ADM1021AARQZ View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1021AARQZ

Low-Cost Microprocessor System Temperature Monitor

Analog Devices 

ADM1021A

This is illustrated in Figure 3. The device address is sent over

the bus followed by R/W set to 0. This is followed by two data

bytes. The first data byte is the address of the internal data reg-

ister to be written to, which is stored in the Address Pointer

Register. The second data byte is the data to be written to the

internal data register.

When reading data from a register there are two possibilities:

1. If the ADM1021A’s Address Pointer Register value is 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 performing a write to the ADM1021A

as before, but only the data byte containing the register read

address is sent, as data is not to be written to the register.

This is shown in Figure 4.

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. This is shown in Figure 5.

2. If the Address Pointer Register is known to be already at the

desired address, data can be read from the corresponding data

register without first writing to the Address Pointer Register,

so Figure 4 can be omitted.

NOTES

1. Although it is possible to read a data byte from a data register

without first writing to the Address Pointer Register, 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.

2. Remember that the ADM1021A registers have different

addresses for read 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 is not 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

The ALERT output goes low whenever an out-of-limit mea-

surement is detected, or if the remote temperature sensor is

open-circuit. It is an open-drain and requires a 10 kΩ pull-up to

VDD. Several ALERT outputs can be wire-ANDed together, so

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

outputs goes low.

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

cessor, 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 6.

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. 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 ADM1021A 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 the

ARA again, and so on until all devices whose ALERT outputs

were low have responded.

LOW POWER STANDBY MODES

The ADM1021A 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 ADM1021A operates normally.

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

so 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 activity,

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 initi-

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

SENSOR FAULT DETECTION

The ADM1021A has a fault detector at the D+ input that detects

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

voltage comparator that trips if the voltage at D+ exceeds

VCC – 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 0°C,

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

can be interpreted as a fault condition.

In this respect, the ADM1021A 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 is shorted

out, the resulting ALERT may be cleared by writing 80h (–128°C)

to the low limit register.

START

ALERT RESPONSE ADDRESS

RD

ACK

DEVICE ADDRESS

NO

ACK

STOP

MASTER SENDS

ARA AND READ

COMMAND

DEVICE SENDS

ITS ADDRESS

Figure 6. Use of SMBALERT

–10–

REV. D

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