AT89C5132
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 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).
18.6.2
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.
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4173E–USB–09/07
ATMEL [ ATMEL ]
