AT89C51CC02CA-RATUM datasheet(7/156 Pages) ATMEL

7 / 156 page

AT/T89C51CC02

4126J–CAN–05/06

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.

Port Structure

Figure 1 shows the structure of Ports, which have internal pull-ups. An external source

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

Note:

1. The internal pull-up can be disabled on P1 when analog function is selected.

LATCH

INTERNAL

WRITE

LATCH

READ

LATCH

ALTERNATE

OUTPUT

FUNCTION

VCC

INTERNAL

PULL-UP (1)

ALTERNATE

INPUT

FUNCTION

BUS

P2.x

P3.x

P4.x

P1.x

(1)

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