ADM1021AARQZ-R datasheet(12/15 Pages) ONSEMI | Low Cost Microprocessor System Temperature Monitor Microcomputer

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ADM1021A

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Figure 15. Writing to the Address Pointer Register Only

R/W

SCLK

SDATA

ACK. BY

ADM1021A

STOP BY

MASTER

START BY

MASTER

ACK. BY

ADM1021A

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

Figure 16. Reading Data from a Previously Selected Register

SCLK

SDATA

NO ACK.

BY MASTER

STOP BY

MASTER

START BY

MASTER

ACK. BY

ADM1021A

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2 DATA BYTE FROM ADM1021A

R/W

When reading data from a register there are two

possibilities:

1. If the ADM1021A’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 ADM1021A as

before, but only the data byte containing the

be written to the register. This is shown in

Figure 15.

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 16.

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 15

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; this is because the first

data byte of a write is always written to the

address pointer register.

2. Remember that the ADM1021A 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

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

pullup to VDD. Several ALERT outputs can be wire−ANDed

together so 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 can be used as an SMBALERT. Slave devices

on the SMBus cannot normally 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

following procedure occurs, as shown in Figure 17.

Figure 17. Use of SMBALERT

MASTER

RECEIVES

SMBALERT

MASTER SENDS

ARA AND READ

COMMAND

ACK

START ALERT RESPONSE ADDRESS RD ACK

DEVICE

ADDRESS

STOP

DEVICE SENDS

ITS ADDRESS

1. SMBALERT is 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 alert response address and the master reads

its device address. The address of the device is

now known and it can be interrogated in the usual

way.

4. If more than one device’s ALERT output is low,

the one with the lowest device address has priority,

in accordance with normal SMBus arbitration.

ADM1021AARQZ-R Datasheet(PDF) 12 Page - ON Semiconductor

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