Rainbow Electronics T89C51CC02 - Manual (Page 47)

Rev.A - May 17, 2001 47
Preliminary
T89C51CC02
11.3. Given Address
Each device has an individual address that is specified in the SADDR register; the SADEN register is a mask byte
that contains don’t-care bits (defined by zeros) to form the device’s given address. The don’t-care bits provide the
flexibility to address one or more slaves at a time. The following example illustrates how a given address is formed.
To address a device by its individual address, the SADEN mask byte must be
1111 1111b .
For example:
SADDR 0101 0110b
SADEN 1111 1100b
Given 0101 01XXb
Here is an example of how to use given addresses to address different slaves:
Slave A: SADDR 1111 0001b
SADEN 1111 1010b
Given 1111 0X0Xb
Slave B: SADDR 1111 0011b
SADEN 1111 1001b
Given 1111 0XX1b
Slave C: SADDR 1111 0010b
SADEN 1111 1101b
Given 1111 00X1b
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 communicate 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 0; for slaves B and C, bit 1 is a don’t care bit. To communicate with slaves A and B, but
not slave C, 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 ).
11.4. 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
SADDR OR SADEN 1111 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: SADDR 1111 0001b
SADEN 1111 1010b
Given 1111 1X11b,
Slave B: SADDR 1111 0011b
SADEN 1111 1001b
Given 1111 1X11B,
Slave C: SADDR= 1111 0010b
SADEN 1111 1101b
Given 1111 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.