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Serial Communication Modules
MOTOROLA
MPC801 USER’S MANUAL
16-37
16
16.4.3 The Port B Registers
Port B has four 16-bit memory-mapped read/write control registers that must all be written
according to their size. No byte-enable mechanism is available.
16.4.3.1 PORT B OPEN-DRAIN REGISTER.
The port B open-drain (PBODR) register
indicates a normal or wired-OR configuration of the port pins. This register is cleared at
system reset.
OD16–OD31—Driver Type
Each pin in port B has a dedicated control bit that selects the driver type. It must decide
between an active or open-drain driver.
0 = The I/O pin is actively driven as an output.
1 = The I/O pin is an open-drain driver. As an output, the pin is actively driven low.
Otherwise, it is three-stated.
16.4.3.2 PORT B DATA REGISTER.
A read of the port B data (PBDAT) register returns
the data to the pin, regardless of whether the pin is defined as an input or output. This allows
the pin to detect output conflicts by comparing the written data with the data on the pin. A
write to the PBDAT register is latched and if that bit in the PBDIR register is configured as
an output, the value latched for that bit is driven to its respective pin. This register can be
read or written to at any time. It is not initialized, but is undefined at reset.
D16–D31—Data
Each bit in the register reflects the pin value, regardless of whether the pin is an input or
output.
PBODR
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
FIELD
OD16
OD17
OD18
OD19
OD20
OD21
OD22
OD23
OD24
OD25
OD26
OD27
OD28
OD29
OD30
OD31
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
PBDAT
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
FIELD
D16
D17
D18
D19
D20
D21
D22
D23
D24
D25
D26
D27
D28
D29
D30
D31
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W