integrated circuit - General "rule of thumb" for unused IC pins

This is somewhat similar to this question about NC pins. In cases where the datasheet doesn't specify what to do with unused IC pins, what is the recommendation to do with these pins? In particular I'm thinking of GPIO pins for the AT32UC3C microcontroller, but also more generally for other IC types (for example multi op-amp IC's).

I can think of a few possible combinations:

1. Provide a solder pad/hole so the chip is secured but leave pin floating.


2. Connect the pin to ground (possible through a resistor/capacitor/etc.)


3. Connect the pin to a supply source



4. Something I can't possibly think of

Answer

1. 
   

   "Provide a solder pad". Of course you will. All the IC's pins should be soldered. Always. Leaving it floating is NOT a good idea. It may change level all the time, which may have undesired effects to the internally connected circuitry. You always want to have predictive behavior. If the input has an internal pull-up resistor this is a good solution, though, as long as you don't forget to enable it.

   
   


2. 
   

   "Connect the pin to ground". That's a good solution, provided that you can guarantee that the pin will never become an output. Output high and you short-circuit the power supply. A resistor would prevent that, but that's an extra cost. Don't use a capacitor; it would leave the pin floating, and the microcontroller doesn't like the capacitive load in case it would become output.

   
   



3. 
   

   "Connect the pin to a supply source". Same as above: if the pin should become output low you'll have a short-circuit.

   
   


4. 
   

   "Leave unconnected, but make the pin output". That's the best solution. Don't use the possible alternate functions, like ADC or serial. A high level is preferred in case you forgot to switch off the internal pull-up resistors, which otherwise would cause a (small) leakage current.

   
   

For opamps the output can be left open, and the inputs to a fixed voltage, but not both to the same! I recently saw in a Linear Technology application note how they connected the non-inverting input to V+, the inverting input to V-. Szymon rightly points out that this can't be used if the inputs have clamping diodes.

The best thing to do with a surplus op-amp it is to use it. There are lots of places in an analog circuit where a buffer amplifier may improve performance - and a unity gain buffer uses no extra components. (from this article, linked to by Szymon)