ADT75 Analog Devices, Inc., ADT75 Datasheet - Page 20

• Manufacturer Part Number: ADT75
• Description: ?1?c Accurate, 12-bit Digital Temperature Sensor
• Manufacturer: Analog Devices, Inc.

ADT75
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 V
SMBus alert outputs can be wire-AND’ e d 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’ e d
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.
START
MASTER
RECEIVES
SMBALERT
ALERT RESPONSE
MASTER SENDS
Figure 20. ADT75 Responds to SMBALERT ARA
ARA AND READ
ADDRESS
COMMAND
RD ACK DEVICE ADDRESS
DEVICE SENDS
ITS ADDRESS
DD
. Several
ACK STOP
NO
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2.
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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
MASTER
RECEIVES
SMBALERT
SMBALERT is pulled low.
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.
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 T
temperature is less than T
known and it can be interrogated in the usual way.
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.
MASTER SENDS
ARA AND READ
ADDRESS
COMMAND
Figure 21. ADT75 Responds to SMBALERT ARA
with Packet Error Checking (PEC)
RD ACK
DEVICE ACK
OS
HYST
, and the LSB is low if the
DEVICE SENDS
ITS ADDRESS
. The address of the device is now
ADDRESS
DEVICE
MASTER
ACK
ACK
DEVICE SENDS
ITS PEC DATA
PEC
MASTER
NACK
ACK
NO
STOP