24AA64/24LC64/24FC64

DS21189L-page 6

© 2007 Microchip Technology Inc.

3.6

Device Addressing

A control byte is the first byte received following the

Start condition from the master device (Figure 3-2).

The control byte consists of a four-bit control code. For

the 24XX64, 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 24XX64 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 ‘1’, a read operation is

selected. When set to a ‘0’, a write operation is

selected. The next two bytes received define the

address of the first data byte (Figure 3-3). Because

only A12...A0 are used, the upper-three address bits

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

transferred first, followed by the Less Significant bits.

Following the Start condition, the 24XX64 monitors the

SDA bus, checking the device-type identifier being

transmitted. Upon receiving a ‘1010’ code and appro-

priate device-select bits, the slave device outputs an

Acknowledge signal on the SDA line. Depending on the

state of the R/W bit, the 24XX64 will select a read or

write operation.

FIGURE 3-2:

CONTROL BYTE FORMAT

3.7

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 512K

bits by adding up to eight 24XX64 devices on the same

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

byte as address bit A13; A1 as address bit A14; and A2

as address bit A15. It is not possible to sequentially

read across device boundaries.

FIGURE 3-3:

ADDRESS SEQUENCE BIT ASSIGNMENTS

10

1

0

A2

A1

A0

SACK

R/W

Control Code

Chip Select

Bits

Slave Address

Acknowledge Bit

Start Bit

Read/Write Bit

1

010

A

2

A

1

A

0 R/W

xx

x

A

11

A

10

A

9

A

7

A

0

A

8

••

•

•

••

A

12

Control Byte

Address High Byte

Address Low Byte

Control

Code

Chip

Select

bits

x

= “don’t care” bit