82C55A

Combinations of MODE 1

Port A and Port B can be individually defined as input or output in Mode 1 to support a wide variety of strobed

IO applications

231256 – 17

Figure 12 Combinations of MODE 1

Operating Modes

MODE 2 (Strobed Bidirectional Bus IO)

This

functional configuration provides a means for com-

municating with a peripheral device or structure on a

single 8-bit bus for both transmitting and receiving

data (bidirectional bus IO) ‘‘Handshaking’’ signals

are provided to maintain proper bus flow discipline in

a similar manner to MODE 1 Interrupt generation

and enabledisable functions are also available

MODE 2 Basic Functional Definitions

bit control port (Port C)

and status for the 8-bit bi-directional bus port

(Port A)

Bidirectional Bus IO Control Signal Definition

INTR (Interrupt Request)

A high on this output can

be used to interrupt the CPU for input or output oper-

ations

Output Operations

OBF (Output Buffer Full)

The OBF output will go

‘‘low’’ to indicate that the CPU has written data out

to port A

ACK (Acknowledge)

A ‘‘low’’ on this input enables

the tri-state output buffer of Port A to send out the

data Otherwise the output buffer will be in the high

impedance state

INTE 1 (The INTE Flip-Flop Associated with

OBF)

Input Operations

STB (Strobe Input)

A ‘‘low’’ on this input loads

data into the input latch

IBF (Input Buffer Full FF)

A ‘‘high’’ on this output

indicates that data has been loaded into the input

latch

INTE 2 (The INTE Flip-Flop Associated with IBF)

13