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ADT7461

the ADT7461. A simple voltage comparator trips if the voltage

at D+ exceeds VDD −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 of the status register (OPEN

flag) is set if a fault is detected. If the ALERT pin is enabled,

setting this flag will cause ALERT to assert low.

If the user does not wish to use an external sensor with the

ADT7461, then in order to prevent the OPEN flag being set

continuously, the user should tie the D+ and D− inputs of the

ADT7461 together.

Most temperature sensing diodes have an operating temperature

range of −55°C to +150°C. Above 150°C, they lose their semicon-

ductor characteristics and approximate conductors instead. This

results in a diode short, setting the OPEN flag. The external diode

in this case will no longer give an accurate temperature meas-

urement. 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 measure-

ment on the external channel may not be accurate.

THE ADT7461 INTERRUPT SYSTEM

The ADT7461 has two interrupt outputs, ALERT and THERM.

Both have different functions and behavior. ALERT is maskable

and responds to violations of software-programmed tempera-

ture limits or an open-circuit fault on the external diode.

THERM is intended as a fail-safe interrupt output that cannot

be masked.

If the external or local temperature exceeds the programmed

high temperature limits, or equals or exceeds the low tempera-

ture limits, the ALERT output is asserted low. An open-circuit

fault on the external 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

The THERM output asserts low if the external or local tempera-

ture 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 limit. The

external THERM limit is set by default to 85°C, as is the local

THERM limit. 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, where the temperature hovers around

the THERM limit, and the fan is constantly being switched.

Table 13. THERM Hysteresis

THERM Hysteresis

Binary Representation

0°C

0 000 0000

1°C

0 000 0001

10°C

0 000 1010

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

Figure 19. Operation of the ALERT and THERM Interrupts

Figure 19

THERM LIMIT

HIGH TEMP LIMIT

THERM LIMIT-HYSTERESIS

RESET BY MASTER

ALERT

THERM

100

°C

TEMPERATURE

°C

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 deasserts (goes high) when the

temperature falls to THERM limit minus hysteresis. In

, the default hysteresis value of 10°C is shown.

The ALERT output deasserts 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 6 on the ADT7461 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 applies to THERM2 also.

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