WS82C55A
Ports A, B, and C
The WS82C55A
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
WS82C55A
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.
INPUT MODE
MASTER
RESET
OR MODE
CHANGE
INTERNAL
DATA IN
INTERNAL
DATA OUT
(LATCHED)
OUTPUT MODE
EXTERNAL
PORT A PIN
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
82C55A
A0-A1
CS
MODE 0
C
B
8
I/O
4
I/O
4
I/O
A
8
I/O
PB7-PB0
MODE 1
PC3-PC0
C
PC7-PC4
PA7-PA0
B
8
I/O
CONTROL CONTROL
OR I/O
OR I/O
C
I/O
A
8
I/O
PB7-PB0
MODE 2
PA7-PA0
B
8
A
BI-
DIRECTIONAL
FIGURE 2A. PORT A BUS-HOLD CONFIGURATION
RESET
OR MODE
CHANGE
V
CC
P
PB7-PB0
CONTROL
PA7-PA0
FIGURE 3. BASIC MODE DEFINITIONS AND BUS INTERFACE
CONTROL WORD
INTERNAL
DATA IN
INTERNAL
DATA OUT
(LATCHED)
OUTPUT MODE
EXTERNAL
PORT B, C
PIN
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
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
WS82C55A
can remain in the input mode with no additional ini-
tialization required. This eliminates the need to pullup or pull-
down resistors in all-CMOS designs. The control word
FIGURE 4. MODE DEFINITION FORMAT
4
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