AT89S8253 datasheet(21/60 Pages) ATMEL

21 / 60 page

3286M–MICRO–10/09

AT89S8253

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 previous 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 SADDR and

SADEN. Zeros in this result are trended as don’t-cares. In most cases, interpreting the don’t-

cares as ones, the broadcast address will be FF hexadecimal.

Upon reset SADDR (SFR address 0A9H) and SADEN (SFR address 0B9H) are loaded with 0s.

This produces a given address of all “don’t cares” as well as a Broadcast address of all “don’t

cares”. This effectively disables the Automatic Addressing mode and allows the microcontroller

to use standard 80C51-type UART drivers which do not make use of this feature.

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