REV. 0

ADM1031

–8–

In the case of the ADM1031, write operations contain either

one or two bytes, and read operations contain one byte, and

perform the following functions.

To write data to one of the device data registers or read data

from it, the Address Pointer Register must be set so that the

correct data register is addressed; data can then be written into

that register or read from it. The first byte of a write operation

always contains an address that is stored in the Address Pointer

Register. If data is to be written to the device, the write opera-

tion contains a second data byte that is written to the register

selected by the address pointer register.

This is illustrated in Figure 2a. The device address is sent over

the bus followed by R/

W set to 0. This is followed by two data

bytes. The first data byte is the address of the internal data

register to be written to, which is stored in the Address Pointer

Register. The second data byte is the data to be written to the

internal data register.

When reading data from a register there are two possibilities:

1. If the ADM1031’s Address Pointer Register value is unknown

or not the desired value, it is first necessary to set it to the

correct value before data can be read from the desired data

register. This is done by performing a write to the ADM1031

as before, but only the data byte containing the register address

is sent, as data is not to be written to the register. This is

shown in Figure 2b.

A read operation is then performed consisting of the serial bus

address, R/

W bit set to 1, followed by the data byte read from

the data register. This is shown in Figure 2c.

2. If the Address Pointer Register is known to be already at the

desired address, data can be read from the corresponding

data register without first writing to the Address Pointer

Register, so Figure 2b can be omitted.

NOTES

1. Although it is possible to read a data byte from a data register

without first writing to the Address Pointer Register, if the

Address Pointer Register is already at the correct value, it is

not possible to write data to a register without writing to the

Address Pointer Register, because the first data byte of a

write is always written to the Address Pointer Register.

2. In Figures 2a to 2c, the serial bus address is shown as the

default value 01011(A1)(A0), where A1 and A0 are set by

the three-state ADD pin.

3. The ADM1031 also supports the Read Byte protocol, as

described in the System Management Bus specification.

R/W

0

SCL

SDA

10

1

1

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1031

START BY

MASTER

19

1

ACK. BY

ADM1031

9

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1031

STOP BY

MASTER

1

9

SCL (CONTINUED)

SDA (CONTINUED)

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

FRAME 3

DATA BYTE

Figure 2a. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

R/W

0

SCL

SDA

10

1

1

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1031

START BY

MASTER

19

1

ACK. BY

ADM1031

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

STOP BY

MASTER

Figure 2b. Writing to the Address Pointer Register Only

R/

W

0

SCL

SDA

10

1

1

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

NO ACK.

BY MASTER

START BY

MASTER

19

1

ACK. BY

ADM1031

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADM1031

STOP BY

MASTER

Figure 2c. Reading Data from a Previously Selected Register