Diamond Systems 16-Channel 12-Bit Analog Output PC/104 Module 82C55A, Operational Description

82C55A

Ports A, B, and C

The 82C55A contains three 8-bit ports (A, B, and C). All can be configured to a wide variety of functional characteristics by the system software but each has its own special features or “personality” to further enhance the power and flexibility of the 82C55A.

Port A One 8-bit data output latch/buffer and one 8-bit data input latch. Both “pull-up” and “pull-down” bus-hold devices are present on Port A. See Figure 2A.

Port B One 8-bit data input/output latch/buffer and one 8-bit data input buffer. See Figure 2B.

Port C One 8-bit data output latch/buffer and one 8-bit data input buffer (no latch for input). This port can be divided into two 4-bit ports under the mode control. Each 4-bit port con- tains a 4-bit latch and it can be used for the control signal output and status signal inputs in conjunction with ports A and B. See Figure 2B.

register will contain 9Bh. During the execution of the system program, any of the other modes may be selected using a single output instruction. This allows a single 82C55A to service a variety of peripheral devices with a simple software maintenance routine. Any port programmed as an output port is initialized to all zeros when the control word is written.

ADDRESS BUS

CONTROL BUS

DATA BUS

RD, WR	D7-D0		A0-A1
	82C55A		CS
MODE 0	C		A
B
8 I/O	4 I/O	4 I/O	8 I/O
PB7-PB0	PC3-PC0	PC7-PC4	PA7-PA0

INPUT MODE

MASTER

RESET

OR MODE

CHANGE

INTERNAL

DATA IN

INTERNAL

DATA OUT

(LATCHED)

OUTPUT MODE

EXTERNAL PORT A PIN

MODE 1		C
B		A
8 I/O		8 I/O
PB7-PB0	CONTROL CONTROL PA7-PA0
	OR I/O	OR I/O
MODE 2		C
B		A
		BI-

FIGURE 2A. PORT A BUS-HOLD CONFIGURATION

RESET	VCC
OR MODE	P
CHANGE

8 I/O	DIRECTIONAL
PB7-PB0	PA7-PA0
	CONTROL

FIGURE 3. BASIC MODE DEFINITIONS AND BUS INTERFACE

INTERNAL	EXTERNAL
DATA IN	PORT B, C
INTERNAL	PIN
DATA OUT
(LATCHED)

OUTPUT MODE

FIGURE 2B. PORT B AND C BUS-HOLD CONFIGURATION

FIGURE 2. BUS-HOLD CONFIGURATION

Operational Description

Mode Selection

There are three basic modes of operation than can be selected by the system software:

Mode 0 - Basic Input/Output Mode 1 - Strobed Input/Output Mode 2 - Bi-directional Bus

When the reset input goes “high”, all ports will be set to the input mode with all 24 port lines held at a logic “one” level by internal bus hold devices. After the reset is removed, the 82C55A can remain in the input mode with no additional ini- tialization required. This eliminates the need to pullup or pull-

CONTROL WORD

D7 D6 D5 D4 D3 D2 D1 D0

GROUP B

PORT C (LOWER) 1 = INPUT

0 = OUTPUT

PORT B

1 = INPUT

0 = OUTPUT

MODE SELECTION 0 = MODE 0

1 = MODE 1

GROUP A

PORT C (UPPER) 1 = INPUT

0 = OUTPUT

PORT A

1 = INPUT

0 = OUTPUT

MODE SELECTION 00 = MODE 0

01 = MODE 1

1X = MODE 2

MODE SET FLAG 1 = ACTIVE

down resistors in all-CMOS designs. The control word

FIGURE 4. MODE DEFINITION FORMAT

4