ADM1023

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13

Figure 17. Reading Data from a Previously Selected Register

SCLK

SDATA

D7

D6

D5

D4

D3

D2

D1

D0

NO ACK.

BY MASTER

START BY

MASTER

9

1

ACK. BY

ADM1023

9

STOP BY

MASTER

A6

A5

A4

A3

A2

A1

A0

1

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADM1023

R/W

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.

For the ADM1023, 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. Data can then 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 15. 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 ADM1023’s address pointer register value is

unknown or not the desired value, it is 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 ADM1023 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 16.

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 17.

2. If the address pointer register is known to be at the

desired address already, data can be read from the

corresponding data register without first writing to

the address pointer register.

NOTES:

without first writing to the address pointer register.

However, it is not possible to write data to a register

without writing to the address pointer register even if

the address pointer register is already at the correct

value. This is because the first data byte of a write is

always written to the address pointer register.

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.

ALERT Output

The ALERT output goes low whenever an out−of−limit

measurement is detected or if the remote temperature sensor

is open−circuit. It is an open drain and requires a 10 k

W

wire−AND’ed together, so that the common line goes low if

one or more of the ALERT outputs goes low.

The ALERT output can be used as an interrupt signal to a

processor, or it may be used as an SMBALERT. Slave

devices on the SMBus normally cannot signal to the master

that they want to talk, but the SMBALERT function allows

them to do so.

One or more ALERT outputs are connected to a common

SMBALERT line connected to the master. When the

SMBALERT line is pulled low by one of the devices, the

procedure shown in Figure 18 occurs.

Figure 18. Use of SMBALERT

MASTER

RECEIVES

SMBALERT

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

NO

ACK

START

ALERT RESPONSE

ADDRESS

RD ACK

DEVICE

ADDRESS

STOP

SMBALERT Process

1. SMBALERT pulled low.

2. Master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. The device whose ALERT output is low responds

to the ARA and the master reads its device

address. The address of the device is now known,

and it can be interrogated in the usual way.