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Am8255A OPERATION CONTROIL WORD FORMAT
The operation contY'ol word (bit j7=1) defines three basic modes of
operation.
•
Mode 0
=
Basic Input/Output
•
Mode 1
=
Strobed Input/Output
•
Mode 2
=
Bidirectional Bus
The modes for port A and B can be separately defined, while port C is
divided into two 4 bit ports as required by the port A and port B
definitions.
Table 3-2 provides a summary of mode definitions and port
restrictions.
The mode control word format is shown in figure 3-7.
Am8255A BIT SET/RESET CONTROL WORD
LJhen operati ngi n mode 1 or 2,
thE~
bits of the port C can be set or
reset using the bit set/reset control word.
The functions of some port
C bi ts are defi ned by port A and B operat ions in modes 1 and 2.
Refer
to table 3-2 for porot C bit definitions in modes 1 and 2.
Figure 3-8
shows the bit set/reset control word format.
~~~
L
Port C,
an.
0-3
1
=
Input
o
=
Output
- - Port B
1
=
Input
o
=
Output
- - - - - Mode Selection
1
=
Mode 1
o
=
Mode 0
L..-._____
Port C. Bits 4-7
1
=
Input
o
=
Output
- - - - - - - Port A
1
=
Input
o
=
Output
Mode
00
=
Mode
0
01
=
Mode 1
1X
=
Mode 2
, - - - - , - - - 1
=
Operation Control Word
Figure 3-7.
Arn8255A OpeY'ation Control Word Format
3-11
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