ADM1021

–10–

REV. 0

ALERT OUTPUT

The

ALERT output goes low whenever an out-of limit measure-

ment is detected, or if the remote temperature sensor is open-

circuit. It is an open-drain and requires a 10 k

Ω pull-up to V

Several

ALERT outputs can be wire-ANDED together, so that

the common line will go low if one or more of the

ALERT out-

puts goes low.

The

ALERT output can be used as an interrupt signal to a

processor, or it may be used as an

SMBALERT. Slave devices

on the SMBus can normally not signal to the master that they

want to talk, but the

SMBALERT function allows them to do

so.

One or more

ALERT outputs are 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 16.

MASTER

RECEIVES

SMBALERT

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

NO

ACK

START

ALERT RESPONSE ADDRESS

RD

ACK

DEVICE ADDRESS

STOP

Figure 16. Use of

SMBALERT

1.

SMBALERT pulled low.

2. Master initiates a read operation and sends the Alert Re-

sponse 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 ad-

dress. 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, will have priority, in accordance

with normal SMBus arbitration.

5. Once the ADM1021 has responded to the Alert Response

Address, it will reset its

ALERT output, provided that the

error condition that caused the

ALERT no longer exists. If

the

SMBALERT line remains low, the master will send

ARA again, and so on until all devices whose

ALERT out-

puts were low have responded.

LOW POWER STANDBY MODES

The ADM1021 can be put into a low power standby mode

using hardware or software, that is by taking the

STBY input

low, or by setting Bit 6 of the Configuration Register. When

STBY is high, or Bit 6 is low, the ADM1021 operates normally.

When

STBY is pulled low or Bit 6 is high, the ADC is inhibited,

any conversion in progress is terminated without writing the

result to the corresponding value register.

The SMBus is still enabled. Power consumption in the standby

mode is reduced to less than 10

µA if there is no SMBus activ-

ity, or 100

µA if there are clock and data signals on the bus.

These two modes are similar but not identical. When

STBY is

low, conversions are completely inhibited. When Bit 6 is set but

STBY is high, a one-shot conversion of both channels can be

initiated by writing XXh to the One-Shot Register (address 0Fh).

SENSOR FAULT DETECTION

The ADM1021 has a fault detector at the D+ input that detects

if the external sensor diode is open-circuit. This is a simple

1 V (typical). The output of this comparator is checked when a

conversion is initiated, and sets Bit 2 of the Status Register if a

fault is detected.

If the remote sensor voltage falls below the normal measuring

range, for example due to the diode being short-circuited, the

ADC will output –128

°C (1000 0000). Since the normal operat-

ing temperature range of the device only extends down to –55

°C,

this output code should never be seen in normal operation, so it

can be interpreted as a fault condition. Since it will be outside

the power-on default low temperature limit (–55

°C) and any

low limit that would normally be programmed, a short-circuit

sensor will cause an SMBus alert.

In this respect, the ADM1021 differs from and improves upon

competitive devices that output zero if the external sensor goes

short-circuit. These devices can misinterpret a genuine 0

°C

measurement as a fault condition.

If the external diode channel is not being used and it is shorted

out, then the resulting

ALERT may be cleared by writing 80h

(–128

°C) to the low limit register.

R/

W

A6

SCLK

SDATA

A5

A4

A3

A2

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

NO ACK.

BY MASTER

STOP BY

MASTER

START BY

MASTER

19

1

ACK. BY

ADM1021

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADM1021

Figure 15. Reading Data from a Previously Selected Register