ON ADT7481 Dual channel temperature sensor and overtemperature alarm Datasheet

ADT7481
Dual Channel Temperature
Sensor and Overtemperature
Alarm
The ADT7481 is a 3-channel digital thermometer and under/ over
temperature alarm, intended for use in PCs and thermal management
systems. It can measure its own ambient temperature or the
temperature of two remote thermal diodes. These thermal diodes can
be located in a CPU or GPU, or they can be discrete diode connected
transistors. The ambient temperature, or the temperature of the remote
thermal diode, can be accurately measured to ±1°C. The temperature
measurement range defaults to 0°C to +127°C, compatible with
ADM1032, but can be switched to a wider measurement range from
−64°C to +191°C.
The ADT7481 communicates over a 2-wire serial interface
compatible with System Management Bus (SMBus) standards. The
SMBus address of the ADT7481 is 0x4C. An ADT7481−1 with an
SMBus address of 0x4B is also available.
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, for example, on/off control of a cooling fan. The
ALERT output can be reconfigured as a second THERM output if
required.
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MSOP−10
CASE 846AC
PIN ASSIGNMENT
VDD
1
D1+
2
10 SCLK
ADT7481
9
SDATA
D1−
3
8
ALERT/THERM2
THERM
4
7
D2+
GND
5
6
D2−
MARKING DIAGRAM
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
Extended, Switchable Temperature Measurement Range
0°C to 127°C (Default) or −64°C to +191°C
2-wire SMBus Serial Interface with SMBus ALERT Support
Programmable Over/Undertemperature Limits
Offset Registers for System Calibration
Up to 2 Overtemperature Fail-Safe THERM Outputs
Small 10-lead MSOP Package
240 mA Operating Current, 5 mA Standby Current
These Devices are Pb-Free, Halogen Free and are RoHS Compliant
Applications
•
•
•
•
•
•
•
10
T0x
AYWG
G
1
T0x
A
Y
W
G
= Refer to Ordering Info Table
= Assembly Location
= Year
= Work Week
= Pb-Free Package
(Note: Microdot may be in either location)
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 19 of this data sheet.
Desktop and Notebook Computers
Industrial Controllers
Smart Batteries
Automotive
Embedded Systems
Burn-In Applications
Instrumentation
© Semiconductor Components Industries, LLC, 2014
September, 2014 − Rev. 8
1
Publication Order Number:
ADT7481/D
ADT7481
ADDRESS POINTER
REGISTER
ONE-SHOT
REGISTER
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
THERM LIMIT REGISTER
2
D1−
3
D2+
7
D2−
6
ANALOG
MUX
11-BIT A-TO-D
CONVERTER
BUSY
RUN/STANDBY
LOCAL TEMPERATURE
LOW LIMIT REGISTER
LIMIT COMPARATOR
D1+
CONVERSION RATE
REGISTER
DIGITAL MUX
ON-CHIP
TEMPERATURE
SENSOR
REMOTE 1 AND 2 TEMP
VALUE REGISTER
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
REMOTE 1 & 2 TEMP.
THERM LIMIT REG.
REMOTE 1 & 2 TEMP.
LOW LIMIT REGISTERS
REMOTE 1 AND 2 TEMP
OFFSET REGISTER
REMOTE 1 & 2 TEMP.
HIGH LIMIT REGISTERS
CONFIGURATION
REGISTER
EXTERNAL DIODE OPEN-CIRCUIT
INTERRUPT
MASKING
STATUS REGISTERS
ADT7481
SMBUS INTERFACE
1
5
9
10
4
VDD
GND
SDATA
SCLK
THERM
8
ALERT/THERM2
Figure 1. Functional Block Diagram
Table 1. ABSOLUTE MAXIMUM RATINGS
Parameter
Positive Supply Voltage (VDD) to GND
D+
D− to GND
Rating
Unit
−0.3 to +3.6
V
−0.3 to VDD + 0.3
V
−0.3 to +0.6
V
SCLK, SDATA, ALERT, THERM
−0.3 to +3.6
V
Input Current, SDATA, THERM
−1 to +50
mA
±1
mA
ESD Rating, All Pins (Human Body Model)
1,500
V
Maximum Junction Temperature (TJ MAX)
150
°C
Storage Temperature Range
−65 to +150
°C
IR Reflow Peak Temperature
220
°C
IR Reflow Peak Temperature for Pb-Free
260
°C
Lead Temperature, Soldering (10 sec)
300
°C
Input Current, D−
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.
Table 2. THERMAL CHARACTERISTICS
Package Type
10-lead MSOP
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2
qJA
qJC
Unit
142
43.74
°C/W
ADT7481
Table 3. PIN ASSIGNMENT
Pin No.
Mnemonic
Description
1
VDD
Positive Supply, 3.0 V to 3.6 V.
2
D1+
Positive Connection to the Remote 1 Temperature Sensor.
3
D1−
Negative Connection to the Remote 1 Temperature Sensor.
4
THERM
5
GND
Supply Ground Connection.
6
D2−
Negative Connection to the Remote 2 Temperature Sensor.
7
D2+
Positive Connection to the Remote 2 Temperature Sensor.
8
ALERT/THERM2
9
SDATA
Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pullup resistor.
10
SCLK
Logic Input, SMBus Serial Clock. Requires pullup resistor.
Open-Drain Output. Requires pullup resistor. Signals overtemperature events, could be used to turn a
fan on/off, or throttle a CPU clock.
Open-Drain Logic Output. Used as interrupt or SMBALERT. This may also be configured as a second
THERM output. Requires pullup resistor.
Table 4. TIMING SPECIFICATIONS (Note 1)
1.
2.
3.
4.
Parameter
Limit at TMIN and TMAX
Unit
fSCLK
400
kHz max
Description
tLOW
1.3
ms min
Clock low period, between 10% points.
tHIGH
0.6
ms 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; STA (Note 2)
600
ns min
Start condition hold time.
tSU; DAT (Note 3)
100
ns min
Data setup time.
tHD; DAT
300
ns min
Data hold time.
tSU; STO (Note 4)
600
ns min
Stop condition setup time.
tBUF
1.3
ms min
Bus free time between stop and start conditions.
Guaranteed by design, not production tested.
Time from 10% of SDATA to 90% of SCLK.
Time for 10% or 90% of SDATA to 10% of SCLK.
Time for 90% of SCLK to 10% of SDATA.
tF
t LOW
t HD; STA
tR
SCLK
t HD; STA
t HD; DAT
t HIGH
t SU; STA
t SU; DAT
t SU; STO
SDATA
t BUF
STOP START
START
Figure 2. Serial Bus Timing
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3
STOP
ADT7481
Table 5. ELECTRICAL CHARACTERISTICS (TA = −40°C to +120°C, VDD = 3.0 V to 3.6 V, unless otherwise noted)
Parameter
Conditions
Min
Typ
Max
Unit
3.0
3.30
3.6
V
Power Supply
Supply Voltage, VDD
Average Operating Supply Current, IDD
Undervoltage Lockout Threshold
0.0625 Conversions/Sec Rate (Note 1)
−
3.0
4.0
mA
Standby Mode
−
5.0
30
mA
VDD Input, Disables ADC, Rising Edge
Power-On-Reset Threshold
−
2.55
−
V
1.0
−
2.5
V
−
−
−
−
−
−
±1
±1.5
±2.5
°C
−
1.0
−
°C
−
−
−
−
−
−
±1
±1.5
±2.5
°C
Temperature-to-Digital Converter
Local Sensor Accuracy (Note 2)
0°C ≤ TA ≤ +70°C
0°C ≤ TA ≤ +85°C
−40 ≤ TA ≤ +100°C
Resolution
Remote Diode Sensor Accuracy (Note 2)
0°C ≤ TA ≤ +70°C, −55°C ≤ TD (Note 3) ≤ +150°C
0°C ≤ TA ≤ +85°C, −55°C ≤ TD (Note 3) ≤ +150°C
−40°C ≤ TA ≤ +100°C, −55°C ≤ TD (Note 3) ≤ +150°C
−
0.25
−
°C
High Level (Note 4)
−
233
−
mA
Low Level (Note 4)
−
14
−
mA
From Stop Bit to Conversion Complete (Both
Channels) One-shot Mode with Averaging
Switched On
−
73
94
ms
One-shot Mode with Averaging Off
(Conversion Rate = 16, 32, or 64 Conversions
per Second)
−
11
14
ms
Resolution
Remote Sensor Source Current
Conversion Time
Open-Drain Digital Outputs (THERM, ALERT/THERM2)
Output Low Voltage, VOL
IOUT = −6.0 mA
−
−
0.4
V
High Level Output Leakage Current, IOH
VOUT = VDD
−
0.1
1.0
mA
Logic Input High Voltage, VIH
SCLK, SDATA
2.1
−
−
V
Logic Input Low Voltage, VIL
SCLK, SDATA
−
−
0.8
V
Hysteresis
−
500
−
mV
SMBus Interface (Notes 4 and 5)
SDA Output Low Voltage, VOL
−
−
0.4
V
Logic Input Current, IIH, IIL
IOUT = −6.0 mA
−1.0
−
+1.0
mA
SMBus Input Capacitance,
SCLK, SDATA
−
5.0
−
pF
SMBus Clock Frequency
−
−
400
kHz
SMBus Timeout (Note 6)
User Programmable
−
25
32
ms
SCLK Falling Edge to SDATA Valid Time
Master Clocking in Data
−
−
1.0
ms
1.
2.
3.
4.
5.
6.
See Table 11 for information on other conversion rates.
Averaging enabled.
Guaranteed by characterization, not production tested.
Guaranteed by design, not production tested.
See Timing Specifications section for more information.
Disabled by default. See the Serial Bus Interface section for details to enable it.
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ADT7481
TYPICAL PERFORMANCE CHARACTERISTICS
3.5
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
2.5
2.0
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 15
DEV 16
MEAN
HIGH 4S
LOW 4S
1.5
1.0
0.5
0
−0.5
−1.0
−50
0
50
100
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR (°C)
3.5
2.5
2.0
1.0
0.5
0
−0.5
−1.0
−50
150
0
50
100
150
TEMPERATURE (°C)
Figure 3. Local Temperature Error vs. Temperature
Figure 4. Remote 1 Temperature Error
vs. Temperature
10
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
DEV 1
DEV 2
DEV 3
DEV 4
DEV 5
DEV 6
DEV 7
3.0
2.5
2.0
DEV 15
DEV 16
MEAN
HIGH 4S
LOW 4S
5
TEMPERATURE ERROR (°C)
3.5
TEMPERATURE ERROR (°C)
DEV 15
DEV 16
HIGH 4S
LOW 4S
1.5
TEMPERATURE (°C)
1.5
1.0
0.5
0
D+ To GND
0
−5
−10
D+ To VCC
−15
−20
−0.5
−25
−1.0
−50
0
50
100
10
1
150
100
LEAKAGE RESISTANCE (MW)
TEMPERATURE (°C)
Figure 5. Remote 2 Temperature Error
vs. Temperature
Figure 6. Temperature Error vs. D+/D− Leakage
Resistance
0
1000
−2
900
−4
800
DEV 2BC
700
−6
IDD (mA)
TEMPERATURE ERROR (°C)
DEV 8
DEV 9
DEV 10
DEV 11
DEV 12
DEV 13
DEV 14
−8
−10
DEV 3
−12
600
500
DEV 4BC
400
300
DEV 2
−14
DEV 3BC
200
DEV 4
−16
100
−18
0
5
10
15
20
0
0.01
25
CAPACITANCE (nF)
0.1
1
10
CONVERTION RATE (Hz)
Figure 7. Temperature Error vs. D+/D− Capacitance
Figure 8. Operating Supply Current
vs. Conversion Rate
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5
100
ADT7481
TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)
422
4.4
420
4.2
DEV 2
DEV 2BC
4.0
418
IDD (mA)
IDD (mA)
DEV 3
416
414
DEV 3BC
3.8
DEV 4
3.6
DEV 4BC
412
3.4
410
3.2
408
3.0
3.1
3.2
3.3
3.4
3.5
3.0
3.0
3.6
3.1
3.2
VDD (V)
Figure 9. Operating Supply Current vs. Voltage
35
TEMPERATURE ERROR (°C)
20
15
10
5
20
3.6
10
100
100 mV
15
10
50 mV
5
0
20 mV
0
1000
0
100
200
300
400
500
600
NOISE FREQUENCY (MHz)
FSCL (kHz)
Figure 11. Standby Supply Current vs. SCLK
Frequency
Figure 12. Temperature Error vs. Common-Mode
Noise Frequency
80
TEMPERATURE ERROR (°C)
ISTBY (mA)
3.5
25
25
1
3.4
Figure 10. Standby Supply Current vs. Voltage
DEV 2BC
DEV 3BC
DEV 4BC
30
3.3
VDD (V)
70
60
100 mV
50
40
30
50 mV
20
10
20 mV
0
−10
0
100
200
300
400
500
600
NOISE FREQUENCY (MHz)
Figure 13. Temperature Error vs. Differential Mode Noise Frequency
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ADT7481
Theory of Operation
This technique requires calibration to null the effect of the
absolute value of VBE, which varies from device to device.
The technique used in the ADT7481 measures the change
in VBE when the device is operated at two different currents.
Figure 14 shows the input signal conditioning used to
measure the output of a remote temperature sensor. This
figure shows the remote sensor as a substrate transistor, but
it could equally be a discrete transistor. If a discrete
transistor is used, the collector is not grounded and is linked
to the base. To prevent ground noise 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. C1 may optionally be added
as a noise filter with a recommended maximum value of
1,000 pF.
To measure DVBE, the operating current through the
sensor is switched among two related currents. The currents
through the temperature diode are switched between I, and
N × I, giving DVBE. The temperature can then be calculated
using the DVBE measurement.
The resulting DVBE waveforms pass 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 DVBE.
The ADC digitizes this voltage producing a temperature
measurement. 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 local
temperature sensor is performed in the same manner.
The ADT7481 is a local and dual remote temperature
sensor and over/under temperature alarm. When the
ADT7481 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 remote diode open circuit will cause 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.
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.
I
N×I
VDD
IBIAS
VOUT+
D+
REMOTE
SENSING
TRANSISTOR
To ADC
C1*
D−
VOUT−
BIAS
DIODE
LOW-PASS FILTER
fC = 65 kHz
*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000 pF MAX
Figure 14. Input Signal Conditioning
Temperature Measurement Results
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.
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.
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ADT7481
In extended temperature mode, the upper and lower
temperatures that can be measured by the ADT7481 are
limited by the remote diode selection. While the temperature
registers can have values from −64°C to +191°C, 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 ADT7481 should not be exposed to
temperatures greater than those specified in the Absolute
section. Furthermore, the device is only guaranteed to operate
as specified at ambient temperatures from −40°C to +120°C.
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)
If Bit 3 of the Configuration 1 register is set to 1, then the
Remote 2 temperature values can be read from the following
register addresses:
Remote 2, MSBs = 0x01
Remote 2, LSBs = 0x10
Temperature Data Format
The ADT7481 has two temperature data formats. When
the temperature measurement range is from 0°C to +127°C
(default), the temperature data format is binary for both local
and remote temperature results. See the Temperature
Measurement Range section for information on how to
switch between the two data formats.
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.
The above is true only when Bit 3 of the Configuration 1
register is set. To read the Remote 1 temperatures, this bit
needs to be switched 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
Table 8. TEMPERATURE DATA FORMAT
(LOCAL AND REMOTE TEMPERATURE HIGH BYTE)
When reading the full remote temperature value,
including both the high and low byte, the two registers
should be read LSB first and then the MSB. This is because
reading the LSB will cause the MSB to be locked until it is
read. This is to guarantee that the two values read are derived
from the same temperature measurement. The MSB register
updates only after it has been read. The MSB will not lock
if a SMBus repeat start is used between reading the two
registers. There needs to be a stop between reading the LSB
and MSB.
If the LSB register is read but not the MSB register, then
fail-safe protection is provided by the THERM and ALERT
signals which update with the latest temperature measurements
rather than the register values.
Offset Binary
(Note 1)
Temperature
Binary
−55°C
0 000 0000
(Note 2)
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 1111
(Note 3)
1 101 0110
1. Offset binary scale temperature values are offset by +64.
2. Binary scale temperature measurement returns 0 for all
temperatures <0°C.
3. Binary scale temperature measurement returns 127 for all
temperatures >127°C.
Temperature Measurement Range
The temperature measurement range for both local and
remote measurements is, by default, 0°C to +127°C.
However, the ADT7481 can be operated using an extended
temperature range. The temperature range in the extended
mode is −64°C to +191°C. The user can 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.
Bit 2 Configuration Register 2 = 0 = 0°C to +127°C = default
Bit 2 Configuration Register 2 = 1 = −64°C to +191°C
The user may switch between measurement ranges at any
time. Switching the range will also switch the data format.
The next temperature result following the switching will be
reported back to the register in the new format. However, the
contents of the limit registers will not change. It is up to the
user to ensure that when the data format changes, the limit
registers are reprogrammed as necessary. More information
on this can be found in the Limit Registers section.
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ADT7481
Registers
Temperature Value Registers
The registers in the ADT7481 are eight bits wide. These
registers are used to store the results of remote and local
temperature measurements, high and low temperature limits,
and to configure and control the device. A description of these
registers follows.
The ADT7481 has five registers to store the results of
local and remote temperature measurements. These
registers can only be written to by the ADC and read by the
user over the SMBus.
• The local temperature value register is at Address 0x00.
• The Remote 1 temperature value high byte register is at
Address 0x01, with the Remote 1 low byte register at
Address 0x10.
• The Remote 2 temperature value high byte register is at
Address 0x30, with the Remote 2 low byte register at
Address 0x33.
• The Remote 2 temperature values can also be read from
Address 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 value for all five registers is 0x00.
Address Pointer Register
The address pointer register does not have, nor does it
require, an address because 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 ADT7481, 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, so if a read operation is performed immediately after
power-on, without first writing to the address pointer, the
value of the local temperature will be returned since its
register address is 0x00.
Table 9. CONFIGURATION 1 REGISTER (READ ADDRESS 0x03, WRITE ADDRESS 0x09)
Bit
Mnemonic
Function
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 ADT7481 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. However THERM
and ALERT are not active in standby mode, and their states in standby mode are not reliable.
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 the THERM2 pin.
4
Reserved
3
Remote
1/2
Setting this bit to 1 enables the user to read the Remote 2 values from the Remote 1 registers. When 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 of −64°C to +191°C. When using the
default = 0, the temperature range is 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.
Reserved for future use.
Table 10. CONFIGURATION 2 REGISTER (ADDRESS 0x24)
Bit
Mnemonic
Function
7
Lock Bit
Setting this bit to 1 locks all lockable registers to their current values. This prevents tampering with settings until
the device is powered down. Default = 0.
<6:0>
Res
Reserved for future use.
Conversion Rate/Channel Selector Register
This register can be written to, and read back from, the
SMBus. The default value of this register is 0x08, giving a
rate of 16 conversions per second. Using slower conversion
times greatly reduces the device power consumption.
Bit 7 in this register can be used to disable averaging of the
temperature measurements. All temperature channels are
measured by default. It is possible to configure the
ADT7481 to measure the temperature of one channel only.
This can be configured using Bit 4 and Bit 5 (see Table 11).
The conversion rate/channel selector register for reads is
at Address 0x04, and at 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 ADT7481 to perform
continuous measurements, set the conversion rate register to
0x0B. For example, a conversion rate of eight
conversions/second means that beginning at 125 ms
intervals, the device performs a conversion on the local and
the remote temperature channels.
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ADT7481
Table 11. CONVERSION RATE/CHANNEL SELECTOR REGISTER (READ ADDRESS 0x04, WRITE ADDRESS 0x0A)
Bit
Mnemonic
7
Averaging
Setting this bit to 1 disables averaging of the temperature measurements at the slower conversion
rates (averaging cannot take place at the three faster rates, so setting this bit has no effect). When
default = 0, averaging is enabled.
Function
6
Reserved
Reserved for future use. Do not write to this bit.
<5:4>
Channel Selector
These bits are used to select the temperature measurement channels:
00 = Round Robin = Default = All Channels Measured
01 = Local Temperature Only Measured
10 = Remote 1 Temperature Only Measured
11 = Remote 2 Temperature Only Measured
<3:0>
Conversion Rates
These bits set how often the ADT7481 measures each temperature channel.
Conversion rates are as follows:
Conversions/sec
0000 = 0.0625
0001 = 0.125
0010 = 0.25
0011 = 0.5
0100 = 1
0101 = 2
0110 = 4
0111 = 8 = Default
1000 = 16
1001 = 32
1010 = 64
1011 = Continuous Measurements
Limit Registers
Time (seconds)
16
8
4
2
1
500 m
250 m
125 m
62.5 m
31.25 m
15.5 m
73 m (Averaging Enabled)
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.
The ADT7481 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 from, the SMBus.
See Table 16 for details of the limit register addresses and
power-on default values.
C will result 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 will result 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 may be reprogrammed.
It is important to remember that the temperature limits
data format is the same as the temperature measurement data
format. So 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 automatically
switch.
The user must reprogram the limit registers 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 being
Status Registers
The status registers are read-only registers, at
Address 0x02 (Status Register 1) and Address 0x23 (Status
Register 2). They contain status information for the
ADT7481.
Table 12. STATUS REGISTER 1 BIT ASSIGNMENTS
Bit
Mnemonic
Function
ALERT
7
BUSY
1 when ADC Converting
No
6
LHIGH
(Note 1)
1 when Local High
Temperature Limit Tripped
Yes
5
LLOW
(Note 1)
1 when Local Low
Temperature Limit Tripped
Yes
4
R1HIGH
1 when Remote 1 High
Temperature Limit Tripped
Yes
3
R1LOW
(Note 1)
1 when Remote 1 Low
Temperature Limit Tripped
Yes
2
D1 OPEN
(Note 1)
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
1. These flags stay high until the status register is read, or they are
reset by POR.
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ADT7481
the ALERT. This bit gets reset when the ALERT output gets
reset. If the ALERT output is masked, then this bit is not set.
Table 13. STATUS REGISTER 2 BIT ASSIGNMENTS
Bit
Mnemonic
7
Res
Reserved for Future Use
Function
ALERT
No
6
Res
Reserved for Future Use
No
5
Res
Reserved for Future Use
No
4
R2HIGH
(Note 1)
1 when Remote 2 High
Temperature Limit Tripped
Yes
3
R2LOW
(Note 1)
1 when Remote 2 Low
Temperature Limit Tripped
Yes
2
D2 OPEN
(Note 1)
1 when Remote 2 Sensor
Open Circuit
Yes
1
R2THRM1
1 when Remote 2 THERM
Limit Tripped
No
0
ALERT
1 when ALERT Condition
Exists
No
Offset Register
Offset errors may 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 Register 0x35 (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.
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 flag is high, the ALERT interrupt latch
is set and the ALERT output goes low (provided that the
flag(s) is/are not masked out).
Reading the Status 1 register will clear the five 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 will clear 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 will be 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. The
user may add hysteresis by programming Register 0x21.
The THERM output will be 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 and 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 Status Register 2 gets set whenever the ALERT
output is asserted low. Thus, the user need only read Status
Register 2 to determine if the ADT7481 is responsible for
Only the upper two bits of the LSB registers are used. The
MSB of the MSB offset register 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 to, or
subtracted from, the measured value of the remote
temperature.
The offset register powers up with a default value of 0°C
and will have no effect unless the user writes a different
value to it.
Table 14. SAMPLE OFFSET REGISTER CODES
Offset Value
0x11/0x34
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
10 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 is used to initiate a conversion and
comparison cycle when the ADT7481 is in standby mode,
after which the device returns to standby. Writing to the
one-shot register address (0x0F) causes the ADT7481 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 0x0F that
causes the one-shot conversion. The data written to this
address is irrelevant and is not stored. However the ALERT
and THERM outputs are not operational in one-shot mode
and should not be used.
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ADT7481
Consecutive ALERT Register
Table 15. CONSECUTIVE ALERT REGISTER BIT
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.
The purpose of this register is to allow the user to perform
some filtering of the output. This is particularly useful at the
fastest three conversion rates, where no averaging takes
place. This register is at Address 0x22. This register has
other functions that are listed in Table 15.
Bit
Name
Description
7
SCL
Timeout
Set to 1, enables the SMBus SCL
timeout bit. Default = 0 = Timeout
disabled. See the Serial Bus Interface
section for more information.
6
SDA
Timeout
Set to 1 to enable the SMBus SDA
Timeout Bit. Default = 0 = Timeout
disabled. See the Serial Bus Interface
section for more information.
5
Mask Local
Setting this bit to 1 masks ALERTs
due to the local temperature
exceeding a programmed limit.
Default = 0.
4
Res
<3:0>
Consecutive
ALERT
Reserved for future use.
These bits set the number of
consecutive out-of-limit
measurements that have to occur
before an ALERT is generated.
000x = 1
001x = 2
011x = 3
111x = 4
Table 16. LIST OF REGISTERS
Read
Address
(Hex)
Write
Address
(Hex)
N/A
N/A
Address Pointer
Undefined
00
N/A
Local Temperature Value
0000 0000 (0x00)
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/Channel Selector
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 Temp High Limit High Byte
0101 0101 (0x55) (85°C)
Bit 3 Conf Reg = 0
Yes
07
0D
Remote 2 Temp High Limit High Byte
0101 0101 (0x55) (85°C)
Bit 3 Conf Reg = 1
Yes
08
0E
Remote 1 Temp Low Limit High Byte
0000 0000 (0x00) (0°C)
Bit 3 Conf Reg = 0
Yes
08
0E
Remote 2 Temp Low Limit High Byte
0000 0000 (0x00) (0°C)
Bit 3 Conf Reg = 1
Yes
N/A
0F
(Note 1)
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
Mnemonic
Power-On Default
Comment
Lock
No
No
One-Shot
N/A
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 Temp High Limit Low Byte
0000 0000
Bit 3 Conf Reg = 0
Yes
13
13
Remote 2 Temp High Limit Low Byte
0000 0000
Bit 3 Conf Reg = 1
Yes
14
14
Remote 1 Temp Low Limit Low Byte
0000 0000
Bit 3 Conf Reg = 0
Yes
14
14
Remote 2 Temp 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
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ADT7481
Table 16. LIST OF REGISTERS (continued)
Read
Address
(Hex)
Write
Address
(Hex)
19
19
Remote 2 THERM Limit
0101 0101 (0x55) (85°C)
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 Temp High Limit High Byte
0101 0101 (0x55) (85°C)
Yes
32
32
Remote 2 Temp 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 Temp High Limit Low Byte
0000 0000 (0x00) (0°C)
Yes
37
37
Remote 2 Temp Low Limit Low Byte
0000 0000 (0x00) (0°C)
Yes
Yes
Mnemonic
Power-On Default
39
39
Remote 2 THERM Limit
0101 0101 (0x55) (85°C)
3D
N/A
Device ID
1000 0001 (0x81)
3E
N/A
Manufacturer ID
0100 0001 (0x41)
Comment
Lock
Bit 3 Conf Reg = 1
Yes
N/A
1. Writing to Address 0F causes the ADT7481 to perform a single measurement. It is not a data register as such, and it does not matter what
data is written to it.
Serial Bus Interface
address of 0x4B (1001 011b). The addresses mentioned in
this datasheet are 7-bit addresses. The R/W bit needs to be
added to arrive at an 8-bit address. Other than the different
SMBus addresses, the ADT7481 and the ADT7481−1 are
functionally identical.
The serial bus protocol operates as follows:
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 a R/W bit,
which determines the direction of the data transfer, that is,
whether data will be written to, or read from, the slave
device. The peripheral with the address corresponding 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 remain idle
while the selected device waits for data to be read from or
written to it. If the R/W bit is 0, the master writes to the slave
device. If the R/W bit is 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 may be interpreted as a stop signal.
Control of the ADT7481 is achieved via the serial bus.
The ADT7481 is connected to this bus as a slave device
under the control of a master device.
The ADT7481 has an SMBus timeout feature. When this
is enabled, the SMBus will typically timeout after 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.
The ADT7481 supports packet error checking (PEC) and
its use is optional. It is triggered by supplying the extra clock
for the PEC byte. The PEC byte is calculated using CRC−8.
The frame check sequence (FCS) conforms to CRC−8 by the
polynomial:
C(x) + x 8 ) x 2 ) x 1 ) 1
Consult the SMBus 1.1 specification
information (www.smbus.org).
(eq. 1)
for
more
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 ADT7481 is available with one device address, 0x4C
(1001 100b). An ADT7481−1 is also available. The only
difference between the ADT7481 and the ADT7481−1 is the
SMBus address. The ADT7481−1 has a fixed SMBus
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ADT7481
changed without starting a new operation. In the case of the
ADT7481, 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.
This procedure is illustrated in Figure 15. The device
address is sent over the bus followed by R/W set to 0 and
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.
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 will
pull the data line high during the tenth clock pulse to assert
a stop condition. In read mode, the master device will
override 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 will then take 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 may 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
1
9
9
1
SCLK
1
SDATA
0
START BY
MASTER
0
1
1
1
0
R/W
D7
FRAME 1
SERIAL BUS ADDRESS BYTE
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7481
FRAME 2
ADDRESS POINTER REGISTER BYTE
ACK. BY
ADT7481
1
9
SCLK (CONTINUED)
SDATA (CONTINUED)
D7
D6
D5
D4
D3
D2
D1
FRAME 3
DATA BYTE
D0
ACK. BY STOP BY
ADT7481 MASTER
Figure 15. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
1
9
9
1
SCLK
SDATA
1
START BY
MASTER
0
0
1
1
0
1
FRAME 1
SERIAL BUS ADDRESS BYTE
D7
R/W
ACK. BY
ADT7481
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7481
FRAME 2
ADDRESS POINTER REGISTER BYTE
STOP BY
MASTER
Figure 16. Writing to the Address Pointer Register Only
9
1
1
9
SCLK
SDATA
START BY
MASTER
1
0
0
1
1
0
1
FRAME 1
SERIAL BUS ADDRESS BYTE
R/W
D7
D6
ACK. BY
ADT7481
D5
D4
D3
D2
FRAME 2
DATA BYTE FROM ADT7481
Figure 17. Reading Data from a Previously Selected Register
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14
D1
D0
ACK. BY
ADT7481
STOP BY
MASTER
ADT7481
When reading data from a register there are two possible
scenarios:
1. If the address pointer register value of the
ADT7481 is unknown or not the desired value, it
is 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
ADT7481 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 16.
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 (shown
in Figure 17).
2. If the address pointer register is 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 16 can be omitted.
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 with a low ALERT output responds to
the alert response address, and the master reads the
address from the responding device. An LSB of 1
is added because the device address is comprised
of seven bits. 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 output,
the one with the lowest device address will have
priority, in accordance with normal SMBus
arbitration.
5. Once the ADT7481 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 sends the ARA again, and
so on until all devices with low ALERT outputs
respond.
NOTES: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.
Masking the ALERT Output
The ALERT output can be masked for local, Remote 1,
Remote 2 or all three channels. This is done by setting the
appropriate mask bits in either the Configuration 1 register
(read address = 0x03, write address = 0x09) or in the
consecutive ALERT register (address = 0x22)
To mask ALERTs due to local temperature, set Bit 5 of the
consecutive ALERT register to 1. Default = 0.
To mask ALERTs due to Remote 1 temperature, set Bit 1 of
the Configuration 1 register to 1. Default = 0.
To mask ALERTs due to Remote 2 temperature, set Bit 0 of
the Configuration 1 register to 1. Default = 0.
To mask ALERTs due to any channel, set Bit 7 of the
Configuration 1 register to 1. Default = 0.
Remember that some of the ADT7481 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.
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 open circuit.
It is an open-drain output and requires a pullup. Several
ALERT outputs can be wire-OR’ed 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
processor, or it may 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 18.
Low Power Standby Mode
The ADT7481 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.
The ADT7481 operates normally when Bit 6 is 0. 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 mA if there is no SMBus activity, or up to 30 mA
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 both channels by writing to
the one-shot register (Address 0x0F), after which the device
will return 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
MASTER RECEIVES SMBALERT
START
ALERT RESPONSE
ADDRESS
MASTER SENDS
ARA AND READ
COMMAND
RD ACK
DEVICE
ADDRESS
NO
STOP
ACK
DEVICE SENDS
ITS ADDRESS
Figure 18. Use of SMBALERT
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ADT7481
the hysteresis value. This applies to both local and remote
measurement channels. The power-on hysteresis default
value is 10°C, but this may be reprogrammed to any value
after powerup.
The hysteresis loop on the THERM outputs is useful when
THERM is used for on/off control of a fan. The user’s
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, a condition wherein the
temperature hovers around the THERM limit, and the fan is
constantly being switched on and off.
standby mode. ALERT and THERM are not available in
standby mode and, therefore, should not be used because the
state of these pins is unreliable.
Sensor Fault Detection
The ADT7481 has internal sensor fault detection circuitry
at its D+ input. This circuit can detect situations where a
remote diode is not connected, or is incorrectly connected,
to the ADT7481. If the voltage at D+ exceeds VDD − 1.0 V
(typical), it signifies an open circuit between D+ and D−, and
consequently, trips the simple voltage comparator. The
output of this comparator is checked when a conversion is
initiated. Bit 2 (D1 open flag) of the Status Register 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 will
cause ALERT to assert low.
If a remote sensor is not used with the ADT7481, then the
D+ and D− inputs of the ADT7481 need to be tied together
to prevent the open flag from being continuously set.
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 will give the last good
temperature measurement. The user should be aware that
while the diode fault is triggered, the temperature
measurement on the remote channels is likely to be
inaccurate.
Table 17. THERM HYSTERESIS
THERM Hysteresis
Binary Representation
0°C
0 000 0000
1°C
0 000 0001
10°C
0 000 1010
Figure 19 shows how the THERM and ALERT outputs
operate. A user may wish to use the ALERT output as a
SMBALERT to signal to the host via the SMBus that the
temperature has risen. The user could use the THERM
output to turn on a fan to cool the system, if the temperature
continues to increase. This method would ensure that there
is a fail-safe mechanism to cool the system, without the need
for host intervention.
TEMPERATURE
1005C
905C
THERM LIMIT
805C
THERM LIMIT − HYSTERESIS
705C
Interrupt System
HIGH TEMP LIMIT
605C
The ADT7481 has two interrupt outputs, ALERT and
THERM. Both outputs 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 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 automatically reset 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
will reset when the temperature falls to the limit value minus
505C
405C
RESET BY MASTER
ALERT
1
4
THERM
2
3
Figure 19. Operation of the ALERT and THERM
Interrupts
• If the measured temperature exceeds the high
•
•
•
temperature limit, the ALERT output will assert low.
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.
The THERM output de-asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 19, the default hysteresis value of 10°C is
shown.
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.
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ADT7481
collector connected to the substrate. Discrete types can be
either a PNP or an NPN transistor 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 ADT7481 is trimmed for an nf value of 1.008. Use
the following equation to calculate the error introduced
at a temperature, T (°C), when using a transistor where
nf does not equal 1.008. Consult the processor data
sheet for the nf values.
Pin 8 on the ADT7481 can be configured as either an
ALERT output or as an additional THERM output.
THERM2 will assert 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 also applies to
THERM2.
Figure 20 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.
TEMPERATURE
905C
THERM LIMIT
DT + ǒn f * 1.008Ǔń1.008
805C
705C
605C
THERM2 LIMIT
505C
405C
•
305C
THERM2
1
THERM
4
2
3
Figure 20. Operation of the THERM and THERM2
Interrupts
When the THERM2 limit is exceeded, the THERM2
signal asserts low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
• , there is no hysteresis value shown.
• 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.
ǒ273.15 Kelvin ) TǓ
(eq. 2)
To factor this in, the user can write the DT value to the
offset register. It will then automatically be added to, or
subtracted from, the temperature measurement by the
ADT7481.
Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7481, IHIGH, is 233ĂmA. The
low level current, ILOW, is 14ĂmA. If the ADT7481
current levels do not match the current levels specified
by the CPU manufacturer, it may become necessary to
remove an offset. The CPU data sheet will advise
whether this offset needs to be removed and how to
calculate it. This offset may 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 ADT7481,
the best accuracy is obtained by choosing devices according
to the following criteria:
• Base-emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature.
• Base-emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature.
• Base resistance less than 100 W.
• Small variation in hFE (say 50 to 150) that indicates
tight control of VBE characteristics.
The temperature measurement could be either the local or
the remote temperature measurement.
Applications Information
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice was to place a capacitor across the D+ and
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 1,000 pF.
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;
otherwise, the thermal inertia caused by the sensor’s mass
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7481 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors will generally be PNP types with the
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ADT7481
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 will either be a substrate transistor in the
processor or a small package device, such as an SOT−23,
placed in close proximity to it.
The on-chip sensor, however, will often be remote from
the processor and only monitors the general ambient
temperature around the package. In practice, the ADT7481
package will be 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 will also affect the
accuracy of the measurement. Self-heating, due to the power
dissipated in the ADT7481 or the remote sensor, causes the
chip temperature of the device (or remote sensor) to rise
above ambient. However, the current forced through the
remote sensor is so small that self-heating is negligible. The
worst-case condition occurs when the ADT7481 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, qJA, of the
MSOP−10 package is about 142°C/W.
3. 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 large
temperature differential between them,
thermocouple voltages should be much less than
200 mV.
4. Place a 0.1 mF bypass capacitor close to the VDD
pin. In extremely noisy environments, an input
filter capacitor may be placed across D+ and D−
close to the ADT7481. This capacitance can affect
the temperature measurement, so care must be
taken to ensure that any capacitance seen at D+
and D− is a maximum of 1,000 pF. This maximum
value includes the filter capacitance, plus any
cable or stray capacitance between the pins and the
sensor diode.
5. 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 of cable
is needed.
For really 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
ADT7481. Leave the remote end of the shield
unconnected to avoid ground loops.
Layout Considerations
Digital boards can be electrically noisy environments, and
the ADT7481 measures very small voltages from the remote
sensor, so care must be taken to minimize noise induced at
the sensor inputs. Take the following precautions:
1. Place the ADT7481 as close as possible to the
remote sensing diode. Provided that the worst
noise sources such as clock generators,
data/address buses, and CRTs are avoided, this
distance can range from 4 to 8 inches.
2. 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.
GND
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 22 shows a typical application circuit for the
ADT7481, using discrete sensor transistors. The pullups on
SCLK, SDATA, and ALERT are required only if they are not
already provided elsewhere in the system.
The SCLK and SDATA pins of the ADT7481 can be
interfaced directly to the SMBus of an I/O controller, such
as the Intel® 820 chipset.
5 MIL
5 MIL
D+
5 MIL
5 MIL
D−
5 MIL
5 MIL
GND
5 MIL
Figure 21. Typical Arrangement of Signal Tracks
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ADT7481
VDD
3.0 V to 3.6 V
0.1 mF
ADT7481
TYP 10 kW
D1+
SCLK
2N3904/06
or
CPU THERMAL
DIODE
D1−
D2+
SMBUS
CONTROLLER
SDATA
ALERT
D2−
VDD
5.0 V or 12 V
THERM
TYP 10 kW
GND
FAN
ENABLE
FAN
CONTROL
CIRCUIT
Figure 22. Typical Application Circuit
Table 18. ORDERING INFORMATION
Device Order Number*
Package Type
Shipping†
Branding
SMBus Address
ADT7481ARMZ−REEL
10-lead MSOP
3,000 Tape & Reel
T08
4C
ADT7481ARMZ−1RL
10-lead MSOP
3,000 Tape & Reel
T0M
4B
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
*The “Z’’ suffix indicates Pb-Free package available.
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ADT7481
PACKAGE DIMENSIONS
MSOP10
CASE 846AC−01
ISSUE O
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION “A” DOES NOT INCLUDE MOLD
FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE
BURRS SHALL NOT EXCEED 0.15 (0.006)
PER SIDE.
4. DIMENSION “B” DOES NOT INCLUDE
INTERLEAD FLASH OR PROTRUSION.
INTERLEAD FLASH OR PROTRUSION
SHALL NOT EXCEED 0.25 (0.010) PER SIDE.
5. 846B−01 OBSOLETE. NEW STANDARD
846B−02
−A−
−B−
K
D 8 PL
0.08 (0.003)
PIN 1 ID
G
SEATING
PLANE
T B
S
A
S
H
L
J
MILLIMETERS
MIN
MAX
2.90
3.10
2.90
3.10
0.95
1.10
0.20
0.30
0.50 BSC
0.05
0.15
0.10
0.21
4.75
5.05
0.40
0.70
DIM
A
B
C
D
G
H
J
K
L
C
0.038 (0.0015)
−T−
M
INCHES
MIN
MAX
0.114
0.122
0.114
0.122
0.037
0.043
0.008
0.012
0.020 BSC
0.002
0.006
0.004
0.008
0.187
0.199
0.016
0.028
SOLDERING FOOTPRINT*
10X
1.04
0.041
0.32
0.0126
3.20
0.126
8X
10X
4.24
0.167
0.50
0.0196
SCALE 8:1
5.28
0.208
mm Ǔ
ǒinches
*For additional information on our Pb-Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,
copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC
reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any
particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without
limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications
and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC
does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where
personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly,
any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture
of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
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ADT7481/D