AT89S8253-24JC datasheet(21/56 Pages) ATMEL

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AT89S8253 [Preliminary]

3286A–MICRO–7/04

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 while 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 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.

AT89S8253-24JC Datasheet(PDF) 21 Page - ATMEL Corporation

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