24AA256/24LC256

DS21203C-page 6

© 1998 Microchip Technology Inc.

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

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

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, and 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 bit is a don’t

care bit. The upper address bits are transferred first, fol-

lowed by the less significant bits.

Following the start condition, the 24xx256 monitors the

SDA bus checking the control byte 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, A0 can be used to expand

the contiguous address space for up to 2 Mbit

by adding up to eight 24xx256's 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 read or write

across device boundaries.

FIGURE 5-2:

ADDRESS SEQUENCE BIT ASSIGNMENTS

1

0

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

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

A

13

A

14