a8255

®
Programmable Peripheral
Interface Adapter
September 1996, ver. 1
Features
n a8255 MegaCore function implementing a programmable
peripheral interface adapter
n Optimized for FLEX
®
and MAX
®
architectures
n 24 programmable inputs/outputs
n Static read/write or handshaking modes
n Direct bit set/reset capability
n Synchronous design
n Uses approximately 194 FLEX logic elements (LEs)
n Functionally based on the Intel 8255A and Harris 82C55A devices,
except as noted in the “Variations & Clarifications” section on
page 56
General
Description
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:
n 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.
n 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.
n 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.
Figure 1. a8255 Symbol
PAEN
PBEN
A8255
CLK
nCS
nRD
Altera Corporation 45
A-DS-A8255-01
DOUT[7..0]
PAOUT[7..0]
PBOUT[7..0]
PCEN[7..0]
PCOUT[7..0]
nWR
RESET
A[1..0]
DIN[7..0]
PAin[7..0]
PBin[7..0]
PCin[7..0]