Question

The port C of 8255 can be configured to work in
1. mode 0, mode 1, mode 2 and BSR
2. mode 0, mode 1 and mode 2
3. mode 2 and BSR
4. BSR mode only

Answer 1

Correct Answer - Option 1 : mode 0, mode 1, mode 2 and BSR

8255:

• It consists of three 8-bit bidirectional I/O ports i.e. PORT A, PORT B and PORT C.
• Port A can work either in mode 0, mode 1 or mode 2 of input-output mode.
• Port B can work in either mode 0 or in mode 1 of input-output mode.
• Port C can work in either BSR (bit set rest) mode or in mode 0, mode 1 and mode 2 of input-output mode of 8255.

 

BSR Mode:

In this mode any of the 8-bits of port C can be set or reset depending on D₀ of the control word.

Mode 0:

• In this mode, Port A and B is used as two 8-bit ports and Port C as two 4-bit ports
• Each port can be programmed in either input mode or output mode where outputs are latched, and inputs are not latched
• Ports do not have interrupt capability

 

Mode 1:

• In this mode, Port A and B is used as 8-bit I/O ports
• They can be configured as either input or output ports
• Each port uses three lines from port C as handshake signals
• Inputs and outputs are latched

 

Mode 2:

• In this mode, Port A can be configured as the bidirectional port and Port B either in Mode 0 or Mode 1
• Port A uses five signals from Port C as handshake signals for data transfer
• The remaining three signals from Port C can be used either as simple I/O or as handshake for port B