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ADT75

Data Sheet

Rev. B | Page 20 of 24

SMBus ALERT

The OS/ALERT pin can behave as a SMBus alert pin when the

SMBus alert function is enabled by setting Bit D7 in the

configuration register. The interrupt mode must also be selected

(Bit D1 in the configuration register). The OS/ALERT pin is an

open-drain output and requires a pull-up to VDD. Several SMBus

alert outputs can be wire-AND’ed together, so that the common

line goes low if one or more of the SMBus alert outputs goes

low. The polarity of the OS/ALERT pin must be set for active

low for a number of outputs to be wire-AND’ed together.

The OS/ALERT output can operate as a SMBALERT function.

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. SMBALERT is used in conjunction with the SMBus general

call address.

One or more SMBus alert outputs can be 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 20.

MASTER

RECEIVES

SMBALERT

START

ALERT RESPONSE

ADDRESS

RD ACK DEVICE ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

ACK

STOP

Figure 20. ADT75 Responds to SMBALERT ARA

1. SMBALERT is pulled low.

2. Master initiates a read operation and sends the SMBus alert

response address (ARA = 0001 100). This reserved SMBus/

I2C address must not be used as a specific device address.

3. The device whose SMBus alert output is low responds to the

SMBus alert response address and the master reads its device

address. As the device address is seven bits long, the ADT75’s

LSB is free to be used as an indicator as to which temperature

limit was exceeded. The LSB is high if the temperature is

greater than or equal to TOS, and the LSB is low if the

temperature is less than THYST. The address of the device

is now known and it can be interrogated in the usual way.

4. If more than one devices’ SMBus alert output is low, the one

with the lowest device address has priority, which is in

accordance with normal SMBus specifications.

Once the ADT75 has responded to the SMBus alert response

address, it resets its SMBus alert output. If the SMBALERT line

remains low, the master sends the ARA again. It continues to do

this until all devices whose SMBALERT outputs were low have

responded.

START

ALERT RESPONSE

ADDRESS

RD ACK

DEVICE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

DEVICE ACK

ACK

PEC

ACK

STOP

MASTER

ACK

MASTER

NACK

DEVICE SENDS

ITS PEC DATA

MASTER

RECEIVES

SMBALERT

Figure 21. ADT75 Responds to SMBALERT ARA with

Packet Error Checking (PEC)

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