Owner manual

33 Rev. F - 15 February, 2001

T89C51RD2

Figure 15. UART Timings in Modes 2 and 3

6.6.2. Automatic Address Recognition

The automatic address recognition feature is enabled when the multiprocessor communication 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, you 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

6.6.3. Given Address

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

that contains don’t-care bits (defined by zeros) to form the 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:

SADDR 0101 0110b

SADEN 1111 1100b

Given 0101 01XXb

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

Slave A: SADDR 1111 0001b

SADEN 1111 1010b

Given 1111 0X0Xb

Slave B: SADDR 1111 0011b

SADEN 1111 1001b

Given 1111 0XX1b

Slave C: SADDR 1111 0010b

SADEN 1111 1101b

Given 1111 00X1b

SMOD0=0

Data byte Ninth

bit

Stop

bit

Start

bit

RXD

D8D7D6D5D4D3D2D1D0

SMOD0=1