AT89C51ED2-RDTUM datasheet(50/137 Pages) ATMEL

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4235K–8051–05/08

AT89C51RD2/ED2

Slave C:SADDR1111 0010b

SADEN1111 1101b

Given1111 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 1; for slaves B and C, bit 1 is a don’t care bit. To communicate with slaves

B and C, but not slave A, 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).

14.2.2

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. :

SADDR0101 0110b

SADEN1111 1100b

Broadcast =SADDR OR SADEN1111 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:SADDR1111 0001b

SADEN1111 1010b

Broadcast1111 1X11b,

Slave B:SADDR1111 0011b

SADEN1111 1001b

Broadcast1111 1X11B,

Slave C:SADDR=1111 0011b

SADEN1111 1101b

Broadcast1111 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 an address FFh. To communicate with slaves A and B, but not

slave C, the master can send and address FBh.

14.2.3

Reset Addresses

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 will reply to any

address, and so, that it is backwards compatible with the 80C51 microcontrollers that do not

support automatic address recognition.

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