 2004 Microchip Technology Inc.

DS21126E-page 5

24FC32

3.6

Device Addressing

A control byte is the first byte received following the

Start condition from the master device. The control byte

consists of a four bit control code; for the 24FC32 this

is set as ‘1010’ binary for read and write operations.

The next three bits of the control byte are the device

select bits (A2, A1, A0). They are used by the master

device to select which of the eight devices are to be

accessed. These bits are in effect the three Most

Significant bits of the word address. The last bit of the

control byte (R/W) 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 3-3). Because

only A11..A0 are used, the upper four address bits must

be zeros. The Most Significant bit of the Most

Significant Byte of the address is transferred first.

Following the Start condition, the 24FC32 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 24FC32 will select a read

or write operation.

FIGURE 3-2:

CONTROL BYTE

ALLOCATION

FIGURE 3-3:

ADDRESS SEQUENCE BIT ASSIGNMENTS

Operation

Control

Code

Device Select

R/W

Read

1010

Device Address

1

Write

1010

Device Address

0

SLAVE ADDRESS

X = Don’t care

1

0

1

0

A2

A1

A0

R/W

A

START

READ/WRITE

CONTROL

BYTE

ADDRESS

BYTE 1

1

010

0000

A

2

A

0

A

1

W

Slave

Address

Device

Select

Bits

A

11

A

10

A

9

A

8

A

7

••

•• •

A

0

•

ADDRESS

BYTE 0