24C32A Datasheet, PDF (5/12 Pages) Microchip Technology – 32K 5.0V I 2 C O Serial EEPROM

3.6 Device Addressing
A control byte is the first byte received following the
start condition from the master device. The control byte
consists of a 4-bit control code; for the 24C32A this is
set as 1010 binary for read and write (R/W) operations.
The next three bits of the control byte are the device
select bits (A2, A1, A0). They are used by the master
device to select which of the eight devices are to be
accessed. These bits are in effect the three most signif-
icant bits of the word address. The last bit of the control
byte defines the operation to be performed. When set
to a one a read operation is selected, and when set to
a zero a write operation is selected. The next two bytes
received define the address of the first data byte
(Figure 3-3). Because only A11...A0 are used, the
upper four address bits must be zeros. The most signif-
icant bit of the most significant byte of the address is
transferred first.
FIGURE 3-2: CONTROL BYTE
ALLOCATION
START
READ/WRITE
SLAVE ADDRESS
R/W A
24C32A
Following the start condition, the 24C32A monitors the
SDA bus checking the device type identifier being
transmitted. Upon receiving a 1010 code and appropri-
ate device select bits, the slave device outputs an
acknowledge signal on the SDA line. Depending on the
state of the R/W bit, the 24C32A will select a read or
write operation.
Operation
Control
Code
Device Select
R/W
Read
Write
1010 Device Address
1
1010 Device Address
0
1
0
1
0
A2 A1 A0
FIGURE 3-3: ADDRESS SEQUENCE BIT ASSIGNMENTS
CONTROL BYTE
1
0
1
0
A
2
A
1
A
0
R/W
SLAVE
ADDRESS
DEVICE
SELECT
BUS
ADDRESS BYTE 1
0
0
0
0
AA
11 10
A
9
A
8
ADDRESS BYTE 0
A
7
•
•
•
••
•
A
0
© 1996 Microchip Technology Inc.
Preliminary
DS21163B-page 5