82C55A
Ports A, B, and C
The 82C55A contains three
Port A One
Port B One
Port C One
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 | |||
| 82C55A | CS | |
|
| ||
MODE 0 | C |
| A |
B |
|
| |
8 I/O | 4 I/O | 4 I/O | 8 I/O |
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 |
CONTROL CONTROL | ||
| 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 |
| |
| 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 -
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
FIGURE 4. MODE DEFINITION FORMAT
4