UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
68 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
Note that SM2_n has no effect in Mode 0, and must be logic 0 in Mode 1.
10.20 Automatic address recognition
Automatic address recognition is a feature which allows the UART to recognize certain
addresses in the serial bit stream by using hardware to make the comparisons. This
feature saves a great deal of software overhead by eliminating the need for the software
to examine every serial address which passes by the serial port. This feature is enabled
by setting the SM2_n bit in SnCON. In the 9 bit UART modes (mode 2 and mode 3), the
Receive Interrupt flag (RI_n) will be automatically set when the received byte contains
either the ‘Given’ address or the ‘Broadcast’ address. The 9 bit mode requires that the 9th
information bit is a 1 to indicate that the received information is an address and not data.
Using the Automatic Address Recognition feature allows a master to selectively
communicate with one or more slaves by invoking the Given slave address or addresses.
All of the slaves may be contacted by using the Broadcast address. Two special Function
Registers are used to define the slave’s address, SnADDR, and the address mask,
SnADEN. SnADEN is used to define which bits in the SnADDR are to be used and which
bits are ‘don’t care’. The SnADEN mask can be logically ANDed with the SnADDR to
create the ‘Given’ address which the master will use for addressing each of the slaves.
Use of the Given address allows multiple slaves to be recognized while excluding others.
The following examples will help to show the versatility of this scheme:
In the above example SnADDR is the same and the SnADEN data is used to differentiate
between the two slaves. Slave 0 requires a 0 in bit 0 and it ignores bit 1. Slave 1 requires
a 0 in bit 1 and bit 0 is ignored. A unique address for Slave 0 would be 1100 0010 since
slave 1 requires a 0 in bit 1. A unique address for slave 1 would be 1100 0001 since a 1 in
bit 0 will exclude slave 0. Both slaves can be selected at the same time by an address
which has bit 0 = 0 (for slave 0) and bit 1 = 0 (for slave 1). Thus, both could be addressed
with 1100 0000.
In a more complex system the following could be used to select slaves 1 and 2 while
excluding slave 0:
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 1110 0110. Slave 1
requires that bit 1 = 0 and it can be uniquely addressed by 1110 and 0101. Slave 2
requires that bit 2 = 0 and its unique address is 1110 0011. To select Slaves 0 and 1 and
exclude Slave 2 use address 1110 0100, 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
Table 60.
Slave 0/1 examples
Example 1
Example 2
Slave 0
SnADDR
=
1100 0000
Slave 1
SnADDR
=
1100 0000
SnADEN
=
1111 1101
SnADEN
=
1111 1110
Given
=
1100 00X0
Given
=
1100 000X
Table 61.
Slave 0/1/2 examples
Example 1
Example 2
Example 3
Slave 0
SnADDR
= 1100 0000
Slave 1
SnADDR
= 1110 0000
Slave 2
SnADDR
= 1100 0000
SnADEN
= 1111 1001
SnADEN
= 1111 1010
SnADEN
= 1111 1100
Given
= 1100 0XX0
Given
= 1110 0X0X
Given
= 1110 00XX
