Datasheet

 2009 Microchip Technology Inc. DS21708K-page 7
24AA04/24LC04B
5.0 DEVICE ADDRESSING
A control byte is the first byte received following the
Start condition from the master device. The control byte
consists of a four-bit control code. For the 24XX04, this
is set as ‘
1010’ binary for read and write operations.
The next two bits of the control byte are “don’t cares” for
the 24XX04. The last bit, B0, is used by the master
device to select which of the two 256-word blocks of
memory are to be accessed. This bit is, in effect, the
Most Significant bit of the word address.
The last bit of the control byte defines the operation to
be performed. When set to ‘
1’, a read operation is
selected. When set to ‘
0’, a write operation is selected.
Following the Start condition, the 24XX04 monitors the
SDA bus checking the device type identifier being
transmitted and, upon receiving a ‘
1010’ code, the
slave device outputs an Acknowledge signal on the
SDA line. Depending on the state of the R/W
bit, the
24XX04 will select a read or write operation.
FIGURE 5-1: CONTROL BYTE
ALLOCATION
FIGURE 5-2: ADDRESS SEQUENCE BIT ASSIGNMENTS
Operation
Control
Code
Block Select R/W
Read
1010
Block Address
1
Write
1010
Block Address
0
10 10
xxB0
R/W
ACK
Start Bit
Read/Write
Bit
x = “don’t care”
S
Slave Address
Acknowledge Bit
Control Code
Block
Select
Bits
1010
x
x
B
0
R/W
A
7
A
0
••••••
Control Byte Address Low Byte
Control
Code
Block
Select
bits
x = “don’t care”