ADT7481ARMZ datasheet(20/24 Pages) AD

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ADT7481

Rev. 0 | Page 20 of 24

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 power-up.

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 16. THERM Hysteresis

THERM Hysteresis

Binary Representation

0°C

0 000 0000

1°C

0 000 0001

10°C

0 000 1010

Figure 20 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.

THERM LIMIT

HIGH TEMP LIMIT

THERM LIMIT-HYSTERESIS

RESET BY MASTER

ALERT

THERM

100

°C

TEMPERATURE

°C

Figure 20. Operation of the ALERT and THERM Interrupts

• 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 tempera-

ture falls to THERM limit minus hysteresis. In Figure 20, the

default hysteresis value of 10°C 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.

Pin 8 on the ADT7481 can be configured as either an ALERT

output or as an additional THERM output. THERM2 will assert

low when the temperature exceeds the programmed local and/or

remote high temperature limits. It is reset in the same manner

as THERM, and it is not maskable. The programmed hysteresis

value also applies to THERM2.

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