82C55A
two 4-bit ports under the mode control. Each 4-bit port
contains 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
MODE 0
RD, WR
D7-D0
82C55A
C
B
8
I/O
4
I/O
4
I/O
A
8
I/O
A0-A1
CS
ADDRESS BUS
CONTROL BUS
DATA BUS
PB7-PB0
MODE 1
PC3-PC0
C
PC7-PC4
PA7-PA0
FIGURE 2A. PORT A BUS-HOLD CONFIGURATION
B
8
P
PB7-PB0
MODE 2
INTERNAL
DATA IN
INTERNAL
DATA OUT
(LATCHED)
OUTPUT MODE
EXTERNAL
PORT B, C
PIN
CONTROL CONTROL
OR I/O
OR I/O
C
I/O
PA7-PA0
I/O
A
8
I/O
RESET
OR MODE
CHANGE
V
CC
B
8
A
BI-
DIRECTIONAL
PB7-PB0
CONTROL
PA7-PA0
FIGURE 2B. PORT B AND C BUS-HOLD CONFIGURATION
FIGURE 2. BUS-HOLD CONFIGURATION
FIGURE 3. BASIC MODE DEFINITIONS AND BUS INTERFACE
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 - Bidirectional 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
initialization required. This eliminates the need to pull-up or
pull-down resistors in all-CMOS designs. The control word
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.
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
FIGURE 4. MODE DEFINITION FORMAT
5
FN2969.10
November 16, 2006
INTERSIL [ INTERSIL CORPORATION ]
