AT89C51RB2 Atmel Corporation, AT89C51RB2 Datasheet - Page 46

• Manufacturer Part Number: AT89C51RB2
• Manufacturer: Atmel Corporation

Specifications of AT89C51RB2

• Flash (kbytes): 16 Kbytes
• Max. Operating Frequency: 60 MHz
• Cpu: 8051-12C
• Max I/o Pins: 32
• Spi: 1
• Uart: 1
• Sram (kbytes): 1.25
• Self Program Memory: API
• Operating Voltage (vcc): 2.7 to 5.5
• Timers: 4
• Isp: UART
• Watchdog: Yes

46
Broadcast Address
Reset Addresses
AT89C51RB2/RC2
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 commu-
nicate 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).
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. :
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:
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.
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.
Slave A:SADDR1111 0001b
Slave B:SADDR1111 0011b
Slave C:SADDR=1111 0011b
Broadcast =SADDR OR SADEN1111 111Xb
SADDR0101 0110b
SADEN1111 1100b
SADEN1111 1010b
Broadcast1111 1X11b,
SADEN1111 1001b
Broadcast1111 1X11B,
SADEN1111 1101b
Broadcast1111 1111b
4180E–8051–10/06