AT/T89C51CC02
I/O Configurations
Each Port SFR operates via type-D latches, as illustrated in Figure 1 for Ports 3 and 4. A
CPU ’write to latch’ signal initiates transfer of internal bus data into the type-D latch. A
CPU ’read latch’ signal transfers the latched Q output onto the internal bus. Similarly, a
’read pin’ signal transfers the logical level of the Port pin. Some Port data instructions
activate the ’read latch’ signal while others activate the ’read pin’ signal. Latch instruc-
tions are referred to as Read-Modify-Write instructions. Each I/O line may be
independently programmed as input or output.
can pull the pin low. Each Port pin can be configured either for general-purpose I/O or
for its alternate input output function.
To use a pin for general-purpose output, set or clear the corresponding bit in the Px reg-
ister (x = 1 to 4). To use a pin for general-purpose input, set the bit in the Px register.
This turns off the output FET drive.
To configure a pin for its alternate function, set the bit in the Px register. When the latch
is set, the ’alternate output function’ signal controls the output level (See Figure 1). The
operation of Ports is discussed further in ’Quasi-Bi-directional Port Operation’
paragraph.
Figure 1.
Ports Structure
VCC
ALTERNATE
OUTPUT
FUNCTION
Port Structure
INTERNAL
PULL-UP (1)
READ
LATCH
INTERNAL
BUS
WRITE
TO
LATCH
D
LATCH
CL
Q
P1.x
P2.x
P3.x
P4.x
(1)
READ
PIN
ALTERNATE
INPUT
FUNCTION
Note:
1. The internal pull-up can be disabled on P1 when analog function is selected.
7
4126L–CAN–01/08
ATMEL [ ATMEL CORPORATION ]
