REV. A

ADM1028

–7–

STOP condition. In READ mode, the master device will

override the acknowledge bit by pulling the data line high

during the low period before the ninth clock pulse. This is

known as No Acknowledge. The master will then take the

data line low during the low period before the tenth clock

pulse, then high during the tenth clock pulse to assert a

STOP condition.

Any number of bytes of data may be transferred over the serial

bus in one operation, but it is not possible to mix read and write

in one operation, because the type of operation is determined at

the beginning and cannot subsequently be changed without

starting a new operation.

In the case of the ADM1028, 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, then data can 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 is only one possibility:

1. The serial bus address is written to the device along with the

address pointer register value. The ADM1028 should then

acknowledge the write by pulling SDA low during the ninth

clock pulse. The master does not generate a STOP condition

but issues a new START condition. The serial bus address

is again sent but with the R/

W bit high, indicating a READ

operation. The ADM1028 will then return the data from the

selected register, and a No Acknowledge is generated to signify

the end of the read operation. The master will then initiate a

STOP condition to end the transaction and release the SMBus.

In Figures 2a and 2b, the serial bus address is shown as the

default value 0101110.

R/

W

0

SCL

SDA

10

1

10

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1028

START BY

MASTER

19

1

ACK. BY

ADM1028

9

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1028

1

9

SCL (CONTINUED)

SDA (CONTINUED)

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

FRAME 3

DATA BYTE

1

STOP BY

MASTER

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

1

0

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1028

START BY

MASTER

19

1

ACK. BY

ADM1028

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

R/

W

0

SCL

SDA

10

1

1

1

0

D7

D6

D5

D4

D3

D2

D1

D0

NO ACK.

BY MASTER

START BY

MASTER

19

1

ACK. BY

ADM1028

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADM1028

STOP BY

MASTER

Figure 2b. Reading Data from the ADM1028