AT89LP51 datasheet(62/113 Pages) ATMEL

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3709C–MICRO–5/11

AT89LP51/52 - Preliminary

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 9th information bit be a “1”

to indicate that the received information is an address and not data.

In Mode 1 (8-bit) the RI flag will be set if SM2 is enabled and the information received has a valid

stop bit following the 8th address bits and the information is either a Given or Broadcast

address. Automatic Address Recognition is not available during Mode 0.

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

while excluding others. The following examples 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 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:

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

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.

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