ADT7481
http://onsemi.com
16
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癈 to +150癈. Above 150癈, 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癈. 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癈, 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 users
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 17. THERM
HYSTERESIS
THERM
Hysteresis
Binary Representation
0癈
0 000 0000
1癈
0 000 0001
10癈
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.
Figure 19. Operation of the ALERT
and THERM
Interrupts
1005C
THERM
LIMIT
905C
805C
705C
605C
505C
405C
THERM
LIMIT HYSTERESIS
HIGH TEMP LIMIT
RESET BY MASTER
TEMPERATURE
1
2
3
4
ALERT
THERM
" If the measured temperature exceeds the high
temperature limit, the ALERT
output will assert low.
" If the temperature continues to increase and exceeds the
THERM
limit, the THERM
output asserts low. This can
be used to throttle the CPU clock or switch on a fan.
" The THERM
output de-asserts (goes high) when the
temperature falls to THERM
limit minus hysteresis. In
Figure 19, the default hysteresis value of 10癈 is
shown.
" The ALERT
output de-asserts only when the
temperature has fallen below the high temperature
limit, and the master has read the device address and
cleared the status register.
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