XRT7250 datasheet(138/463 Pages) EXAR

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DS3/E3 FRAMER IC

XRT7250

REV. 1.1.1

119

The 8051 Microcontroller

The 8051 family of microcontrollers is manufactured

by Intel and comes with a variety of amenities. Some

of these amenities include:

• On-chip Serial Port

• Four (4) 8-bit I/O ports (P0 - P3)

• 4k bytes of ROM

• 128 bytes of RAM

The 8051 Microcontroller consists of 4 - 8-bit I/O

Ports. Some of these ports have alternate functions

as will be discussed below.

PORT 0 (P0.0 - P0.7)

This port is a dual-purpose port on pins 32-39 of the

8051 IC. In minimal component designs, it is used as

a general purpose I/O port. For larger designs with

external memory, it becomes a multiplexed address

and data bus (AD0 - AD7).

PORT 1 (P1.0 - P1.7)

Port 1 is a dedicated port on pins 1 - 8. The pins,

designated at P1.0, P1.1, ... P1.7 are available for in-

terfacing as required. No alternative functions are as-

signed for Port 1 pins, thus they are used solely for in-

terfacing external devices. Exceptions are the 8032

and 8052 ICs, which use P1.0 and P1.1 as either as I/

O lines or as extenal inputs to the third timer.

PORT 2 (P2.0 - P2.7)

Port 2 (pins 21 - 28) is a dual-purpose port that can

function as a general purpose I/O, or as the high-byte

of the address bus for designs with external code

memory of more than 256 bytes of external data

memory (A8 - A15).

PORT 3

Port 3 is a dual purpose port on pins 10 - 17. In addi-

tion to functioning as general purpose I/O, these pins

have multiple functions. Each of these pins have an

alternate purpose, as listed in Table 12 below.

The 8051 also has numerous additional pins which

are relevant to interfacing to the XRT7250 DS3/E3

UNI or other peripherals. These pins are:

ALE - Address Latch Enable

If Port 0 is used in its alternative mode -as the data

bus and the lower byte of the address bus -- ALE is

the signal that latches the address into an external

this is done, Port 0 lines are then available for data in-

put or output during the second half of the memory

cycle, when the data transfer takes place.

INT0* (P3.2) and INT1* (P3.3)

INT0* and INT1* are external interrupt request inputs

to the 8051 Microcontroller. Each of these interrupt

pins support direct interrupt processing. In this case,

the term direct means that if one of these inputs are

asserted, then program control will automatically

branch to a specific (fixed) location in code memory.

This location is determined by the circuit design of the

8051 Microcontroller IC and cannot be changed.

Table 13 presents the location (in code memory)

where the program control will branch to, if either of

these inputs are asserted.

TABLE 12: ALTERNATE FUNCTIONS OF PORT 3 PINS

BIT

NAME

ALTERNATE FUNCTION

P3.0

RXD

Receive Data for Serial Port

P3.1

TXD

Transmit Data for Serial Port

P3.2

INT0*

External Interrupt 0

P3.3

INT1*

External Interrupt 1

P3.4

Timer/Counter 0 External Input

P3.5

Timer/Counter 1 External Input

P3.6

WR*

External Data/Memory Write Strobe

P3.7

RD*

External Data/Memory Read Strobe

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