AT89C51AC3-RDTUM datasheet(9/140 Pages) ATMEL

9 / 140 page

AT89C51AC3

4383D–8051–02/08

Figure 3. Port 2 Structure

Notes:

1. Port 2 is precluded from use as general-purpose I/O Ports when as address/data bus

drivers.

2. Port 2 internal strong pull-ups FET (P1 in FiGURE) assist the logic-one output for

memory bus cycle.

When Port 0 and Port 2 are used for an external memory cycle, an internal control signal

switches the output-driver input from the latch output to the internal address/data line.

Read-Modify-Write

Instructions

Some instructions read the latch data rather than the pin data. The latch based instruc-

tions read the data, modify the data and then rewrite the latch. These are called "Read-

Modify-Write" instructions. Below is a complete list of these special instructions (see

Table ). When the destination operand is a Port or a Port bit, these instructions read the

latch rather than the pin:

It is not obvious the last three instructions in this list are Read-Modify-Write instructions.

These instructions read the port (all 8 bits), modify the specifically addressed bit and

P2.X

LATCH

INTERNAL

WRITE

LATCH

READ

LATCH

P2.x (1)

ADDRESS HIGH/ CONTROL

BUS

VDD

INTERNAL

PULL-UP (2)

Instruction

Description

Example

ANL

logical AND

ANL P1, A

ORL

logical OR

ORL P2, A

XRL

logical EX-OR

XRL P3, A

JBC

jump if bit = 1 and clear bit

JBC P1.1, LABEL

CPL

complement bit

CPL P3.0

INC

increment

INC P2

DEC

decrement

DEC P2

DJNZ

decrement and jump if not zero

DJNZ P3, LABEL

MOV Px.y, C

move carry bit to bit y of Port x

MOV P1.5, C

CLR Px.y

clear bit y of Port x

CLR P2.4

SET Px.y

set bit y of Port x

SET P3.3

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