ADT7482ARMZ datasheet(19/24 Pages) AD

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ADT7482

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ALERT OUTPUT

Pin 8 can be configured as an ALERT output. The ALERT

output goes low whenever an out-of-limit measurement is

detected, or if the remote temperature sensor is an open circuit.

It is an open-drain output and requires a pull-up to VDD. Several

ALERT outputs can be wire-OR’ed together, so that the common

line goes low if one or more of the ALERT outputs goes low.

The ALERT output can be used as an interrupt signal to a

processor, or it can be used as an SMBALERT. Slave devices on

the SMBus cannot normally signal to the bus master that they

want to talk, but the SMBALERT function allows them to do so.

One or more ALERT outputs can be connected to a common

SMBALERT line connected to the master. When the SMBALERT

line is pulled low by one of the devices, the following procedure

occurs as illustrated in Figure 20.

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

START

ACK

DEVICE

ADDRESS

ACK STOP

MASTER

RECEIVES

SMBALERT

Figure 20. Use of SMBALERT

SMBALERT is pulled low.

Master initiates a read operation and sends the alert

response address (ARA = 0001 100). This is a general call

address that should not be used as a specific device

address.

The device whose ALERT output is low responds to the

alert response address and the master reads its device

address. As the device address is seven bits, an LSB of 1 is

added. The address of the device is now known and it can

be interrogated in the usual way.

If more than one device has a low ALERT, the one with the

lowest device address has priority, in accordance with

normal SMBus arbitration.

Once the ADT7482 has responded to the alert response

address, it resets its ALERT output, provided that the error

condition that caused the ALERT no longer exists. If the

SMBALERT line remains low, the master sends the ARA

again, and so on until all devices with low ALERT

outputs respond.

LOW POWER STANDBY MODE

The ADT7482 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. When Bit 6

is 0, the ADT7482 operates normally. 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 μA if there is no SMBus activity or up to 30 μA 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 all channels by writing to the one-shot

standby. It does not matter what is written to the one-shot

write new values to the limit register while in standby mode. If

the values stored in the temperature value registers are now

outside the new limits, an ALERT is generated, even though the

ADT7482 is still in standby mode.

SENSOR FAULT DETECTION

The ADT7482 has sensor fault detection circuitry internally at

its D+ inputs. This circuit can detect situations where a remote

diode is not connected, or is incorrectly connected, to the

ADT7482. 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 (D1 OPEN flag) of the

StatusRegister 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 causes

ALERT to assert low.

If a remote sensor is not used with the ADT7482, then the D+

and D− inputs of the ADT7482 need to be tied together to

prevent the OPEN flag from being set continuously.

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 gives the

last good temperature measurement. Be aware that while the

diode fault is triggered, the temperature measurement on the

remote channels may not be accurate.

INTERRUPT SYSTEM

The ADT7482 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 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

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