a8255

`a8255 MegaCore function implementing a programmable peripheral interface adapter
`Optimized for FLEX®and MAX®architectures
`24 programmable inputs/outputs
`Static read/write or handshaking modes
`Direct bit set/reset capability
`Synchronous design
`Uses approximately 194 FLEX logic elements (LEs)
`Functionally based on the Intel 8255A and Harris 82C55A devices,except as noted in the "Variations & Clarifications" section on page 56

The a8255 MegaCore function implements a programmable peripheral interface adapter (see Figure 1). The a8255 has 24 I/O signals that can be programmed in two groups of 12. This MegaCore function operates in the following three modes:
` Mode 0: Basic Input/Output
   -Port A, port B, and port C (upper and lower) can be independently configured as inputs or outputs to read or hold static data. Outputs are registered; inputs are not registered.
` Mode 1: Strobed Input/Output
   -Port A and port B can be independently configured as strobed input or output buses. Signals from port C are dedicated as control signals for data handshaking.
` Mode 2: Bidirectional Bus
   -Port A can be configured as a bidirectional bus with the majority of port C providing the control signals. In this configuration, port B can still implement mode 0 or mode 1.