24LC256T-ESLASHCS16K datasheet(10/42 Pages) MICROCHIP

10 / 42 page

24AA256/24LC256/24FC256

DS20001203W-page 10

 1998-2019 Microchip Technology Inc.

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 4-bit control code. For the 24XX256, 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 24XX256 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, in effect, are

the three Most Significant bits of the word address. The

combination of the 4-bit control code and the next three

bits are called the slave address.

For the MSOP package, the A0 and A1 pins are not

connected. During device addressing, the A0 and A1

Chip Select bits (Figures 5-1 and 5-2) should be set

to ‘0’. Only two 24XX256 MSOP packages can be

connected to the same bus.

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. The next two bytes

received define the address of the first data byte

(Figure 5-2). Because only A14…A0 are used, the

upper address bits are a “don’t care.” The upper

address bits are transferred first, followed by the Least

Significant bits.

Following the Start condition, the 24XX256 monitors

the SDA bus checking the device type identifier being

transmitted. Upon receiving a ‘

1010’ code and

appropriate device select bits, the slave device outputs

an Acknowledge signal on the SDA line. Depending on

the state of the R/W bit, the 24XX256 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 2 Mbit

by adding up to eight 24XX256 devices on the same

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

byte as address bit A15; A1 as address bit A16; and A2

as address bit A17. It is not possible to sequentially

read across device boundaries.

For the MSOP package, up to two 24XX256 devices

can be added for up to 512 Kbit of address space. In

this case, software can use A2 of the control byte as

address bit A17. Bits A0 (A15) and A1 (A16) of the

control byte must always be set to a logic ‘0’ for the

MSOP.

FIGURE 5-2:

ADDRESS SEQUENCE BIT ASSIGNMENTS

ACK

R/W

Control Code

Chip Select

Bits

Slave Address

Acknowledge Bit

Start Bit

Read/Write Bit

010

0 R/W

••

•

••

Control Byte

Address High Byte

Address Low Byte

Control

Code

Chip

Select

Bits

x = “don’t care” bit

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