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ADM1021ARQ

Low Cost Microprocessor System Temperature Monitor

Analog Devices 

ADM1021

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 ADM1021, write operations contain either

one or two bytes, while 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 a valid address that is stored in the Address

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

operation contains a second data byte that is written to the

register selected by the address pointer register.

This is illustrated in Figure 13. 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 ADM1021’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 ADM1021

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

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

This is shown in Figure 14.

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 15.

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 14 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. Don’t forget that the ADM1021 registers have different

addresses for read and write operations. The write address of

a register must be written to the Address Pointer if data is to

be written to that register, but it is not possible to read data

from that address. The read address of a register must be

written to the Address Pointer before data can be read from

that register.

1

9

1

9

SCLK

SDATA

A6

START BY

MASTER

A5 A4 A3 A2 A1 A0

FRAME 1

SERIAL BUS ADDRESS BYTE

R/W

D7

ACK. BY

ADM1021

1

SCL (CONTINUED)

D6 D5 D4 D3 D2 D1 D0

FRAME 2

ADDRESS POINTER REGISTER BYTE

ACK. BY

ADM1021

9

SDA (CONTINUED)

D7 D6 D5 D4 D3 D2 D1 D0

FRAME 3

DATA BYTE

ACK. BY STOP BY

ADM1021 MASTER

Figure 13. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

1

9

1

9

SCLK

SDATA

A6

START BY

MASTER

A5 A4 A3 A2 A1 A0

FRAME 1

SERIAL BUS ADDRESS BYTE

R/W

D7

ACK. BY

ADM1021

D6 D5 D4 D3 D2 D1 D0

FRAME 2

ADDRESS POINTER REGISTER BYTE

ACK. BY

ADM1021

STOP BY

MASTER

Figure 14. Writing to the Address Pointer Register Only

REV. 0

–9–

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