Salient features of a parallel programmable interface-8255, Computer Engineering

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Determine the salient features of a parallel programmable interface, 8255.

  • 24 I/O lines in 3 8-bit port groups - A, B, C
  • A, B can be 8-bit input or output ports
  • C can serve as 2 4-bit input or output ports
  • 3 modes of operation:

- Mode 0: A, B, C simple input or output level sensitive ports

- Mode 1: A, B input or output ports with strobe control in C

- Mode 2: A is bidirectional with control/handshake in B and C

  • A, B can only change 1 byte at a time
  • C has individual bit set/reset capability
  • Advantage is non-dedicated circuit can change port configuration with software and no 'glue logic'
  • Ports A, B, and C are used for I/O data.
  • The control register is programmed to select the operation mode of the three ports A, B, and C.
  • Mode0: simple I/O mode
  • Any of the ports A, B, CL and CU can be programmed as input or output.
  • No control of individual bits (all bits are out or all bits are in)
  • Mode 1: Ports A and B can be used as input or output ports with handshaking.
  • Mode2: Port A can be used as bidirectional I/O port with handshaking

 


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