Hi Joseph,

from your question it is not clear what kind of pins you are talking about.

- If you mean unused INPUT pins of logic circuits like 40xx series or 74xx series, than you have to pull them up or down, else your circuit can behave wrong. Leave unused outputs open.

- If you mean unused I/O pins of an 8051 kind of device than it depends on the type of pin. After a hardware reset all I/O pins are defined as INPUTS. The pins of ports 1, 2 and 3 have internal pull-up resistors, so the pins can be left open. The pins of port 0 on the other hand do NOT have internal pull-up resistors, when these are configured as inputs, they are floating. Like San said, it is recommended to use here resistors to pull the pins up or down, so IF accidently the pins are programmed as output, the outputstage will not fry.

To be sure about how input pins are build internally, always READ THE DATASHEETS ! For example: the AT89C1051 and AT89C2051 have NO internal pull-up resistors at P1.0 and P1.1 (for they are also inputs of a comparator...) so in this case you MUST add external pull-ups.

Henk