ADT7421

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When reading data from a register there are two

possibilities.

unknown or not the desired value, it is first necessary to

set it to the correct value before data can be read from

the desired data register. This is done by writing to the

ADT7421 as before, but only the data byte containing

the register read address is sent, because data is not to

be written to the register (see Figure 11).

A read operation is then performed consisting of the

serial bus address, R/W bit set to 1, followed by the

data byte read from the data register (see Figure 12).

desired address, data can be read from the

corresponding data register without first writing to the

address pointer register and the bus transaction shown

in Figure 11 can be omitted.

Notes

without first writing to the address pointer register.

However, if the address pointer register is already at the

correct value, it is not possible to write data to a register

without writing to the address pointer register because

the first data byte of a write is always written to the

address pointer register.

and write operations. The write address of a register

must be written to the address pointer if data is to be

written to that register, but it may not be possible to

read data from that address. The read address of a

register must be written to the address pointer before

data can be read from that register.

ALERT Output

This is applicable when Pin 6 is 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 open circuit. It is an open−drain output

and requires a pullup resistor. 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 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 that is connected to the master.

When the SMBALERT line is pulled low by one of the

devices, the following procedure occurs (see Figure 13):

Figure 13. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

RD

START

ACK

DEVICE

ADDRESS

NO

ACK STOP

MASTER

RECEIVES

SMBALERT

1. SMBALERT is pulled low.

2. Master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. 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.

4. If more than one device’s ALERT output is low,

the one with the lowest device address takes

priority, in accordance with normal SMBus

arbitration.

Once the ADT7421 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 whose ALERT outputs

were low have responded.

Low Power Standby Mode

The ADT7421 can be put into low power standby mode

by setting Bit 6 of the configuration register. When Bit 6 is

low, the ADT7421 operates normally. When Bit 6 is high,

the ADC is inhibited, and any conversion in progress is

terminated without writing the result to the corresponding

value register. However, the SMBus is still enabled. Power

consumption in the standby mode is reduced to 10

mA.

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

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

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

returns 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. If the values stored in the temperature value

registers are outside the new limits, an ALERT is generated,

even though the ADT7421 is still in standby.

Sensor Fault Detection

At its D+ input, the ADT7421 contains internal sensor

fault detection circuitry. This circuit can detect situations

where an external remote transistor is either not connected

or incorrectly connected to the ADT7421. 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

ADT7421, tie the D+ and D− inputs together to prevent

continuous setting of the open flag.