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2. Port 4 Data Register (P4DR)—H'FF87
Bit
Initial value
Read/Write
P4DR is an 8-bit register containing the data for pins P4
At a reset and in the hardware standby mode, P4DR is initialized to H'00.
When the CPU reads P4DR, for output pins it reads the value in the P4DR latch, but for input
pins, it obtains the pin status directly.
9.5.3 Pin Functions in Each Mode
Port 4 has different functions in the expanded modes (modes 1, 2, 3, 4) and the single-chip mode
(mode 7). Separate descriptions are given below.
Pin Functions in Expanded Modes: In the expanded modes (modes 1, 2, 3, and 4), port 4 is
used for output of the low bits (A
output. Figure 9-9 shows the pin functions for the expanded modes.
Pin Functions in Single-Chip Mode: In the single-chip mode (mode 7), each of the port 4
pins can be designated as an input pin or an output pin, as indicated in figure 9-10, by setting
the corresponding bit in P4DDR to "1" for output or clearing it to "0" for input.
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7
6
5
P4
P4
P4
7
6
0
0
0
R/W
R/W
R/W
– A
7
Port
4
Figure 9-9 Port 4 Pin Functions in Expanded Modes
4
3
P4
P4
5
4
3
0
0
R/W
R/W
to P4
7
) of the address bus. P4DDR is automatically set for
0
A
(output)
7
A
(output)
6
A
(output)
5
A
(output)
4
A
(output)
3
A
(output)
2
A
(output)
1
A
(output)
0
156
2
1
P4
P4
2
1
0
0
R/W
R/W
.
0
0
P4
0
0
R/W
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