ADT7481

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16

Low Power Standby Mode

The ADT7481 can be put into low power standby mode

by setting Bit 6 (Mon/STBY bit) of the Configuration 1

register (Read Address 0x03, Write Address 0x09) to 1. The

ADT7481 operates normally when Bit 6 is 0. When Bit 6 is

1, the ADC is inhibited, and any conversion in progress is

terminated without writing the result to the corresponding

value register.

The SMBus is still enabled in low power standby mode.

Power consumption in this standby mode is reduced to a

typical of 5

mA if there is no SMBus activity, or up to 30 mA

if there are clock and data signals on the bus.

When the device is in standby mode, it is still possible to

initiate a one−shot conversion of both channels by writing to

the one−shot register (Address 0x0F), after which the device

will return to standby. It does not matter what is written to

the one−shot register, all data written to it is ignored. It is also

possible to write new values to the limit register while in

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,

(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

°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 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 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 12. 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. 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.