STC89LE52RC datasheet(120/165 Pages) STC

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8.4 Automatic Address Recognition

Automatic Address Recognition is a future 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 bit in SCON. In the 9-bit UART modes, Mode 2 and Mode 3, the Receive interrupt

flag(RI) will be automatically set when the received byte contains either the “Given” address or the “Broadcast”

address. The 9-bit mode requires that the 9

th information bit is a “1” to indicate that the received information is an

address and not data.

The 8-bit mode is called Mode 1. In this mode the RI flag will be set if SM2 is enabled and the information

received has a valid stop bit following the 8 address bits and the information is either a Given or Broadcast

address.

Mode 0 is the Shift Register mode and SM2 is ignored.

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, SADDR, and the address mask,

SADEN. SADEN is used to define which bits in the SADDR are to be used and which bits are “don’t care”. The

SADEN mask can be logically ANDed with the SADDR 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 which

excluding others. The following examples will help to show the versatility of this scheme :

Slave 0

SADDR = 1100 0000

SADEN = 1111 1101

GIVEN = 1100 00x0

Slave 1

SADDR = 1100 0000

SADEN = 1111 1110

GIVEN = 1100 000x

In the previous example SADDR is the same and the SADEN 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 11000010 since slave 1 requires a “0” in bit 1. A unique address for slave 1 would

be 11000001 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 salve 1). Thus, both could be addressed with 11000000.

In a more complex system the following could be used to select slaves 1 and 2 while excluding slave 0:

Slave 0

SADDR = 1100 0000

SADEN = 1111 1001

GIVEN = 1100 0xx0

Slave 1

SADDR = 1110 0000

SADEN = 1111 1010

GIVEN = 1110 0x0x

Slave 2

SADDR = 1110 0000

SADEN = 1111 1100

GIVEN = 1110 00xx

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