24AA256UID datasheet(7/28 Pages) MICROCHIP

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DS20005215A-page 7

24AA256UID

5.0

DEVICE ADDRESSING

A control byte is the first byte received following the

Start condition from the master device (Figure 5-1).

The control byte consists of a 4-bit control code. For the

24AA256UID, 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 24AA256UID 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 of the word

address.

The last bit of the control byte 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 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 Less Significant bits.

Following the Start condition, the 24AA256UID moni-

tors 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 24AA256UID 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 24AA256UID 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.

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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