24AA04_09 datasheet(7/34 Pages) MICROCHIP

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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

Write

1010

Block Address

B0 R/W ACK

Start Bit

Read/Write Bit

x = “don’t care”

Slave Address

Acknowledge Bit

Control Code

Block

Select

Bits

010

0 R/W

••

•

Control Byte

Address Low Byte

Control

Code

Block

Select

bits

x = “don’t care”

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