I wonder if unused pins of an IC, lets say an NE555 f.e., should be connected to ground via a resistor to prevent noise or not. I have an AVR with analog inputs and just realized there's huge noise, if not connected. So I'm not sure if it is OK to leave unused Pins of other IC's like the 555 Timer unconnected, or better pull them to ground via 10kΩ or 100kΩ resistor?
In other words: could it get trouble, if unused pins of a ne555 left open?
If a pin is labelled N/C by a manufacturer then this almost always means that it should be NOT CONNECTED to anything.
ALL standard 8 pin xx555 timer IC have NO NC pins.
ALL pins have an assigned role.
Some should be left open circuit in some designs - but that is done as part of the design, as required.
Finding an NE555FE datasheet proved wondrous hard.
This page shows NE555FE and NE555F.
http://pdf1.alldatasheet.com/datasheet-pdf/view/17984/PHILIPS/NE555F.html
The NE555FE shown is a standard 8 pin package pinout and has NO NC pins.
The NE555F is an abomination which I have never seen before and which I did not know existed. If you have such an IC you should drive a stake through it's heart, wrap it in garlic, bury it at the nearest cross roads and/or send it to a local technology museum. I would also love a sample for my museum.
If you had one of these and insisted on using it then leaving the NCs disconnected would be safest. But, I'd advise using one of the many many many other 8 pin ones as this is an orphan.
I have seen pins which are labelled NC on a device's datasheet but which DO have internal connections. In some cases, connecting such to ground or supply is 'fatal'. Some modern LEDS have metallic connections on their thermal pad which MUST NOT be connected as if they were LED Cathode or Anode, as they are neither, but are not electrically isolated. Doing this seems very very unwise, but it's done.
When deciding what to do with NC pins, it may be helpful to consult the data sheets of the chips in question and those of possible alternate parts. It's possible that a pin might have no die connection on a particular part, but perform some potentially-bothersome function a similar part. If the latter part becomes more common in the marketplace, it might be desirable to allow substitution; if the pin is used for routing, or is strapped in a way inconsistent with that other part's function for it, substitution could be difficult.
On the other hand, in other cases it may be a good idea to make provisions to pull an unused pin high or low (e.g. with an optional resistor) or, occasionally, to strap it in some other way. For example, if one is using a particular size of parallel flash chip which has some NC pins, and if the corresponding pins on the next larger size are address pins, it may not be a bad idea to examine the data sheet of the larger chip and determine whether the pins could be connected in such a way as to permit the larger and smaller chips to be used interchangeably. As larger chips become more common, they can sometimes become cheaper than smaller ones; even if one wouldn't have any use for the extra capacity of the larger chip, if it would be suitable in the design it may be worth allowing it to be used for at least the smaller capacity. Depending upon the chip and the system design, it may be better to strap them high or low, or it may be better to strap them to some other address bit.
Had the same problem with the Microchip MRF24WB0MA RF transceiver, which has several NC pins, which I diligently tried not to connect.
Howeverm due to the difficulty in soldering this part (IMO), some of the NC pins got grounded through solder slivers, and the chip subsequently failed to initialise, sinking current heavily until it failed altogether.
So unfortunately, NC does not imply that the pin is not connected in the inside, simply that it should not be connected from the outside.!
Maybe NC pins are used in debugging / production test? Otherwise why have them there?
It depends on the characteristics of the pin. If the datasheet marks the pin as N/C or "do not connect" then don't make any connection. Outputs in general should be left floating. For input pins you need to refer to the datasheet's recommendations. If the input is a high-impedance digital input for example, then a pull-up or pull-down is usually required. Of course if there is already an internal pull-down, adding an external pull-up resistor may not a good idea - hence the need to consult the datasheet. Unused op-amp or other analogue inputs should normally by tied either directly or through a capacitor to the reference rail.
Some pins may need special attention. For example the NE555's CTRL pin is at a defined dc voltage and should be bypassed to 0V with a capacitor if not used (as per the datasheet).
