AT83SND2CMP3B datasheet(176/241 Pages) ATMEL

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4341F–MP3–03/06

AT8xC51SND2C/MP3B

SADEN = 1111 1010b

Given = 1111 0X0Xb

Slave B:SADDR = 1111 0011b

SADEN = 1111 1001b

Given = 1111 0XX1b

Slave C:SADDR = 1111 0011b

SADEN = 1111 1101b

Given = 1111 00X1b

The SADEN Byte is selected so that each slave may be addressed separately.

For slave A, bit 0 (the LSB) is a don’t-care bit; for slaves B and C, bit 0 is a 1. To communicate

with slave A only, the master must send an address where bit 0 is clear (e.g. 1111 0000B).

For slave A, bit 1 is a 0; for slaves B and C, bit 1 is a don’t care bit. To communicate with slaves

A and B, but not slave C, the master must send an address with bits 0 and 1 both set (e.g.

1111 0011B

To communicate with slaves A, B and C, the master must send an address with bit 0 set, bit 1

clear, and bit 2 clear (e.g. 1111 0001B).

20.6.2

Broadcast Address

A broadcast address is formed from the logical OR of the SADDR and SADEN registers with

zeros defined as don’t-care bits, e.g.:

SADDR = 0101 0110b

SADEN = 1111 1100b

(SADDR | SADEN)=1111 111Xb

The use of don’t-care bits provides flexibility in defining the broadcast address, however in most

applications, a broadcast address is FFh.

The following is an example of using broadcast addresses:

Slave A:SADDR = 1111 0001b

SADEN = 1111 1010b

Given = 1111 1X11b,

Slave B:SADDR = 1111 0011b

SADEN = 1111 1001b

Given = 1111 1X11b,

Slave C:SADDR = 1111 0010b

SADEN = 1111 1101b

Given = 1111 1111b,

For slaves A and B, bit 2 is a don’t care bit; for slave C, bit 2 is set. To communicate with all of

the slaves, the master must send the address FFh.

To communicate with slaves A and B, but not slave C, the master must send the address FBh.

20.6.3

Reset Address

On reset, the SADDR and SADEN registers are initialized to 00h, i.e. the given and broadcast

addresses are XXXX XXXXb (all don’t care bits). This ensures that the Serial Port is backwards

compatible with the 80C51 microcontrollers that do not support automatic address recognition.

AT83SND2CMP3B Datasheet(PDF) 176 Page - ATMEL Corporation

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