ADT7461A

http://onsemi.com

15

Sensor Fault Detection

At its D+ input, the ADT7461A 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 ADT7461A. 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

ADT7461A, tie the D+ and D− inputs together to prevent

continuous setting of the open flag.

The ADT7461A Interrupt System

The ADT7461A 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 and local limits are set by

default to 85

°C. A hysteresis value can be programmed; in

which case, 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 can be

reprogrammed to any value after powerup.

The hysteresis loop on the THERM outputs is useful when

THERM is used, for example, as an on/off controller for a

fan. The user’s system can be set up so that when THERM

asserts, a fan is 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 switched.

Table 10. 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. The ALERT output can be used as a SMBALERT

to signal to the host via the SMBus that the temperature has

risen. The user can use the THERM output to turn on a fan

to cool the system, if the temperature continues to increase.

This method ensures 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

1

3

2

4

HIGH TEMP LIMIT

RESET BY MASTER

ALERT

THERM

1005C

TEMPERATURE

905C

805C

705C

605C

505C

405C

THERM LIMIT

THERM LIMIT−HYSTERESIS

temperature limit, the ALERT output asserts low.

THERM limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

temperature falls to THERM limit minus hysteresis. In ,

the default hysteresis value of 10

°C is shown.

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

ALERT output or as an additional THERM output.

programmed local and/or remote high temperature

limits. It is reset in the same manner as THERM and is

not maskable.

THERM2.

Figure 20 shows how THERM and THERM2 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 is used to turn on a

fan. If the temperature continues to rise and exceeds the

THERM limits, the THERM output provides additional

cooling by throttling the CPU.