Port Configuration
Each port or half-port is configured as an input or an output according to
the value of the control word in the
PPI
control register. The control word
is 1-byte word programmed at Base A3. Refer to Table 4-1 for the
complete 8255
PPI
address map.
Table 4-1. 8255 PPI Address Map
Address
Base A0
Base A1
Base A2
Base A3
Function
Port A (PA)
Port B (PB)
Port C (PC)
Control Word
Type
Read/Write
Read/Write
Read/Write
Write only
The control word also specifies one of the following three operating
modes (for a complete description of the
PPI
operating modes, refer to the
Intel 8255-5 data sheet):
•
M o d e
0 - Basic I/O; all ports are I/O ports.
•
M o d e
1 - Strobed I/O; part of Port C controls data transfer.
•
M o d e
2 - Bidirectional I/O on Port A only; part of Port C controls
data transfer.
A power-up or reset automatically configures all ports as inputs. This
configuration remains intact until changed by a write to the control
register. The initialization section of your program should set the port
4-2
P
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a
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