WS82C55A
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.
CONTROL WORD
D7 D6 D5 D4 D3 D2 D1 D0
X
X
DON’T
CARE
X
BIT SET/RESET
1 = SET
0 = RESET
BIT SELECT
0 1 2 3 4
0 1 0 1 0
0 0 1 1 0
0 0 0 0 1
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
MODE 0 PORT DEFINITION
A
D4
0
0
0
0
D3
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
D1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
B
D0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
GROUP A
PORTC
PORT A (Upper)
Output
Output
Output
Output
Output
Output
Output
Output
Input
Input
Input
Input
Input
Input
Input
Input
Output
Output
Output
Output
Input
Input
Input
Input
Output
Output
Output
Output
Input
Input
Input
Input
#
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
GROUP B
PORTC
PORT B (Lower)
Output
Output
Input
Input
Output
Output
Input
Input
Output
Output
Input
Input
Output
Output
Input
Input
Output
Input
Output
Input
Output
Input
Output
Input
Output
Input
Output
Input
Output
Input
Output
Input
5
1
0
1
6
0
1
1
7
1 B0
1 B1
1 B2
0
0
0
BIT SET/RESET FLAG
0 = ACTIVE
0
1
1
1
1
1
1
1
1
FIGURE 5. BIT SET/RESET FORMAT
Interrupt Control Functions
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.
When WS82C55A
5
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