MS51
Nov. 28, 2019 Page 371 of 491 Rev 1.00
MS51 32K SERIES TECHNICAL REFERENCE MANUAL
In a more complex system the following could be used to select slaves 1 and 2 while excluding slave
0:
Example 1, slave 0:
SADDR = 11000000b
SADEN = 11111001b
Given = 11000XX0b
Example 2, slave 1:
SADDR = 11100000b
SADEN = 11111010b
Given = 11100X0Xb
Example 3, slave 2:
SADDR = 11000000b
SADEN = 11111100b
Given = 110000XXb
In the above example the differentiation among the 3 slaves is in the lower 3 address bits. Slave 0
requires that bit 0 = 0 and it can be uniquely addressed by 11100110b. Slave 1 requires that bit 1 = 0
and it can be uniquely addressed by 11100101b. Slave 2 requires that bit 2 = 0 and its unique address
is 11100011b. To select Slaves 0 and 1 and exclude Slave 2 use address 11100100b, since it is
necessary to make bit 2 = 1 to exclude slave 2.
The “Broadcast” address for each slave is created by taking the logical OR of SADDR and SADEN.
Zeros in this result are treated as “don’t-cares”, e.g.:
SADDR = 01010110b
SADEN = 11111100b
Broadcast = 1111111Xb
The use of don’t-care bits provides flexibility in defining the Broadcast address, however in most
applications, interpreting the “don’t-cares” as all ones, the broadcast address will be FFH.
On reset, SADDR and SADEN are initialized to 00H. This produces a “Given” address of all “don’t
cares” as well as a “Broadcast” address of all XXXXXXXXb (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 standard 80C51
microcontrollers that do not support automatic address recognition.
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