© 1996 Microchip Technology Inc.

DS21126B-page 3-5

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 appropri-

ate 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

Operation

Control

Code

Device Select

R/W

Read

1010

Device Address

1

Write

1010

Device Address

0

SLAVE ADDRESS

X = Don’t care

101

0A2

A1

A0

R/W

A

START

READ/WRITE

FIGURE 3-3:

ADDRESS SEQUENCE BIT ASSIGNMENTS

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