AT89LP428 datasheet(39/149 Pages) ATMEL

39 / 149 page

3654A–MICRO–8/09

AT89LP428/828

10.3

Port Read-Modify-Write

A read from a port will read either the state of the pins or the state of the port register depending

on which instruction is used. Simple read instructions will always access the port pins directly.

Read-modify-write instructions, which read a value, possibly modify it, and then write it back, will

always access the port register. This includes bit write instructions such as CLR or SETB as they

actually read the entire port, modify a single bit, then write the data back to the entire port. See

Table 10-4 for a complete list of Read-modify-write instructions which may access the ports.

10.4

Port Alternate Functions

Most general-purpose digital I/O pins of the AT89LP428/828 share functionality with the various

I/Os needed for the peripheral units. Table 10-6 lists the alternate functions of the port pins.

Alternate functions are connected to the pins in a logic AND fashion. In order to enable the

alternate function on a port pin, that pin must have a “1” in its corresponding port register bit,

otherwisethe input/output will always be “0”. However, alternate functions may be temporarily

forced to “0” by clearing the associated port bit, provided that the pin is not in input-only mode.

Furthermore, each pin must be configured for the correct input/output mode as required by its

peripheral before it may be used as such. Table 10-5 shows how to configure a generic pin for

use with an alternate function.

Table 10-4.

Port Read-Modify-Write Instructions

Mnemonic

Instruction

Example

ANL

Logical AND

ANL P1, A

ORL

Logical OR

ORL P1, A

XRL

Logical EX-OR

XRL P1, A

JBC

Jump if bit set and clear bit

JBC P3.0, LABEL

CPL

Complement bit

CPL P3.1

INC

Increment

INC P1

DEC

Decrement

DEC P3

DJNZ

Decrement and jump if not zero

DJNZ P3, LABEL

MOV PX.Y, C

Move carry to bit Y of Port X

MOV P1.0, C

CLR PX.Y

Clear bit Y of Port X

CLR P1.1

SETB PX.Y

Set bit Y of Port X

SETB P3.2

Table 10-5.

Alternate Function Configurations for Pin y of Port x

PxM0.y

PxM1.y

Px.y

I/O Mode

Bidirectional (internal pull-up)

Output

Input

Bidirectional (external pull-up)

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