Datasheet

 2007-2019 Microchip Technology Inc. DS20001708M-page 8
24AA04/24LC04B/24FC04
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 combina-
tion of the 4-bit control code and the next three bits are
called the slave address.
The last bit of the control byte is the Read/
Write (R/W)
bit and it 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. Upon
receiving a valid slave address and the R/
W bit, 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.
The next byte received defines the address of the first
data byte within the selected block (Figure 5-2). The
word address byte uses all eight bits.
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
1 010
x
x
R/W
•••••
Control Byte
Word Address Byte
Control
Code
Block
Select
Bits
x = “don’t care”
B0
A7
•
A0