AT89C51ED2-RDTIM datasheet(50/137 Pages) ATMEL

50 / 137 page

AT89C51RD2/ED2

4235E–8051–04/04

Figure 24. UART Timings in Modes 2 and 3

Automatic Address

Recognition

The automatic address recognition feature is enabled when the multiprocessor commu-

nication feature is enabled (SM2 bit in SCON register is set).

Implemented in hardware, automatic address recognition enhances the multiprocessor

communication feature by allowing the serial port to examine the address of each

incoming command frame. Only when the serial port recognizes its own address, the

receiver sets RI bit in SCON register to generate an interrupt. This ensures that the CPU

is not interrupted by command frames addressed to other devices.

If desired, the user may enable the automatic address recognition feature in mode 1.In

this configuration, the stop bit takes the place of the ninth data bit. Bit RI is set only when

the received command frame address matches the device’s address and is terminated

by a valid stop bit.

To support automatic address recognition, a device is identified by a given address and

a broadcast address.

Note:

The multiprocessor communication and automatic address recognition features cannot

be enabled in mode 0 (i. e. setting SM2 bit in SCON register in mode 0 has no effect).

Given Address

Each device has an individual address that is specified in SADDR register; the SADEN

device’s given address. The don’t-care bits provide the flexibility to address one or more

slaves at a time. The following example illustrates how a given address is formed.

To address a device by its individual address, the SADEN mask byte must be 1111

1111b

For example:

SADDR0101 0110b

SADEN1111 1100b

Given0101 01XXb

The following is an example of how to use given addresses to address different slaves:

Slave A:SADDR1111 0001b

SADEN1111 1010b

Given1111 0X0Xb

Slave B:SADDR1111 0011b

SADEN1111 1001b

Given1111 0XX1b

Slave C:SADDR1111 0010b

SADEN1111 1101b

Given1111 00X1b

SMOD0=0

Data byte

Ninth

bit

Stop

bit

Start

bit

RXD

SMOD0=1

AT89C51ED2-RDTIM 数据表(PDF) 50 Page - ATMEL Corporation

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