ADT7482 Datasheet, PDF (16/20 Pages) Analog Devices – Dual Channel Temperature Sensor and Overtemperature Alarm

ADT7482
3. The device whose ALERT output is low responds
to the alert response address and the master reads
its device address. As the device address is seven
bits, an LSB of 1 is added. The address of the
device is now known and it can be interrogated in
the usual way.
4. If more than one device has a low ALERT, the one
with the lowest device address has priority, in
accordance with normal SMBus arbitration.
5. Once the ADT7482 has responded to the alert
response address, it resets its ALERT output,
provided that the error condition that caused the
ALERT no longer exists. If the SMBALERT line
remains low, the master sends the ARA again, and
so on until all devices with low ALERT outputs
respond.
Low Power Standby Mode
The ADT7482 can be put into low power standby mode
by setting Bit 6 (Mon/STBY bit) of the Configuration 1
register (Read Address = 0x03, Write Address = 0x09) to 1.
When Bit 6 is 0, the ADT7482 operates normally. When Bit
6 is 1, the ADC is inhibited, and any conversion in progress
is terminated without writing the result to the corresponding
value register.
The SMBus is still enabled in low power standby mode.
Power consumption in this standby mode is reduced to a
typical of 5 mA if there is no SMBus activity or up to 30 mA
if there are clock and data signals on the bus.
When the device is in standby mode, it is still possible to
initiate a one−shot conversion of all channels by writing to
the one−shot register (Address 0x0F), after which the device
returns to standby. It does not matter what is written to the
one−shot register as all data written to it is ignored. It is also
possible to write new values to the limit register while in
standby mode. If the values stored in the temperature value
registers are now outside the new limits, an ALERT is
generated, even though the ADT7482 is still in standby
mode.
Sensor Fault Detection
The ADT7482 has sensor fault detection circuitry
internally at its D+ inputs. This circuit can detect situations
where a remote diode is not connected, or is incorrectly
connected, to the ADT7482. A simple voltage comparator
trips if the voltage at D+ exceeds V_DD −1 V (typical),
signifying an open circuit between D+ and D−. The output
of this comparator is checked when a conversion is initiated.
Bit 2 (D1 OPEN flag) of the Status Register 1 (Address
0x02) is set if a fault is detected on the Remote 1 channel. Bit
2 (D2 OPEN flag) of the Status Register 2 (Address 0x23)
is set if a fault is detected on the Remote 2 channel. If the
ALERT pin is enabled, setting this flag causes ALERT to
assert low.
If a remote sensor is not used with the ADT7482, then the
D+ and D− inputs of the ADT7482 need to be tied together
to prevent the OPEN flag from being set continuously.
Most temperature sensing diodes have an operating
temperature range of −55°C to +150°C. Above 150°C, they
lose their semiconductor characteristics and approximate
conductors instead. This results in a diode short, setting the
open flag. The remote diode in this case no longer gives an
accurate temperature measurement. A read of the
temperature result register gives the last good temperature
measurement. Be aware that while the diode fault is
triggered, the temperature measurement on the remote
channels may not be accurate.
Interrupt System
The ADT7482 has two interrupt outputs, ALERT and
THERM. Both have different functions and behavior.
ALERT is maskable and responds to violations of software
programmed temperature limits or an open−circuit fault on
the remote diode. THERM is intended as a fail−safe
interrupt output that cannot be masked.
If the Remote 1, Remote 2, or local temperature exceeds
the programmed high temperature limits, or equals or
exceeds the low temperature limits, the ALERT output is
asserted low. An open−circuit fault on the remote diode also
causes ALERT to assert. ALERT is reset when serviced by
a master reading its device address, provided the error
condition has gone away, and the status register has been
reset.
The THERM output asserts low if the Remote 1,
Remote 2, or local temperature exceeds the programmed
THERM limits. The THERM temperature limits should
normally be equal to or greater than the high temperature
limits. THERM is reset automatically when the temperature
falls back within the (THERM − hysteresis) limit. The local
and remote THERM limits are set by default to 85°C. A
hysteresis value can be programmed, in which case,
THERM resets when the temperature falls to the limit value
minus the hysteresis value. This applies to both local and
remote measurement channels. The power−on hysteresis
default value is 10°C, but this can be reprogrammed to any
value after powerup.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The system can
be set up so that when THERM asserts, a fan can be switched
on to cool the system. When THERM goes high again, the
fan can be switched off. Programming a hysteresis value
protects from fan jitter, where the temperature hovers
around the THERM limit, and the fan is constantly being
switched on and off.
Table 12. THERM Hysteresis
THERM Hysteresis
Binary Representation
0°C
1°C
10°C
0 000 0000
0 000 0001
0 000 1010
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