IO ports

There are uni-directional ports as well as bi-directional ports. Bi-directional ports are complex. As we have limited ports we may want to use part of the ports as output and part of it as input, because of that there is a functionality of deciding port direction bit by bit.

PORT A

PORT B

PORT C

This is another 8–bit port. Port-C is shared among a number of peripheral devices including communication, timer, Timer and PWM. These peripheral devises have the higher priority over the PORT C; therefore, if we use them we cannot use port-C.

PORT E

3-pin register, with individual pins configurable as output or Schmitt Trigger inputs (we know that TTL logic has a forbidden range of voltage but in Schmitt trigger inputs there is no forbidden range). Therefore if we cannot sure about the range of the input (in TTL range of not) we can use PORT-D. Also used as control / handshake signals for Port D when such is used in Parallel Slave Port mode.

PORT D

Can be used as bit addressable Digital IO port or as, 8-bit microprocessor parallel port compatible with standard types of microprocessor busses.

Below diagram shows PORT D in PSP (parallel slave port) mode

PSP – This is a parallel port which reads other devices (That’s why it is called as slave)
Those external devises may use interrupt or polling, but there is no way that notify a external devise that we have written something to port.