CS82C55A datasheet(5/26 Pages) HARRIS

5 / 26 page

The modes for Port A and Port B can be separately deﬁned,

while Port C is divided into two portions as required by the

Port A and Port B deﬁnitions. All of the output registers,

including the status ﬂip-ﬂops, will be reset whenever the

mode is changed. Modes may be combined so that their

functional deﬁnition can be “tailored” to almost any I/O

structure. For instance: Group B can be programmed in

Mode 0 to monitor simple switch closings or display compu-

tational results, Group A could be programmed in Mode 1 to

monitor a keyboard or tape reader on an interrupt-driven

basis.

The mode deﬁnitions and possible mode combinations may

seem confusing at ﬁrst, but after a cursory review of the

complete device operation a simple, logical I/O approach will

surface. The design of the 82C55A has taken into account

things such as efﬁcient PC board layout, control signal deﬁ-

nition vs. PC layout and complete functional ﬂexibility to sup-

port almost any peripheral device with no external logic.

Such design represents the maximum use of the available

pins.

Single Bit Set/Reset Feature (Figure 5)

Any of the eight bits of Port C can be Set or Reset using a

single Output instruction. This feature reduces software

requirements in control-based applications.

When Port C is being used as status/control for Port A or B,

these bits can be set or reset by using the Bit Set/Reset

operation just as if they were output ports.

Interrupt Control Functions

When the 82C55A is programmed to operate in mode 1 or

mode 2, control signals are provided that can be used as

interrupt request inputs to the CPU. The interrupt request

signals, generated from port C, can be inhibited or enabled

by setting or resetting the associated INTE ﬂip-ﬂop, using the

bit set/reset function of port C.

This function allows the programmer to enable or disable a

CPU interrupt by a speciﬁc I/O device without affecting any

other device in the interrupt structure.

INTE Flip-Flop Deﬁnition

(BIT-SET)-INTE is SET - Interrupt Enable

(BIT-RESET)-INTE is Reset - Interrupt Disable

NOTE:

All Mask ﬂip-ﬂops are automatically reset during mode se-

lection and device Reset.

Operating Modes

Mode 0 (Basic Input/Output). This functional conﬁguration

provides simple input and output operations for each of the

three ports. No handshaking is required, data is simply writ-

ten to or read from a speciﬁc port.

Mode 0 Basic Functional Deﬁnitions:

• Two 8-bit ports and two 4-bit ports

• Any Port can be input or output

• Outputs are latched

• Input are not latched

• 16 different Input/Output conﬁgurations possible

FIGURE 5. BIT SET/RESET FORMAT

D7 D6 D5

D4 D3 D2 D1 D0

BIT SET/RESET

1 = SET

0 = RESET

BIT SELECT

BIT SET/RESET FLAG

CONTROL WORD

DON’T

CARE

0 = ACTIVE

1234567

01010101

00110011

00001111

MODE 0 PORT DEFINITION

GROUP A

GROUP B

PORT A

PORTC

(Upper)

PORT B

PORTC

(Lower)

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

Output

Input

82C55A

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