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