ADT7481
http://onsemi.com
16
Low Power Standby Mode
The ADT7481 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. The
ADT7481 operates normally when Bit 6 is 0. 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 both channels by writing to
the one−shot register (Address 0x0F), after which the device
will return to standby. It does not matter what is written to
the one−shot register, all data written to it is ignored. It is also
possible to write new values to the limit register while in
standby mode. ALERT
and THERM are not available in
standby mode and, therefore, should not be used because the
state of these pins is unreliable.
Sensor Fault Detection
The ADT7481 has internal sensor fault detection circuitry
at its D+ input. This circuit can detect situations where a
remote diode is not connected, or is incorrectly connected,
to the ADT7481. If the voltage at D+ exceeds V
DD
− 1.0 V
(typical), it signifies an open circuit between D+ and D−, and
consequently, trips the simple voltage comparator. 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 will
cause ALERT
to assert low.
If a remote sensor is not used with the ADT7481, then the
D+ and D− inputs of the ADT7481 need to be tied together
to prevent the open flag from being continuously set.
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 will give the last good
temperature measurement. The user should be aware that
while the diode fault is triggered, the temperature
measurement on the remote channels is likely to be
inaccurate.
Interrupt System
The ADT7481 has two interrupt outputs, ALERT and
THERM
. Both outputs 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 automatically reset 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
will reset 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 may 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 user’s
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, a condition wherein 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 0 000 0000
1°C 0 000 0001
10°C 0 000 1010
Figure 19 shows how the THERM and ALERT outputs
operate. A user may wish to use the ALERT
output as a
SMBALERT
to signal to the host via the SMBus that the
temperature has risen. The user could use the THERM
output to turn on a fan to cool the system, if the temperature
continues to increase. This method would ensure that there
is a fail−safe mechanism to cool the system, without the need
for host intervention.
