AT80C51RB2-3CSIM datasheet(36/83 Pages) ATMEL

36 / 83 page

AT80C51RD2/AT83C51Rx2

4113B–8051–03/05

The SADEN byte is selected so that each slave may be addressed separately.

For slave A, bit 0 (the LSB) is a don’t-care bit; for slaves B and C, bit 0 is a 1. To com-

municate with slave A only, the master must send an address where bit 0 is clear (e.g.

1111 0000b

For slave A, bit 1 is a 1; for slaves B and C, bit 1 is a don’t care bit. To communicate with

slaves B and C, but not slave A, the master must send an address with bits 0 and 1 both

set (e.g. 1111 0011b).

To communicate with slaves A, B and C, the master must send an address with bit 0 set,

bit 1 clear, and bit 2 clear (e.g. 1111 0001b).

Broadcast Address

A broadcast address is formed from the logical OR of the SADDR and SADEN registers

with zeros defined as don’t-care bits, e.g.:

SADDR 0101 0110b

SADEN 1111 1100b

Broadcast

= SADDR OR SADEN1111 111Xb

The use of don’t-care bits provides flexibility in defining the broadcast address, however

in most applications, a broadcast address is FFh. The following is an example of using

broadcast addresses:

Slave A:SADDR1111 0001b

SADEN1111 1010b

Broadcast1111 1X11b,

Slave B:SADDR1111 0011b

SADEN1111 1001b

Broadcast1111 1X11B,

Slave C:SADDR

= 1111 0010b

SADEN1111 1101b

Broadcast1111 1111b

For slaves A and B, bit 2 is a don’t care bit; for slave C, bit 2 is set. To communicate with

all of the slaves, the master must send an address FFh. To communicate with slaves A

and B, but not slave C, the master can send and address FBh.

Reset Addresses

On reset, the SADDR and SADEN registers are initialized to 00h, i.e. the given and

broadcast addresses are XXXX XXXXb (all don’t-care bits). This ensures that the serial

port will reply to any address, and so, that it is backwards compatible with the 80C51

microcontrollers that do not support automatic address recognition.

Table 19. SADEN Register

SADEN - Slave Address Mask Register (B9h)

Reset Value = 0000 0000b

Not bit addressable

765

43210

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