X24128
DEVICE ADDRESSING
Following a start condition, the master must output the
address of the slave it is accessing. The first four bits
of the Slave Address Byte are the device type identifier
bits. These must equal “1010”. The next 3 bits are the
device select bits S
0
, S
1
, and S
2
. This allows up to 8
devices to share a single bus. These bits are
compared to the S
0
, S
1
, and S
2
device select input
pins. The last bit of the Slave Address Byte defines the
operation to be performed. When the R/W bit is a one,
then a read operation is selected. When it is zero then
a write operation is selected. Refer to figure 4. After
loading the Slave Address Byte from the SDA bus, the
device compares the device type bits with the value
“1010” and the device select bits with the status of the
Figure 4. Device Addressing
device select input pins. If the compare is not successful,
no acknowledge is output during the ninth clock cycle
and the device returns to the standby mode.
The word address is either supplied by the master or
obtained from an internal counter, depending on the
operation. The master must supply the two Word
Address Bytes as shown in figure 4.
The internal organization of the E
2
array is 512 pages by
32 bytes per page. The page address is partially
contained in the Word Address Byte 1 and partially in
bits 7 through 5 of the Word Address Byte 0. The byte
address is contained in bits 4 through 0 of the Word
Address Byte 0. See figure 4.
DEVICE TYPE
IDENTIFIER
DEVICE
SELECT
1
0
1
0
S2
S1
S0
R/ W
SLAVE ADDRESS BYTE
HIGH ORDER WORD ADDRESS
0
0
A13
A12
A11 A10
A9
A8
X24128 WORD ADDRESS BYTE 1
LOW ORDER WORD ADDRESS
A7
A6
A5
A4
A3
A2
A1
A0
WORD ADDRESS BYTE 0
D7
D6
D5
D4
D3
D2
D1
D0
DA BYTE
TA
7027 FM 06
5
XICOR [ XICOR INC. ]
