Unused pins on microcontroller

[Gondulf]

Hi!<p>I'm using an 8051 for a communication board but I am not using all the port lines.
Some lines are open-collector types and others have internal pull-ups.<p>Do I need to connect all of those unused pins to a pull-down resistor? The board will be operating in a very noisy environment.<p>Thanks!

[haklesup]

In general (and I have not looked at your part number) It is permissable to float any unused output pins. It is also OK to float an input pin if there is an internal pulldown (PD) or pullup (PU)load. If it is an "input only" pin it should be biased to gnd or Vcc through a resistor. I/O pins can also usually be floated.<p>You can sometimes float unused inputs on a bipolar device and only the associated output would be undetermined. If you float input pins on a CMOS device you will almost always end up with an increase in static supply current as well as underermined logic.<p>An exception to floating the PU/PD pins comes when you want to force that pin to the other logic state. For example, a floated input with a pullup will be seen by the device as being connected to logic high. If you want that pin low, you need to connect it to gnd.<p>As for the noisy enviornment, make sure to use sufficient bypass capacitors on the supply close to the chips and don't design the board to be an antenna (i.e. adequate shielding and a ground plane, don't parallel noisy signal lines etc.) and you will be fine.

[jollyrgr]

You can float the outputs but inputs should not be floated. Even though you may have pullups inside the device, save yourself any headache and either pull up the unused pins or pull them down; depending on which works best for your application.

[Gondulf]

Thanks for your help!