Rainbow Electronics T89C51RD2 - Manual (Page 34)

Rev. F - 15 February, 2001 34
T89C51RD2
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 ).
6.6.4. 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 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
Broadcast 1111 1X11b,
Slave B: SADDR 1111 0011b
SADEN 1111 1001b
Broadcast 1111 1X11B,
Slave C: SADDR= 1111 0010b
SADEN 1111 1101b
Broadcast 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 an address FFh. To communicate with slaves A and B, but not slave C, the master can send
and address FBh.
6.6.5. 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.
SADEN - Slave Address Mask Register (B9h)
Reset Value = 0000 0000b
Not bit addressable
SADDR - Slave Address Register (A9h)
Reset Value = 0000 0000b
Not bit addressable
Table 15. SCON Register
7 6 5 4 3 2 1 0
7 6 5 4 3 2 1 0