Control Word 8255

Control Word Format 8255

The 8255A has an 8 bit control register.  The contents of this register called, the control word, decides the I/O function for each port.  This register is not accessible for a read operation.

control word for I/O modes

 When D0 = 0,  port C Lower - output port

                        D0 = 1,  port C Lower - input port.

            The bit D1 configures port B as an 8 bit wide input or output

            When   D1 = 0,  port B is output port.

                        D1 = 1,  port B is input port.

            The bit D2 is the mode select bit for port B and the lower 4 bits of port C.

                        D2 = 0, selects Mode 0

                        D2 = 1, selects Mode 1.

            The bits D3 through D6 in the control register correspond to the group A control.

            Bits D3 and D4 of the control register are used to configure the operation of the upper half of port C and complete port A.

            When   D3 = 0,  port C upper is output port.

            or         D3 = 1,  port C upper is input port.

            Similarly when D4 = 0, port A is output port.

                              or   D4 = 1, port A is input port.

            The bits D5 and D6 are used to select among the three modes of operation names mode 0, mode 1 and mode 2.

                        D6 D5             = 00 selects mode 0

                                    = 01 selects mode 1

                                    = 1x selects mode 2.

            The last control register bit D7 is the mode set flag, which is shown in Fig.3.

                        D7       = 1 selects I/O mode and

                        D7       = 0 selects BSR mode.

BSR Mode Control Word Register Format

 The bit set/reset function is not really a control word.  Instead, it allows individual bits of port C to be set or reset.  But only one bit can be set or reset at a time.  One of the advantages of this mode is that individual bits of port C can be changed without changing any of the others.  This is needed when port C is used to control the ON/OFF status of several external devices.

            For example, the device connected to PC4 can be turned ON without affecting the status of any devices connected to the other seven outputs.

            The bit set/reset function is used in mode 1 and 2 to enable interrupt outputs available in these modes.

OPERATIONAL MODES OF 8255