Dual Channel Temperature Sensor
and Overtemperature Alarm
ADT7482
Rev. 0
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responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
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Tel: 781.329.4700 www.analog.com
Fax: 781.461.3113 ©2006 Analog Devices, Inc. All rights reserved.
FEATURES
1 local and 2 remote temperature sensors
0.25°C resolution/1°C accuracy on remote channels
1°C resolution/1°C accuracy on local channel
Automatically cancels up to 1.5 kΩ (typ) of resistance in
series with the remote sensors
Extended, switchable temperature measurement range 0°C
to +127°C (default) or −55°C to +150°C
2-wire SMBus serial interface with SMBus alert support
Programmable overtemperature/undertemperature limits
Offset registers for system calibration
Up to 2 overtemperature fail-safe THERM outputs
Small, 10-lead MSOP package
240 μA operating current, 5 μA standby current
APPLICATIONS
Desktop and notebook computers
Industrial controllers
Smart batteries
Automotive
Embedded systems
Burn-in applications
Instrumentation
GENERAL DESCRIPTION
The ADT74821 is a three-channel digital thermometer and
undertemperature/overtemperature alarm for PCs and thermal
management systems. It can measure the temperature in two
remote locations, such as in the remote thermal diode in a CPU
or GPU, or using a discrete diode connected transistor. This
device also measures its own ambient temperature.
One feature of the ADT7482 is series resistance cancellation
where up to 1.5 kΩ (typical) of resistance in series with each of
the temperature monitoring diodes can be automatically
cancelled from the temperature result, allowing noise filtering.
The temperature of the remote thermal diodes and ambient
temperature can be measured accurate to ±1°C. The tempera-
ture measurement range, which defaults to 0°C to 127°C, can be
switched to a wider measurement range of from −55°C to +150°C.
The ADT7482 communicates over a 2-wire serial interface
compatible with system management bus (SMBus) standards.
The default address of the ADT7482 is 0x4C. An ALERT output
signals when the on-chip or remote temperature is outside the
programmed limits. The THERM output is a comparator output
that allows on/off control of a cooling fan. The ALERT output can
be reconfigured as a second THERM output if required.
1 Protected by U.S. Patents 5,195,827, 5,867,012, 5,982,221, 6,097,239,
6,133,753, 6,169,442, 7,010,440, other patents pending.
FUNCTIONAL BLOCK DIAGRAM
11-BIT ADC
RUN/STANDBYBUSY
SRC
LIMIT COMPARATOR
STATUS REGISTERS
INTERRUPT
MASKING
LOCAL TEMPERATURE
VALUE REGISTER
ON-CHIP
TEMPERATURE
SENSOR
ANALOG
MUX
EXTERNAL DIODES OPEN-CIRCUIT
SMBus INTERFACE
ADT7482
6
7
101 5 9
V
DD
GND SDATA SCLK
8
D2+
D2–
3
2
D1+
D1–
ALERT/THERM2
CONFIGURATION
REGISTERS
LOCAL TEMPERATURE
HIGH-LIMIT REGISTER
LOCAL TEMPERATURE
LOW-LIMIT REGISTER
LOCAL TEMPERATURE
THERM-LIMIT REGISTER
CONVERSION RATE
REGISTER
ONE-SHOT
REGISTER
ADDRESS POINTER
REGISTER
DIGITAL MUX
4
THERM
0
6150-001
REMOTE 1 AND 2 TEMP
VALUE REGISTERS
REMOTE 1 AND 2 TEMP
THERM-LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
LOW-LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
HIGH-LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
VALUE REGISTERS
Figure 1.
ADT7482
Rev. 0 | Page 2 of 24
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications....................................................................................... 1
General Description......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Specifications .................................................................. 4
Absolute Maximum Ratings............................................................ 5
Thermal Resistance ...................................................................... 5
ESD Caution.................................................................................. 5
Pin Configuration and Function Descriptions............................. 6
Typical Performance Characteristics ............................................. 7
Theory of Operation ........................................................................ 9
Series Resistance Cancellation.................................................... 9
Temperature Measurement Method .......................................... 9
Temperature Measurement Results.......................................... 10
Temperature Measurement Range ........................................... 10
Temperature Data Format......................................................... 11
Registers........................................................................................... 12
Address Pointer Register ........................................................... 12
Configuration Registers............................................................. 12
Temperature Value Registers .................................................... 12
Conversion Rate Register.......................................................... 13
Limit Registers............................................................................ 13
Status Registers ........................................................................... 14
Offset Register ............................................................................ 14
One-Shot Register ...................................................................... 15
Consecutive ALERT Register ................................................... 15
Serial Bus Interface......................................................................... 17
Addressing the Device ............................................................... 17
ALERT Output............................................................................ 19
Low Power Standby Mode......................................................... 19
Sensor Fault Detection .............................................................. 19
Interrupt System......................................................................... 19
Applications Information.............................................................. 22
Noise Filtering............................................................................. 22
Factors Affecting Diode Accuracy........................................... 22
Thermal Inertia and Self-Heating............................................ 22
Layout Considerations............................................................... 23
Application Circuit..................................................................... 23
Outline Dimensions ....................................................................... 24
Ordering Guide .......................................................................... 24
REVISION HISTORY
6/06—Revision 0: Initial Version
ADT7482
Rev. 0 | Page 3 of 24
SPECIFICATIONS
TA = −40°C to +120°C , VDD = 3 V to 3.6 V, unless otherwise noted.
Table 1.
Parameter Min Typ Max Unit Test Conditions
POWER SUPPLY
Supply Voltage, VDD 3.0 3.30 3.6 V
Average Operating Supply Current, IDD 240 350 μA 0.0625 conversions/sec rate1
5 30 μA Standby mode
Undervoltage Lockout Threshold 2.55 V VDD input, disables ADC, rising edge
Power-On Reset Threshold 1 2.5 V
TEMPERATURE-TO-DIGITAL CONVERTER
Local Sensor Accuracy ±1 °C 0°C ≤ TA ≤ +70°C
±1.5 °C 0°C ≤ TA ≤ +85°C
±2.5 °C −40°C ≤ TA ≤ +100°C
Resolution 1 °C
Remote Diode Sensor Accuracy ±1 °C 0°C ≤ TA ≤ +70°C, −55°C ≤ TDD
2 ≤ +150°C
±1.5 °C 0°C ≤ TA ≤ +85°C, −55°C ≤ TD2 ≤ +150°C
±2.5 °C −40°C ≤ TA ≤ +100°C, −55°C ≤ TD2 ≤ +150°C
Resolution 0.25 °C
Remote Sensor Source Current 220 μA High level3
82 μA Mid level3
13.5 μA Low level2
Maximum Series Resistance Cancelled 1.5 kΩ Resistance split evenly on D+ and D− lines
Conversion Time 71 93 ms From stop bit to conversion complete (all channels) one-shot mode
with averaging switched on
11.5 15 ms
One-shot mode with averaging off (that is, conversion rate = 16, 32,
or 64 conversions per second)
OPEN-DRAIN DIGITAL OUTPUTS
(THERM, ALERT/THERM2)
Output Low Voltage, VOL 0.4 V IOUT = −6.0 mA
High Level Output Leakage Current, IOH 0.1 1 μA VOUT = VDD
SMBus INTERFACE3, 4
Logic Input High Voltage, VIH 2.1 V
SCLK, SDATA
Logic Input Low Voltage, VIL 0.8 V
SCLK, SDATA
Hysteresis 500 mV
SDA Output Low Voltage, VOL 0.4 V IOUT = −6.0 mA
Logic Input Current, IIH, IIL −1 +1 μA
SMBus Input Capacitance, SCLK, SDATA 5 pF
SMBus Clock Frequency 400 kHz
SMBus Timeout5 25 64 ms User programmable
SCLK Falling Edge to SDATA Valid Time 1 μs Master clocking in data
1 See Table 11 for conversion rates.
2 Guaranteed by characterization, but not production tested.
3 Guaranteed by design, but not production tested.
4 See the Timing Specifications section for more information.
5 Disabled by default. For details on enabling the SMBus, see the Serial Bus Interface section.
ADT7482
Rev. 0 | Page 4 of 24
TIMING SPECIFICATIONS
Table 2. SMBus Timing Specifications1
Parameter Limit at TMIN, TMAX Unit Description
fSCLK 400 kHz max
tLOW 1.3 μs min Clock low period, between 10% points.
tHIGH 0.6 μs min Clock high period, between 90% points.
tR 300 ns max Clock/data rise time.
tF 300 ns max Clock/data fall time.
tSU; STA 600 ns min Start condition setup time.
tHD; STA2 600 ns min Start condition hold time.
tSU; DAT3 100 ns min Data setup time.
tSU; STO4 600 ns min Stop condition setup time.
tBUF 1.3 μs min Bus free time between stop and start conditions.
1 Guaranteed by design, but not production tested.
2 Time from 10% of SDATA to 90% of SCLK.
3 Time for 10% or 90% of SDATA to 10% of SCLK.
4 Time for 90% of SCLK to 10% of SDATA.
SCLK
S
DAT
A
t
R
t
F
t
LOW
t
HD;DAT
t
HD;STA
t
HIGH
t
SU;DAT
STOP START STOPSTART
t
SU;STA
t
SU;STO
t
HD;STA
t
BUF
06150-002
Figure 2. Serial Bus Timing
ADT7482
Rev. 0 | Page 5 of 24
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Rating
Positive Supply Voltage (VDD) to GND −0.3 V to +3.6 V
D+ −0.3 V to VDD + 0.3 V
D− to GND −0.3 V to +0.6 V
SCLK, SDATA, ALERT, THERM −0.3 V to +3.6 V
Input Current, SDATA, THERM −1 mA, to +50 mA
Input Current, D− ±1 mA
ESD Rating, All Pins (Human Body Model) 2000 V
Maximum Junction Temperature (TJ Max) 150°C
Storage Temperature Range −65°C to +150°C
IR Reflow Peak Temperature 220°C
IR Reflow Peak Temperature Pb-Free 260°C
Lead Temperature (Soldering 10 sec) 300°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
THERMAL RESISTANCE
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
Table 4. Thermal Resistance
Package Type θJA θ
JC Unit
10-Lead MSOP 142 43.7 °C/W
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
ADT7482
Rev. 0 | Page 6 of 24
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
V
DD 1
D1+
2
D1–
3
THERM
4
GND
5
SCLK
10
SDATA
9
ALERT/THERM2
8
D2+
7
D2–
6
ADT7482
TOP VIEW
(Not to Scale)
06150-003
Figure 3. Pin Configuration
Table 5. Pin Function Descriptions
Pin No. Mnemonic Description
1 VDD Positive Supply, 3 V to 3.6 V.
2 D1+ Positive Connection to the First Remote (Remote 1) Temperature Sensor.
3 D1− Negative Connection to the First Remote (Remote 1) Temperature Sensor.
4 THERM Open-Drain Output. This pin can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pull-up resistor.
5 GND Supply Ground Connection.
6 D2− Negative Connection to the Second Remote (Remote 2) Temperature Sensor.
7 D2+ Positive Connection to the Second Remote (Remote 2) Temperature Sensor.
8 ALERT
/THERM2
Open-Drain Logic Output. This pin is used as interrupt or SMBus alert. May also be configured as a second
THERM output. Requires pull-up resistor.
9 SDATA Logic Input/Output, SMBus Serial Data. Open-drain output. Requires pull-up resistor.
10 SCLK Logic Input, SMBus Serial Clock. Requires pull-up resistor.
ADT7482
Rev. 0 | Page 7 of 24
TYPICAL PERFORMANCE CHARACTERISTICS
3.5
–1.0
–50 150
TEMPERATURE (°C)
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
050100
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4Σ
LOW 4Σ
06150-004
Figure 4. Local Temperature Error vs. Temperature
3.5
–1.0
–50 150
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
050100
TEMPERATURE (°C)
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
HIGH 4Σ
LOW 4Σ
06150-005
Figure 5. Remote 1 Temperature Error vs. Temperature
3.5
–1.0
–50 150
05466-021
TEMPERATURE ERROR
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
050100
TEMPERATURE (°C)
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4Σ
LOW 4Σ
06150-006
Figure 6. Remote 2 Temperature Error vs. Temperature
10
–25
1100
LEAKAGE RESISTANCE (MΩ)
TEMPERATURE ERROR (°C)
5
0
–5
–10
–15
–20
10
D+ TO GND
D+ TO V
CC
06150-007
Figure7. Temperature Error vs. D+/D− Leakage Resistance
0
–18
025
CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
–2
–4
–6
–8
–10
–12
–14
–16
5101520
DEV 3
DEV 2
DEV 4
06150-008
Figure 8. Temperature Error vs. D+/D− Capacitance
1000
0
0.01 100
CONVERSION RATE (Hz)
I
DD
(µA)
900
800
700
600
500
400
300
200
100
0.1 1 10
DEV 4BC
DEV 3BC
DEV 2BC
06150-009
Figure 9. Operating Supply Current vs. Conversion Rate
ADT7482
Rev. 0 | Page 8 of 24
422
408
3.0 3.6
V
DD
(V)
I
DD
(µA)
420
418
416
414
412
410
3.1 3.2 3.3 3.4 3.5
DEV 3BC
DEV 2BC
DEV 4BC
06150-010
Figure 10. Operating Supply Current vs. Voltage
4.4
3.0
3.0 3.6
V
DD
(V)
I
DD
(µA)
4.2
4.0
3.8
3.6
3.4
3.2
3.1 3.2 3.3 3.4 3.5
DEV 3
DEV 2
DEV 4
06150-011
Figure 11. Standby Supply Current vs. Voltage
35
0
11000
FSCL (kHz)
I
STBY
(µA)
30
25
20
15
10
5
10 100
DEV 2BC
DEV 3BC
DEV 4BC
06150-012
Figure 12. Standby Supply Current vs. Clock Frequency
0
5
10
15
25
100 200 300 400 500 600
NOISE FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
20mV
20
0
100mV
50mV
06150-013
Figure 13. Temperature Error vs. Common Mode Noise Frequency
–10
0
20
40
80
100 200 300 400 500 600
NOISE FREQUENCY (MHz)
TEMPERATURE ERROR (°C)
20mV
60
0
100mV
10
30
70
50
50mV
06150-014
Figure 14. Temperature Error vs. Differential Mode Noise Frequency
0
80
70
60
50
40
30
20
10
2500
TOTAL SERIES RESISTANCE ON D+/D– LINES (Ω)
TEMPERATURE ERROR (°C)
0 500 1000 1500 2000
06150-015
Figure 15. Temperature Error vs. Series Resistance
ADT7482
Rev. 0 | Page 9 of 24
THEORY OF OPERATION
The ADT7482 is a local and 2× remote temperature sensor and
overtemperature/undertemperature alarm. When the ADT7482
is operating normally, the on-board ADC operates in a free-
running mode. The analog input multiplexer alternately selects
either the on-chip temperature sensor to measure its local
temperature or either of the remote temperature sensors. The
ADC digitizes these signals and the results are stored in the
local, Remote 1, and Remote 2 temperature value registers.
The local and remote measurement results are compared with
the corresponding high, low, and THERM temperature limits,
stored in on-chip registers. Out-of-limit comparisons generate
flags that are stored in the status register. A result that exceeds
the high temperature limit, the low temperature limit, or a
remote diode open circuit causes the ALERT output to assert
low. Exceeding THERM temperature limits causes the THERM
output to assert low. The ALERT output can be reprogrammed
as a second THERM output.
The limit registers can be programmed, and the device
controlled and configured, via the serial SMBus. The contents
of any register can also be read back via the SMBus.
Control and configuration functions consist of switching the
device between normal operation and standby mode, selecting
the temperature measurement scale, masking or enabling the
ALERT output, switching Pin 8 between ALERT and THERM2,
and selecting the conversion rate.
SERIES RESISTANCE CANCELLATION
Parasitic resistance to the D+ and D− inputs to the ADT7482,
seen in series with the remote diode, is caused by a variety of
factors, including PCB track resistance and track length. This
series resistance appears as a temperature offset in the remote
sensor temperature measurement. This error typically causes a
0.5°C offset per ohm of parasitic resistance in series with the
remote diode.
The ADT7482 automatically cancels out the effect of this series
resistance on the temperature reading, providing a more
accurate result, without the need for user characterization of
this resistance. The ADT7482 is designed to automatically cancel
typically up to 1.5 kΩ of resistance. By using an advanced
temperature measurement method, this is transparent to the
user. This feature allows resistances to be added to the sensor
path to produce a filter, allowing the part to be used in noisy
environments. See the Noise Filtering section for more details.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the
base-emitter voltage (VBE) of a transistor operated at constant
current. However, this technique requires calibration to null out
the effect of the absolute value of VBE, which varies from device
to device.
The technique used in the ADT7482 is to measure the change
in VBE when the device is operated at three different currents.
Previous devices have used only two operating currents. The
use of a third current allows automatic cancellation of
resistances in series with the external temperature sensor.
Figure 16 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the collec-
tor is not grounded and should be linked to the base. To prevent
ground noise from interfering with the measurement, the more
negative terminal of the sensor is not referenced to ground, but
is biased above ground by an internal diode at the D− input.
Capacitor C1 can be added as a noise filter (a recommended
maximum value of 1000 pF). However, a better option in noisy
environments is to add a filter, as described in the Noise
Filtering section. See the Layout Considerations section for
more information.
To me asure ΔVBE, the operating current through the sensor is
switched among three related currents. Shown in Figure 16,
N1 × I and N2 × I are different multiples of the current, I. The
currents through the temperature diode are switched between I
and N1 × I, giving ΔVBE1, and then between I and N2 × I, giving
ΔVBE2. The temperature can then be calculated using the two
ΔVBE measurements. This method can also be shown to cancel
the effect of any series resistance on the temperature measurement.
The resulting ΔVBE waveforms are passed through a 65 kHz
low-pass filter to remove noise and then to a chopper-stabilized
amplifier. This amplifies and rectifies the waveform to produce
a dc voltage proportional to ΔVBE. The ADC digitizes this vol-
tage and a temperature measurement is produced. To reduce the
effects of noise, digital filtering is performed by averaging the
results of 16 measurement cycles for low conversion rates. At
rates of 16, 32, and 64 conversions/second, no digital averaging
takes place.
Signal conditioning and measurement of the internal tempera-
ture sensor are performed in the same manner.
ADT7482
Rev. 0 | Page 10 of 24
IN1
×
IN2
×
II
BIAS
V
DD
V
OUT+
TO ADC
V
OUT–
REMOTE
SENSING
TRANSISTOR
1
CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
D+
D–
C1
1
BIAS
DIODE LOW-PASS FILTER
f
C
= 65kHz
0
6150-016
Figure 16. Input Signal Conditioning
TEMPERATURE MEASUREMENT RESULTS
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and are compared with limits programmed into the local and
remote high and low limit registers.
The local temperature measurement is an 8-bit measurement
with 1°C resolution. The remote temperature measurements are
10-bit measurements, with the 8 MSBs stored in one register
and the 2 LSBs stored in another register. Table 6 is a list of the
temperature measurement registers.
Table 6. Register Address for the Temperature Values
Temperature
Channel
Register Address,
MSBs
Register Address,
LSBs
Local 0x00 N/A
Remote 1 0x01 0x10 (2 MSBs)
Remote 2 0x30 0x33 (2 MSBs)
Set Bit 3 of the Configuration 1 register to 1, to read the Remote
2 temperature values from the following register addresses:
• Remote 2, MSBs = 0x01
• Remote 2, LSBs = 0x10
The above is true only when Bit 3 of the Configuration 1
register is set. To read the Remote 1 temperatures, switch this
bit back to 0.
Only the two MSBs in the remote temperature low byte are
used. This gives the remote temperature measurement a
resolution of 0.25°C. Table 7 shows the data format for the
remote temperature low byte.
Table 7. Extended Temperature Resolution
(Remote Temperature Low Byte)
Extended Resolution Remote Temperature Low Byte
0.00°C 0 000 0000
0.25°C 0 100 0000
0.50°C 1 000 0000
0.75°C 1 100 0000
When reading the full remote temperature value, both the high
and low byte, the two registers should be read LSB first and then
MSB. Reading the LSB causes the MSB to be locked until it is
read. This guarantees that the two values are read as a result of
the same temperature measurement.
TEMPERATURE MEASUREMENT RANGE
The temperature measurement range for both local and remote
measurements is, by default, 0°C to +127°C. However, the
ADT7482 can be operated using an extended temperature
range. It can measure the full temperature range of a remote
thermal diode, from −55°C to +150°C. Switch between these
two temperature ranges by setting or clearing Bit 2 in the
Configuration 1 register. A valid result is available in the next
measurement cycle after changing the temperature range.
In extended temperature mode, the upper and lower
temperature measured by the ADT7482 is limited by the remote
diode selection. The temperature registers themselves can have
values from −64°C to +191°C. However, most temperature-
sensing diodes have a maximum temperature range of
−55°C to +150°C.
Note that while both local and remote temperature measurements
can be made while the part is in extended temperature mode, the
ADT7482 itself should not be exposed to temperatures greater than
those specified in the Absolute Maximum Ratings section.
Further, the device is only guaranteed to operate as specified at
ambient temperatures from −40°C to +120°C.
ADT7482
Rev. 0 | Page 11 of 24
TEMPERATURE DATA FORMAT
The ADT7482 has two temperature data formats. When the temperature measurement range is from 0°C to 127°C (default), the
temperature data format for both local and remote temperature results is binary.
When the measurement range is in extended mode, an offset binary data format is used for both local and remote results. Temperature
values in the offset binary data format are offset by +64. Examples of temperatures in both data formats are shown in Table 8.
Switching between measurement ranges can be done at any time. Switching the range also switches the data format. The next temperature
result following the switching is reported back to the register in the new format. However, the contents of the limit registers do not
change. Ensure that when the data format changes, the limit registers are reprogrammed as necessary. For more information, refer to the
Limit Registers section.
Table 8. Temperature Data Format (Local and Remote Temperature High Byte)
Temperature Binary Offset Binary1
–55°C 0 000 00002 0 000 1001
0°C 0 000 0000 0 100 0000
+1°C 0 000 0001 0 100 0001
+10°C 0 000 1010 0 100 1010
+25°C 0 001 1001 0 101 1001
+50°C 0 011 0010 0 111 0010
+75°C 0 100 1011 1 000 1011
+100°C 0 110 0100 1 010 0100
+125°C 0 111 1101 1 011 1101
+127°C 0 111 1111 1 011 1111
+150°C 0 111 11113 1 101 0110
1 Offset binary scale temperature values are offset by +64.
2 Binary scale temperature measurement returns 0 for temperature < 0°C.
3 Binary scale temperature measurement returns 127 for temperature > 127°C.
ADT7482
Rev. 0 | Page 12 of 24
REGISTERS
The registers in the ADT7482 are 8-bits wide. These registers
are used to store the results of remote and local temperature
measurements and high and low temperature limits and to
configure and control the device. A description of these registers
follows.
ADDRESS POINTER REGISTER
The address pointer register itself does not have, or require, an
address, as the first byte of every write operation is automatically
written to this register. The data in this first byte always contains
the address of another register on the ADT7482, which is stored
in the address pointer register. It is to this register address that
the second byte of a write operation is written to or to which a
subsequent read operation is performed.
The power-on default value of the address pointer register is
0x00. Therefore, if a read operation is performed immediately
after power-on, without first writing to the address pointer, the
value of the local temperature is returned, since its register
address is 0x00.
CONFIGURATION REGISTERS
There are two configuration registers used to control the
operation of the ADT7482. Configuration 1 register is at
Address 0x03 for reads and Address 0x09 for writes. See Table 9
for details regarding the operation of this register. Configuration 2
Register is at Address 0x24 for both reads and writes. Setting Bit
7 of this register locks all lockable registers. The affected
registers can only be modified if the ADT7482 is powered down
and powered up again. See Tabl e 16 for a list of the registers
affected by the lock bit.
TEMPERATURE VALUE REGISTERS
The ADT7482 has five registers to store the results of local and
remote temperature measurements. These registers can only be
written to by the ADC and can be read over the SMBus.
• The local temperature value register is at Address 0x00.
• The Remote 1 temperature value high byte register is at
Address 0x01; the Remote 1 low byte register is at
Address 0x10.
• The Remote 2 temperature value high byte register is at
Address 0x30; the Remote 2 low byte register is at
Address 0x33.
• The Remote 2 temperature values can be read from
Addresses 0x01 for the high byte and Address 0x10 for the
low byte if Bit 3 of Configuration Register 1 is set to 1.
• To read the Remote 1 temperature values, set Bit 3 of
Configuration Register 1 to 0.
• The power-on default for all five registers is 0x00.
Table 9. Configuration 1 Register (Read Address = 0x03, Write Address = 0x09)
Bit Mnemonic Description
7 Mask Setting this bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT enabled. This applies only if Pin 8 is
configured as ALERT, otherwise it has no effect.
6 Mon/STBY
Setting this bit to 1 places the ADT7482 in standby mode (that is, it suspends all temperature measurements (ADC). The
SMBus remains active and values can be written to and read from the registers. THERM and ALERT are also active in
standby mode. Changes made to the limit registers in standby mode that effect the THERM or ALERT outputs cause
these signals to be updated. Default = 0 = temperature monitoring enabled.
5 AL/TH This bit selects the function of Pin 8. Default = 0 = ALERT. Setting this bit to 1 configures Pin 8 as THERM2 pin.
4 Reserved Reserved for future use.
3 Remote 1
/Remote 2
Setting this bit to 1 enables Remote 2 values to be read from the Remote 1 registers. Default = 0 = Remote 1
temperature values and limits are read from these registers.
2 Temp
Range
Setting this bit to 1 enables the extended temperature measurement range (−50°C to +150°C).
Default = 0 = (0°C to +127°C).
1 Mask R1
Setting this bit to 1 masks ALERTs due to the Remote 1 temperature exceeding a programmed limit. Default = 0.
0 Mask R2
Setting this bit to 1 masks ALERTs due to the Remote 2 temperature exceeding a programmed limit. Default = 0.
Table 10. Configuration 2 Register (Read/Write Address = 0x24)
Bit Mnemonic Description
7 Lock Bit
Setting this bit to 1 locks all lockable registers to their current values. This prevents settings from being tampered with
until the device is powered down. Default = 0.
<6:0> Res Reserved for future use.
ADT7482
Rev. 0 | Page 13 of 24
CONVERSION RATE REGISTER
The conversion rate register is at Address 0x04 for reads and
Address 0x0A for writes. The four LSBs of this register are used
to program the conversion times from 15.5 ms (Code 0x0A) to
16 seconds (Code 0x00). To program the ADT7482 to perform
continuous measurements, set the conversion rate register to
0x0B. For example, a conversion rate of 8 conversions/second
means that, beginning at 125 ms intervals, the device performs
a conversion on the local and the remote temperature channels.
The four MSBs of this register are reserved and should not be
written to.
This register can be written to and read back over the SMBus.
The default value of this register is 0x07, giving a rate of 8
conversions per second. Use of slower conversion times greatly
reduces the device power consumption.
LIMIT REGISTERS
The ADT7482 has three limits for each temperature channel:
high, low, and THERM temperature limits for local, Remote 1,
and Remote 2 temperature measurements. The remote
temperature high and low limits span two registers each, to
contain an upper and lower byte for each limit. There is also a
THERM hysteresis register. All limit registers can be written to
and read back over the SMBus. See Table 16 for limit register
addresses and power-on default values.
When Pin 8 is configured as an ALERT output, the high limit
registers perform a > comparison while the low limit registers
perform a ≤ comparison. For example, if the high limit register
is programmed with 80°C, then measuring 81°C results in an
out-of-limit condition, setting a flag in the status register. If the
low limit register is programmed with 0°C, measuring 0°C or
lower results in an out-of-limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 8 is configured as THERM2, exceeding
either the local or remote high limit asserts THERM2 low. A
default hysteresis value of 10°C is provided that applies to both
THERM channels. This hysteresis value can be reprogrammed.
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. If the temperature measurement uses the default binary
scale, then the temperature limits also use the binary scale. If
the temperature measurement scale is switched, however, the
temperature limits do not switch automatically.
The limit registers must be reprogrammed to the desired value
in the correct data format. For example, if the remote low limit
is set at 10°C and the default binary scale is used, the limit
register value should be 0000 1010b. If the scale is switched to
offset binary, the value in the low temperature limit register
should be reprogrammed to be 0100 1010b.
Table 11. Conversion Rate Register (Read Address = 0x04, Write Address = 0x0A)
Bit Mnemonic Function
7 Reserved Reserved for future use. Do not write to this bit.
6 Reserved Reserved for future use . Do not write to this bit.
5 Reserved Reserved for future use . Do not write to this bit.
4 Reserved Reserved for future use. Do not write to this bit.
<3:0> Conversion rates These bits set how often the ADT7482 measures each temperature channel.
Conversions/sec Time (seconds)
0000 = 0.0625 16
0001 = 0.125 8
0010 = 0.25 4
0011 = 0.5 2
0100 = 1 1
0101 = 2 500 m
0110 = 4 250 m
0111 = 8 = default 125 m
1000 = 16 62.5 m
1001 = 32 31.25 m
1010 = 64 15.5 m
ADT7482
Rev. 0 | Page 14 of 24
STATUS REGISTERS
The status registers are read-only registers at Addresses 0x02
(Status Register 1) and Address 0x23 (Status Register 2). They
contain status information for the ADT7482.
Table 12. Status Register 1 Bit Assignments
Bit Name Function ALERT
7 BUSY 1 when ADC converting No
6 LHIGH1 1 when local high temperature limit
tripped
Yes
5 LLOW1 1 when local low temperature limit
tripped
Yes
4 R1HIGH1 1 when Remote 1 high temperature
limit tripped
Yes
3 R1LOW1 1 when Remote 1 low temperature
limit tripped
Yes
2 D1 OPEN1 1 when Remote 1 sensor open
circuit
Yes
1 R1THRM1 1 when Remote1 THERM limit
tripped
No
0 LTHRM1 1 when local THERM limit tripped No
These flags stay high until the status register is read, or they are reset by PO R
1 These flags stay high until the status register is read, or they are reset by POR.
Table 13. Status Register 2 Bit Assignments
Bit Name Function ALERT
7 Res Reserved for future use No
6 Res Reserved for future use No
5 Res Reserved for future use No
4 R2HIGH1 1 when Remote 2 high temperature
limit tripped Yes
3 R2LOW1 1 when Remote 2 low temperature
limit tripped Yes
2 D2 OPEN1 1 when Remote 2 sensor open
circuit Yes
1 R2THRM1 1 when Remote2 THERM limit
tripped No
0 ALERT 1 when ALERT condition exists. No
1 These flags stay high until the status register is read, or they are reset by POR.
The eight flags that can generate an ALERT are NOR’d together.
When any flags are high, the ALERT interrupt latch is set and
the ALERT output goes low (provided they are not
masked out).
Reading the Status 1 register clears the 5 flags, (Bit 6 through
Bit 2) in Status Register 1, provided the error conditions that
caused the flags to be set have gone away. Reading the Status 2
Register clears the three flags, (Bit 4 through Bit 2) in Status
Register 2, provided the error conditions that caused the flags to
be set have gone away. A flag bit can only be reset if the
corresponding value register contains an in-limit measurement
or if the sensor is good.
The ALERT interrupt latch is not reset by reading the status
register. It is reset when the ALERT output has been serviced by
the master reading the device address, provided the error
condition has gone away and the status register flag bits have
been reset.
When Flag 1 and/or Flag 0 of Status Register 1 or Flag 1 of
Status Register 2 are set, the THERM output goes low to
indicate that the temperature measurements are outside the
programmed limits. The THERM output does not need to be
reset, unlike the ALERT output. Once the measurements are
within the limits, the corresponding status register bits are reset
automatically, and the THERM output goes high. To add
hysteresis, program Register 0x21. The THERM output is reset
only when the temperature falls below the THERM limit minus
hysteresis.
When Pin 8 is configured as THERM2, only the high
temperature limits are relevant. If Flag 6 or Flag 4 of Status
Register 1 or Flag 4 of Status Register 2 are set, the THERM2
output goes low to indicate that the temperature measurements
are outside the programmed limits. Flag 5 and Flag 3 of Status
Register 1 and Flag 3 of Status Register 2 have no effect on
THERM2. The behavior of THERM2 is otherwise the same
as THERM.
Bit 0 of the Status Register 2 is set whenever the ADT7482
ALERT output is asserted low. Read Status Register 2 to
determine if the ADT7482 is responsible for the ALERT. This
bit is reset when the ALERT output is reset. If the ALERT
output is masked, then this bit is not set.
OFFSET REGISTER
Offset errors can be introduced into the remote temperature
measurement by clock noise or by the thermal diode being
located away from the hot spot. To achieve the specified
accuracy on this channel, these offsets must be removed.
The offset values are stored as 10-bit, twos complement values.
• The Remote 1 Offset MSBs are stored in Register 0x11 and
the LSBs are stored in Register 0x12 (low byte, left
justified). The Remote 2 Offset MSBs are stored in Register
0x34 and the LSBs are stored in Register0x35 (low byte, left
justified).
• The Remote 2 Offset can be written to or read from the
Remote 1 Offset Registers if Bit 3 of the Configuration 1
register is set to 1. This bit should be set to 0 (default) to
read the Remote 1 offset values.
Only the upper 2 bits of the LSB registers are used. The MSB of
MSB offset registers is the sign bit. The minimum offset that
can be programmed is −128°C, and the maximum is +127.75°C.
The value in the offset register is added or subtracted to the
measured value of the remote temperature.
The offset register powers up with a default value of 0°C and has
no effect unless a different value is written to it.
ADT7482
Rev. 0 | Page 15 of 24
Table 14. Sample Offset Register Codes
Offset Value 0x11/0×34 0x12/0x35
−128°C 1000 0000 00 00 0000
−4°C 1111 1100 00 00 0000
−1°C 1111 1111 00 000000
−0.25°C 1111 1111 11 00 0000
0°C 0000 0000 00 00 0000
+0.25°C 0000 0000 01 00 0000
+1°C 0000 0001 00 00 0000
+4°C 0000 0100 00 00 0000
+127.75°C 0111 1111 11 00 0000
ONE-SHOT REGISTER
The one-shot register initiates a conversion and comparison
cycle when the ADT7482 is in standby mode, after which the
device returns to standby. Writing to the one-shot register
address (0×0F) causes the ADT7482 to perform a conversion
and comparison on both the local and the remote temperature
channels. This is not a data register as such, and it is the write
operation to Address 0×0F that causes the one-shot conversion.
The data written to this address is irrelevant and is not stored.
CONSECUTIVE ALERT REGISTER
The value written to this register determines how many out-of-
limit measurements must occur before an ALERT is generated.
The default value is that one out-of-limit measurement generates
an ALERT. The maximum value that can be chosen is 4. This
register allows some filtering of the output. This is particularly
useful at the fastest three conversion rates, where no averaging
takes place. This register address is 0x22. For more information,
refer to Table 15.
Table 15. Consecutive ALERT Register Bit
Register Value1
Amount of Out-of-Limit
Measurements Required
yza× 000x 1
yza× 001x 2
yza× 011x 3
yza× 111x 4
1 Notes:
y = SMBus SCL timeout bit. Default = 0. See the Serial Bus Interface section
for more information.
z = SMBus SDA timeout bit. Default = 0. See the Serial Bus Interface section
for more information.
a = Mask Internal ALERTs.
x = Don’t care bit.
ADT7482
Rev. 0 | Page 16 of 24
Table 16. List of Registers
Read Address
(Hex)
Write Address
(Hex) Name Power-On Default Comment Lock
N/A N/A Address Pointer Undefined No
00 N/A Local Temperature Value 0000 0000 (0x00) No
01 N/A Remote 1 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf. Reg. = 0 No
01 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf. Reg. = 1 No
02 N/A Status Register 1 Undefined No
03 09 Configuration Register 1 0000 0000 (0x00) Yes
04 0A Conversion Rate 0000 0111 (0x07) Yes
05 0B Local Temperature High Limit 0101 0101 (0x55) (85°C) Yes
06 0C Local Temperature Low Limit 0000 0000 (0x00) (0°C) Yes
07 0D Remote 1 Temperature High Limit High Byte 0101 0101 (0x55) (85°C) Bit 3 Conf. Reg. = 0 Yes
07 0D Remote 2 Temperature High Limit High Byte 0101 0101 (0x55) (85°C) Bit 3 Conf. Reg. = 1 Yes
08 0E Remote 1 Temperature Low Limit High Byte 0000 0000 (0x00) (0°C) Bit 3 Conf. Reg. = 0 Yes
08 0E Remote 2 Temperature Low Limit High Byte 0000 0000 (0x00) (0°C) Bit 3 Conf. Reg. = 1 Yes
N/A 0F1 One Shot N/A
10 N/A Remote 1 Temperature Value Low Byte 0000 0000 Bit 3 Conf. Reg. = 0 No
10 N/A Remote 2 Temperature Value Low Byte 0000 0000 Bit 3 Conf. Reg. = 1 No
11 11 Remote 1 Temperature Offset High Byte 0000 0000 Bit 3 Conf. Reg. = 0 Yes
11 11 Remote 2 Temperature Offset High Byte 0000 0000 Bit 3 Conf. Reg. = 1 Yes
12 12 Remote 1 Temperature Offset Low Byte 0000 0000 Bit 3 Conf. Reg. = 0 Yes
12 12 Remote 2 Temperature Offset Low Byte 0000 0000 Bit 3 Conf. Reg. = 1 Yes
13 13 Remote 1 Temperature High Limit Low Byte 0000 0000 Bit 3 Conf. Reg. = 0 Yes
13 13 Remote 2 Temperature High Limit Low Byte 0000 0000 Bit 3 Conf. Reg. = 1 Yes
14 14 Remote 1 Temperature Low Limit Low Byte 0000 0000 Bit 3 Conf. Reg. = 0 Yes
14 14 Remote 2 Temperature Low Limit Low Byte 0000 0000 Bit 3 Conf. Reg. = 1 Yes
19 19 Remote 1 THERM Limit 0101 0101 (0x55) (85°C) Bit 3 Conf. Reg. = 0 Yes
19 19 Remote 2 THERM Limit 0101 0101 (0x55) (85°C) Bit 3 Conf. Reg. = 1 Yes
20 20 Local THERM Limit 0101 0101 (0x55) (85°C) Yes
21 21 THERM Hysteresis 0000 1010 (0x0A) (10°C) Yes
22 22 Consecutive ALERT 0000 0001 (0x01) Yes
23 N/A Status Register 2 0000 0000 (0x00) No
24 24 Configuration 2 Register 0000 0000 (0x00) Yes
30 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) No
31 31 Remote 2 Temperature High Limit High Byte 0101 0101 (0x55) (85°C) Yes
32 32 Remote 2 Temperature Low Limit High Byte 0000 0000 (0x00) (0°C) Yes
33 N/A Remote 2 Temperature Value Low Byte 0000 0000 (0x00) No
34 34 Remote 2 Temperature Offset High Byte 0000 0000 (0x00) Yes
35 35 Remote 2 Temperature Offset Low Byte 0000 0000 (0x00) Yes
36 36 Remote 2 Temperature High Limit Low Byte 0000 0000 (0x00) (0°C) Yes
37 37 Remote 2 Temperature Low Limit Low Byte 0000 0000 (0x00) (0°C) Yes
39 39 Remote 2 THERM Limit 0101 0101 (0x55) (85°C) Yes
FE N/A Manufacturer ID 0100 0001 (0x41) N/A
FF N/A Die Revision Code 0110 0101 (0x65) N/A
1 Writing to address 0F causes the ADT7482 to perform a single measurement. It is not a data register as such and it does not matter what data is written to it.
ADT7482
Rev. 0 | Page 17 of 24
SERIAL BUS INTERFACE
Control of the ADT7482 is achieved via the serial bus. The
ADT7482 is connected to this bus as a slave device, under the
control of a master device.
The ADT7482 has an SMBus timeout feature. When this is
enabled, the SMBus times out after typically 25 ms of no
activity. However, this feature is not enabled by default. Set Bit 7
(SCL Timeout Bit) of the consecutive alert register (Address =
0x22) to enable the SCL Timeout. Set Bit 6 (SDA Timeout Bit)
of the consecutive alert register (Address = 0x22) to enable the
SDA Timeout.
Consult the SMBus 1.1 specification for more information.
ADDRESSING THE DEVICE
In general, every SMBus device has a 7-bit device address,
except for some devices that have extended, 10-bit addresses.
When the master device sends a device address over the bus,
the slave device with that address responds. The ADT7482 is
available with one device address, 0x4C (1001 100b). The
address mentioned in this data sheet is a 7-bit address. The R/W
bit needs to be added to arrive at an 8-bit address.
Serial Bus Protocol Operation
The master initiates data transfer by establishing a start
condition, defined as a high-to-low transition on the serial data
line SDATA while the serial clock line SCLK remains high. This
indicates that an address/data stream follows.
All slave peripherals connected to the serial bus respond to the
start condition and shift in the next eight bits, consisting of a
7-bit address (MSB first) plus an R/W bit, which determines the
direction of the data transfer, that is, whether data is to be
written to or read from the slave device. The peripheral whose
address corresponds to the transmitted address responds by
pulling the data line low during the low period before the ninth
clock pulse, known as the acknowledge bit. All other devices on
the bus now remain idle while the selected device waits for data
to be read from or written to it. If the R/W bit is a 0, the master
writes to the slave device. If the R/W bit is a 1, the master reads
from the slave device.
Data is sent over the serial bus in a sequence of nine clock
pulses, eight bits of data followed by an acknowledge bit from
the slave device. Transitions on the data line must occur during
the low period of the clock signal and remain stable during the
high period, since a low-to-high transition when the clock is
high can be interpreted as a stop signal. The number of data
bytes that can be transmitted over the serial bus in a single read
or write operation is limited only by what the master and slave
devices can handle.
When all data bytes have been read or written, stop conditions
are established. In write mode, the master pulls the data line
high during the tenth clock pulse to assert a stop condition. In
read mode, the master device overrides the acknowledge bit by
pulling the data line high during the low period before the
ninth clock pulse. This is known as no acknowledge. The
master then takes the data line low during the low period before
the tenth clock pulse, then high during the tenth clock pulse to
assert a stop condition.
Any number of bytes of data can be transferred over the serial
bus in one operation, but it is not possible to mix read and write
in one operation because the type of operation is determined at
the beginning and cannot subsequently be changed without
starting a new operation. In the case of the ADT7482, write
operations contain either one or two bytes, while read
operations contain one byte.
To write data to one of the device data registers or to read data
from it, the address pointer register must be set so that the
correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device, the
write operation contains a second data byte that is written to the
register selected by the address pointer register.
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 register 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.
ADT7482
Rev. 0 | Page 18 of 24
1
191
00 1 1 0 0R/W D7
D6 D5 D4 D3 D2 D1 D0
SCL
SD
9
A
START BY
MASTER
ACK. BY
ADT7482
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
FRAME 3
DATA BYTE
SCL (CONTINUED)
SDA (CONTINUED)
ACK. BY
ADT7482
ACK. BY
ADT7482
STOP BY
MASTER
91
D7 D6 D5 D4 D3 D2 D1 D0
06150-017
Figure 17. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1
191
00 1 1 00R/W D7
D6 D5 D4 D3 D2 D1 D0
SCL
9
S
D
A
START BY
MASTER
ACK. BY
ADT7482
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
ACK. BY
ADT7482
STOP BY
MASTER
06150-018
Figure 18. Writing to the Address Pointer Register Only
SCL
SDA
START BY
MASTER
ACK. BY
ADT7482
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
DATA BYTE FROM ADT7482
NO ACK.
BY MASTER
STOP BY
MASTER
191 9
1 0 0 1 1 0 0 R/W D7 D6 D5 D4 D3 D2 D1 D0
06150-019
Figure 19. Reading from a Previously Selected Register
Reading Data from a Register
When reading data from a register there are two possibilities:
• If the ADT7482 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
ADT7482 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 18.
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 19.
• 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 and the bus transaction shown in Figure 18 can be
omitted.
When reading data from a register, it is important to note the
following points:
1. It is possible to read a data byte from a data register
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. This is
because the first data byte of a write is always written to the
address pointer register.
2. Remember that some of the ADT7482 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 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.
ADT7482
Rev. 0 | Page 19 of 24
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
RDSTART ACK DEVICE
ADDRESS
NO
ACK STOP
MASTER
RECEIVES
SMBALERT
0
6150-020
Figure 20. Use of 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 should 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 has a low ALERT, the one with the
lowest device address has priority, in accordance with
normal SMBus arbitration.
5. 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
register (Address 0x0F), after which the device returns to
standby. It does not matter what is written to the one-shot
register as 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 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
ADT7482
Rev. 0 | Page 20 of 24
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 reset
automatically 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 resets 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 can be
reprogrammed to any value after power-up.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The 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, where the temperature hovers around the THERM limit,
and the fan is constantly being switched on and off.
Table 17. THERM Hysteresis
THERM Hysteresis Binary Representation
0°C 0 000 0000
1°C 0 000 0001
10°C 0 000 1010
If the ADT7482 is in the default temperature range (0°C to 127°C),
then THERM hysteresis must be less than the THERM limit.
Figure 21 shows how the THERM and ALERT outputs operate.
If desired, use the ALERT output as a SMBALERT to signal to
the host via the SMBus that the temperature has risen. Use the
THERM output to turn on a fan to cool the system, if the
temperature continues to increase. This method ensures that
there is a fail-safe mechanism to cool the system, without the
need for host intervention.
1
32
4
THERM LIMIT
HIGH TEMP LIMIT
THERM LIMIT-HYSTERESIS
RESET BY MASTER
ALERT
THERM
100°C
T
EMPERATURE
90°C
80°C
70°C
60°C
50°C
40°C
0
6150-021
Figure 21. Operation of the ALERT and THERM Interrupts
1. If the measured temperature exceeds the high temperature
limit, the ALERT output asserts low.
2. If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low. This can be
used to throttle the CPU clock or switch on a fan.
3. The THERM output de-asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 21, the default hysteresis value of 10°C is shown.
4. The ALERT output de-asserts only when the temperature
has fallen below the high temperature limit, and the master
has read the device address and cleared the status register.
Pin 8 on the ADT7482 can be configured as either an ALERT
output or as an additional THERM output. THERM2 asserts
low when the temperature exceeds the programmed local
and/or remote high temperature limits. It is reset in the same
manner as THERM, and it is not maskable. The programmed
hysteresis value applies to THERM2 also.
Figure 22 shows how THERM and THERM2 might operate
together to implement two methods of cooling the system. In
this example, the THERM2 limits are set lower than the
THERM limits. The THERM2 output could be used to turn on
a fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output could provide additional
cooling by throttling the CPU.
ADT7482
Rev. 0 | Page 21 of 24
THERM2 LIMIT
THERM LIMIT
THERM2
1
3
2
4
TEMPERATURE
THERM
90°C
80°C
70°C
60°C
50°C
40°C
30°C
06150-022
Figure 22. Operation of the THERM and THERM2 Interrupts
1. When the THERM2 limit is exceeded, the THERM2 signal
asserts low.
2. If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
3. The THERM output de-asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 22, there is no hysteresis value shown.
4. As the system cools further, and the temperature falls
below the THERM2 limit, the THERM2 signal resets.
Again, no hysteresis value is shown for THERM2.
The temperature measurement could be either the local or the
remote temperature measurement.
ADT7482
Rev. 0 | Page 22 of 24
APPLICATIONS INFORMATION
NOISE FILTERING
For temperature sensors operating in noisy environments, the
previous practice was to place a capacitor across the D+ pin and
the D− pins to help combat the effects of noise. However, large
capacitances affect the accuracy of the temperature measurement,
leading to a recommended maximum capacitor value of 1000 pF.
While this capacitor reduces the noise, it does not eliminate it,
making it difficult to use the sensor in a very noisy environment.
The ADT7482 has a major advantage over other devices for
eliminating the effects of noise on the external sensor. The
series resistance cancellation feature allows a filter to be
constructed between the external temperature sensor and the
part. The effect of any filter resistance seen in series with the remote
sensor is automatically cancelled from the temperature result.
The construction of a filter allows the ADT7482 and the remote
temperature sensor to operate in noisy environments. Figure 23
shows a low-pass R-C-R filter, with the following values:
R = 100 Ω and C = 1 nF
This filtering reduces both common-mode noise and
differential noise.
04110-0-009
D+
1nF
100Ω
REMOTE
T
EMPERATURE
SENSOR
D–
100Ω
Figure 23. Filter Between Remote Sensor and ADT7482
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7482 is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN
transistors connected as a diode (base shorted to collector). If
an NPN transistor is used, the collector and base are connected
to D+ and the emitter to D−. If a PNP transistor is used, the
collector and base are connected to D− and the emitter to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
• The ideality factor, nf, of the transistor is a measure of the
deviation of the thermal diode from ideal behavior. The
ADT7482 is trimmed for an nf value of 1.008. The
following equation can be used to calculate the error
introduced at a temperature T (°C), when using a transistor
whose nf does not equal 1.008. Consult the processor data
sheet for the nf values.
(
)
(
)
TKelvinnT f+×=Δ 15.273008.1/008.1–
To factor this in, write the ΔT value to the offset register. It
is then automatically added to or subtracted from the
temperature measurement by the ADT7482.
• Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7482, IHIGH, is 220 μA and the low level current,
ILOW, is 13.5 μA. If the ADT7482 current levels do not
match the current levels specified by the CPU manufacturer,
it may be necessary to remove an offset. The CPU data
sheet advises whether this offset needs to be removed and
how to calculate it. This offset can be programmed to the
offset register. It is important to note that if more than one
offset must be considered, the algebraic sum of these
offsets must be programmed to the offset register.
If a discrete transistor is being used with the ADT7482, the best
accuracy is obtained by choosing devices according to the
following criteria:
• Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
• Base-emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature.
• Base resistance less than 100 Ω.
• Small variation in hFE (such as 50 to 150) that indicates
tight control of VBE characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
THERMAL INERTIA AND SELF-HEATING
Accuracy depends on the temperature of the remote sensing
diode and/or the local temperature sensor being at the same
temperature as that being measured. A number of factors can
affect this. Ideally, the sensor should be in good thermal contact
with the part of the system being measured. If it is not, the
thermal inertia caused by the sensor’s mass causes a lag in the
response of the sensor to a temperature change. In the case of
the remote sensor, this should not be a problem, since it is
either a substrate transistor in the processor or a small package
device, such as SOT-23, placed in close proximity to it.
The on-chip sensor, however, is often remote from the
processor and only monitors the general ambient temperature
around the package. In practice, the ADT7482 package is in
electrical, and hence thermal, contact with a PCB and may also
be in a forced airflow. How accurately the temperature of the
board and/or the forced airflow reflects the temperature to be
measured also affects the accuracy. Self-heating due to the
power dissipated in the ADT7482 or the remote sensor causes
the chip temperature of the device or remote sensor to rise
ADT7482
Rev. 0 | Page 23 of 24
above ambient. However, the current forced through the remote
sensor is so small that self-heating is negligible. In the case of
the ADT7482, the worst-case condition occurs when the device
is converting at 64 conversions per second while sinking the
maximum current of 1 mA at the ALERT and THERM output.
In this case, the total power dissipation in the device is about
4.5 mW. The thermal resistance, θJA, of the MSOP-10 package is
about 142°C/W.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and the
ADT7482 is measuring very small voltages from the remote
sensor, so care must be taken to minimize noise induced at the
sensor inputs. Take the following precautions:
• Place the ADT7482 as close as possible to the remote
sensing diode. Provided that the worst noise sources, that
is, clock generators, data/address buses, and CRTs, are
avoided, this distance can be 4 inches to 8 inches.
• Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. To minimize
inductance and reduce noise pick-up, a 5 mil track width
and spacing is recommended. Provide a ground plane
under the tracks,if possible.
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
GND
D+
D–
GND
06150-024
Figure 24. Typical Arrangement of Signal Tracks
• Try to minimize the number of copper/solder joints that
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+ and
D− path and at the same temperature.
• Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
Unless there are two thermocouples with a big temperature
differential between them, thermocouple voltages should
be much less than 200 mV.
• Place a 0.1 μF bypass capacitor close to the VDD pin. In
extremely noisy environments, an input filter capacitor can
be placed across D+ and D−, close to the ADT7482. This
capacitance can effect the temperature measurement, so
care must be taken to ensure that any capacitance seen at
D+ and D− is a maximum of 1000 pF. This maximum
value includes the filter capacitance, plus any cable or stray
capacitance between the pins and the sensor diode.
• If the distance to the remote sensor is more than 8 inches,
the use of twisted pair cable is recommended. A total of 6
feet to 12 feet is needed.
• For long distances (up to 100 feet), use shielded twisted
pair, such as Belden No. 8451 microphone cable. Connect
the twisted pair to D+ and D− and the shield to GND close
to the ADT7482. Leave the remote end of the shield
unconnected to avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable or filter capacitance can affect the
measurement. When using long cables, the filter capacitance
can be reduced or removed.
APPLICATION CIRCUIT
Figure 25 shows a typical application circuit for the ADT7482,
using discrete sensor transistors. The pull-ups on SCLK,
SDATA, and ALERT are required only if they are not already
provided elsewhere in the system.
The SCLK pin and the SDATA pin of the ADT7482 can be
interfaced directly to the SMBus of an I/O controller, such as
the Intel® 820 chipset.
5V OR 12V
SMBUS
CONTROLLER
FAN CONTROL
CIRCUIT
2N3904/06
OR
CPU THERMAL
DIODE
D1+
D1–
D2+
D2–
V
DD
SCLK
SDATA
ALERT
THERM
GND
ADT7482
0.1µF
V
DD
TYP 10kΩ
FAN ENABLE
3V TO 3.6V
TYP 10kΩ
06150-025
Figure 25. Typical Application Circuit
ADT7482
Rev. 0 | Page 24 of 24
OUTLINE DIMENSIONS
0.23
0.08
0.80
0.60
0.40
8°
0°
0.15
0.00
0.27
0.17
0.95
0.85
0.75
SEATING
PLANE
1.10 MAX
10 6
5
1
0.50 BSC
3.00 BSC
3.00 BSC
4.90 BSC
PIN 1
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 26. 10-Lead Mini Small Outline Package (MSOP)
RM-10
Dimensions shown in millimeters
ORDERING GUIDE
Model Temperature Range Package Description Package Option Branding SMBus Address
ADT7482ARMZ1−40°C to +125°C 10-Lead MSOP RM-10 T0A 4C
ADT7482ARMZ-REEL −40°C to +125°C 10-Lead MSOP RM-10 T0A 4C
ADT7482ARMZ-REEL7 −40°C to +125°C 10-Lead MSOP RM-10 T0A 4C
1 Z = Pb-free part.
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D06150-0-6/06(0)
