ADT7461

http://onsemi.com

15

Sensor Fault Detection

At its D+ input, the ADT7461 contains internal sensor

fault detection circuitry. This circuit can detect situations

where an external remote diode is either not connected or

incorrectly connected to the ADT7461. A simple voltage

(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 causes ALERT to assert low.

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

ADT7461, then to prevent continuous setting of the OPEN

flag, the user should tie the D+ and D− inputs together.

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 temperature 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 temperature 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 register has been reset.

The THERM output asserts low if the external or local

temperature exceeds the programmed THERM limits.

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 limit is set by default to 85

°C,

as is the local THERM limit. A hysteresis value can be

programmed so that 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 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 an

hovers around the THERM limit, and the fan is constantly

being switched.

Table 14. 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 choose to use the ALERT output as an

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 there is a

fail-safe mechanism to cool the system without the need for

host intervention.

Figure 20. 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

23

4

ALERT

THERM

1. If the measured temperature exceeds the high

temperature limit, the ALERT output asserts low.

2. 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.

3. The THERM output deasserts (goes high) when

the temperature falls to THERM limit minus

hysteresis. The default hysteresis value of 10

°C is

shown in Figure 20.

4. The ALERT output deasserts only when the

temperature falls 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 asserts 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.

Figure 21 shows how THERM and THERM2

might operate together to implement two methods

of cooling the system. In this example, the

THERM2 limits are set lower than the THERM

limits. The THERM2 output could be used to turn

on a fan. If the temperature continues to rise and

exceeds the THERM limits, the THERM output

could provide additional cooling by throttling the

CPU.