Hi Mohit,

the port structure of the 8051 is "wired-and" of the port latch and the alternate function. So there is no need to select the one or the other and both usable together.
A low from the port override a high from the alternate function and reverse.

Also the output stage is open drain with pull up. So there is no need to select input or output mode and both usable together.
a low from port or alternate function override a high from an external source connected to the pin and reverse.

The only exception was P0 and P2 during external memory access. Then the state of the P0 and P2 latches was ignored.


There is an 8051 users manual on the Atmel web, which describes the port architecture more verbose. Or the Philips book IC20.


Peter