AT89C51AC2 datasheet(52/121 Pages) ATMEL

52 / 121 page

A/T89C51AC2

4127G–8051–05/06

Figure 29. UART Timing in Mode 1

Figure 30. UART Timing 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 the hardware, automatic address recognition enhances the multiproces-

sor 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 will the

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

the CPU is not interrupted by command frames addressed to other devices.

If necessary, you can 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).

Data byte

SMOD0=X

Stop

bit

Start

bit

RXD

SMOD0=1

SMOD0=0

Data byte

Ninth

bit

Stop

bit

Start

bit

RXD

SMOD0=1

AT89C51AC2 Datasheet(PDF) 52 Page - ATMEL Corporation