24LC024 datasheet(9/41 Pages) MICROCHIP

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DS20001210Q-page 9

24AA024/24LC024/24AA025/24LC025

5.0

DEVICE ADDRESSING

A control byte is the first byte received following the

Start condition from the host device. The control byte

consists of a 4-bit control code. For the 24AA024/

24LC024/24AA025/24LC025, this is set as ‘1010’

binary for read and write operations. The next three bits

of the control byte are the Chip Select bits (A2, A1, A0).

The Chip Select bits allow the use of up to eight

24AA024/24LC024/24AA025/24LC025 devices on the

same bus and are used to select which device is

accessed. The Chip Select bits in the control byte must

correspond to the logic levels on the corresponding A2,

A1 and A0 pins for the device to respond. These bits

are, in effect, the three Most Significant bits (MSb) of

the word address. The combination of the 4-bit control

code and the next three bits are called the client

address.

For the SOT-23 package, the A2 address pin is not

available. During device addressing, the A2 Chip

Select bit should be set to ‘0’.

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 a one, a read operation is selected. When set to

a zero, a write operation is selected. The next byte

received defines the address of the first data byte

(Figure 5-2). Following the Start condition, the

24AA024/24LC024/24AA025/24LC025 monitors the

SDA bus checking the control byte being transmitted.

Upon receiving a valid client address and the R/W bit,

the client device outputs an Acknowledge signal on the

SDA line. Depending on the state of the R/W bit, the

24AA024/24LC024/24AA025/24LC025 will select a

read or write operation.

FIGURE 5-1:

CONTROL BYTE FORMAT

5.1

Contiguous Addressing Across

Multiple Devices

The Chip Select bits A2, A1 and A0 can be used to

expand the contiguous address space for up to

16 Kbits by adding up to eight 24AA024/24LC024/

24AA025/24LC025 devices on the same bus. In this

case, software can use A0 of the control byte as

address bit A8; A1 as address bit A9; and A2 as

address bit A10. It is not possible to sequentially read

across device boundaries.

For the SOT-23 package, up to four 24AA025/24LC025

devices can be added for up to 8 Kbits of address

space. In this case, software can use A0 of the control

byte as address bit A8, and A1 as address bit A9. It is

not possible to sequentially read across device

boundaries.

FIGURE 5-2:

ADDRESS SEQUENCE BIT ASSIGNMENTS

SACK

R/W

Control Code

Chip Select

Bits

Client Address

Acknowledge Bit

Start Bit

Read/Write Bit

•

010

R/W

••

•

Control Byte

Word Address Byte

Control

Code

Chip

Select

bits

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