AD7997/AD7998

Rev. 0 | Page 24 of 32

WRITING TO THE AD7997/AD7998

Depending on the register being written to, there are three

different writes for the AD7997/AD7998.

WRITING TO THE ADDRESS POINTER REGISTER

FOR A SUBSEQUENT READ

In order to read from a particular register, the address pointer

register must first contain the address of that register. If it does

not, the correct address must be written to the address pointer

register by performing a single-byte write operation, as shown

in Figure 26. The write operation consists of the serial bus

address followed by the address pointer byte. No data is written

to any of the data registers. A read operation may be subsequently

performed to read the register of interest.

WRITING A SINGLE BYTE OF DATA TO THE ALERT

STATUS REGISTER OR CYCLE REGISTER

The alert status register and cycle register are both 8-bit registers,

so only one byte of data can be written to each. Writing a single

byte of data to one of these registers consists of the serial bus

write address, the chosen data register address written to the

address pointer register, followed by the data byte written to the

selected data register. See Figure 27.

WRITING TWO BYTES OF DATA TO A LIMIT,

HYSTERESIS, OR CONFIGURATION REGISTER

Each of the four limit registers are 16-bit registers, so two bytes

of data are required to write a value to any one of them. Writing

two bytes of data to one of these registers consists of the serial

bus write address, the chosen limit register address written to

the address pointer register, followed by two data bytes written

to the selected data register. See Figure 28.

If the master is write addressing the AD7997/AD7998, it can

write to more than one register without readdressing the ADC.

After the first write operation has completed for the first data

register, during the next byte the master simply writes to the

address pointer byte to select the next data register for a write

operation. This eliminates the need to readdress the device in

order to write to another data register.

SDA

START BY

MASTER

ACK. BY

AD7997/AD7998

ACK. BY

AD7997/AD7998

STOP BY

MASTER

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

19

1

9

C4

C3

C2

P2

P1

P0

A0

A1

A2

A3

0

0

SCL

1

C1

P3

R/W

Figure 26. Writing to the Address Pointer Register to Select a Register for a Subsequent Read Operation

SDA

ACK. BY

AD7997/AD7998

11

9

9

C4

C3

C2

P2

P1

P0

R/W

A0

A1

A2

A3

0

0

SCL

1

C1

P3

91

9

D7

D6

D5

D2

D1

D0

D4

D3

FRAME 3

DATA BYTE

SCL (CONTINUED)

SDA (CONTINUED)

START BY

MASTER

ACK. BY

AD7997/AD7998

ACK. BY

AD7997/AD7998

STOP BY

MASTER

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

Figure 27. Single-Byte Write Sequence