Should unused pins on the MCS 251 Microcontroller, 8xC251SX, be tied to Vcc or Vss?
Ports 1, 2 and 3 have fixed internal s and are referred to as quasi-bi-directional ports. Port 0 is a true bi-directional port (open drain); i.e., the port pins float when configured as input pins. If port 0 is not used, it is advisable to terminate the pins with pull-up resistors or pull the pins to ground in order to avoid floating. All other control pins have internal pull-up resistors. It is up to the user to determine if unused pins require a pull down to Vss. This may be advisable in a noisy environment.
