is the basis that leads to the specific resistor values you suggest? A couple of answers already suggest reading the datasheet first; this is the best approach. \$\endgroup\$ – Krauss Jan 11 at 4:37
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    \$\begingroup\$ @kwc Because it's generally more common to need a pull-up than a pull-down. Because reading and following the datasheet is absolutely not something which should happen during testing. Because it uses PCB real estate. Because fitting unneeded resistors takes extra money and time. Sometimes of course you do want to break out unused gates in case you need them later, but it is bad practise to do it blindly for every pin. \$\endgroup\$ – Graham Jan 11 at 8:38
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    \$\begingroup\$ @Graham; your points wrt pullups and power are correct with regard to TTL bipolar logic, but there's not much of that in new designs nowadays. And I admire your confidence in the clarity, completeness and correctness of datasheets. With regard to "fitting unneeded resistors" ... indeed if they are unneeded, don't fit them! \$\endgroup\$ – Brian Drummond Jan 11 at 12:48
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Unused output pins should just be left disconnected.

For unused input or I/O pins you need to read the datasheet and/or make some discisions. As another answer points out the datasheet for your simcom device says you should leave them floating, so that is what I would do in the first instance.

As a general rule though, a CMOS input should not be just left floating. Doing so can significantly increase the current draw of the chip as the transistors in the input buffer start to conduct.

One option for IO pins is to configure the pin as an output. Then you can just leave it disconnected. Downside is if you make a mistake as to which pins you should set as outputs you may end up shorting an output driver.

Some input pins may have internal pull-ups and/or pull-downs which can be enabled, these will hold the pin in a known state with less risk of mistakes than setting them as an output. In the case of a module there may also be pull-ups/pull-downs on the module PCB.

If you aren't hugely constrained for cost or space then an external pull up/down is often the way to go. This can also double up as a conviniant place to solder on a bodge wire when you find out you needed the pin after all.

Tieing inputs hard to the power rail or ground is something I preffer to avoid where possible. It may lead to high currents if a pin is inadverantly set as an output, and often makes it harder to modify the board when you realise you need the input after-all.

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Generally no input should be left floating. Check the data sheet to determine the logic level required for the application you have. If the input should be logic zero, ground it. If it has to be logic one, tie it to logic one. If the status does not affect your operation, check the name of it has a bar on it, meaning an active low input, connect it to positive supply through proper pull up resistor, otherwise connect it ground. Do not live them open because they may pick up noise and change state and these transients may affect the circuit operation and cause undeterministic behaviour.

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  • \$\begingroup\$ Note however that you shouldn't connect outputs to Vcc or gnd, either directly or through a pull up/down resistor. Unless stated by the datasheet, outputs should be left alone. (This is received wisdom for me so it may be not entirely correct, but I think it is) \$\endgroup\$ – Jachdich Jan 12 at 9:03

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