Atmel 8051 User Manual

Common Features Description
2-103 Atmel 8051 Microcontrollers Hardware Manual
4316B–8051–02/04
Slave B:SADDR1111 0011b
SADEN
1111 1001b
Given1111 0XX1b
Slave C:SADDR1111 0011b
SADEN
1111 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 com-
municate 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).
2.15.3 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
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
SADEN
1111 1010b
Broadcast1111 1X11b,
Slave B:SADDR1111 0011b
SADEN
1111 1001b
Broadcast1111 1X11B,
Slave C:SADDR=1111 0010b
SADEN
1111 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.
2.15.4 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 thus, that it is backwards compatible with the 80C51
microcontrollers that do not support automatic address recognition.
Table 2-16. SADEN Register
SADEN - Slave Address Mask Register (B9h)
76543210

Atmel 8051 Specifications

General

Brand	Atmel
Model	8051
Category	Microcontrollers
Language	English