Document Outline
- Specifications
- Pinout
- Package drawings
- Ordering Guide
- Features
- Applications
- Product Description
- Absolute Maximum Ratings
- Functional Block Diagram
- Pin Function Description
- Typical Characteristics
- THERMAL CHARACTERISTICS
- FUNCTIONAL DESCRIPTION
- MEASUREMENT METHOD
- TEMPERATURE DATA FORMAT
- DIFFERENCES BETWEEN THE ADM1021 AND THE ADM1021A
- REGISTERS
- SERIAL BUS INTERFACE
- ADDRESS PINS
- NOTES
- ALERT OUTPUT
- LOW POWER STANDBY MODES
- SENSOR FAULT DETECTION
- LAYOUT CONSIDERATIONS
- APPLICATIONS INFORMATION FACTORS AFFECTING ACCURACY
- APPLICATION CIRCUITS
- DIAGRAMS
- Diagram for Serial Bus Timing
- Input Signal Conditioning
- Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
- Writing to the Address Pointer Register Only
- Reading Data from a Previously Selected Register
- Use of SMBALERT
- Typical ADM1021A Application Circuit
- Typical System Using ADM1021A
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no 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. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
a
ADM1021A
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2001
FUNCTIONAL BLOCK DIAGRAM
ON-CHIP TEMP.
SENSOR
ANALOG MUX
A-TO-D
CONVERTER
LOCAL TEMPERATURE
VALUE REGISTER
REMOTE TEMPERATURE
VALUE REGISTER
BUSY
RUN/STANDBY
LOCAL TEMPERATURE
LOW LIMIT COMPARATOR
STATUS REGISTER
REMOTE TEMPERATURE
LOW LIMIT COMPARATOR
REMOTE TEMPERATURE
HIGH LIMIT COMPARATOR
ADDRESS POINTER
REGISTER
ONE-SHOT
REGISTER
CONVERSION RATE
REGISTER
LOCAL TEMPERATURE
LOW LIMIT REGISTER
LOCAL TEMPERATURE
HIGH LIMIT REGISTER
REMOTE TEMPERATURE
LOW LIMIT REGISTER
LOCAL TEMPERATURE
HIGH LIMIT COMPARATOR
REMOTE TEMPERATURE
HIGH LIMIT REGISTER
CONFIGURATION
REGISTER
INTERRUPT
MASKING
EXTERNAL DIODE OPEN-CIRCUIT
SMBUS INTERFACE
ADM1021A
NC
V
DD
NC
GND
NC
NC
NC
D+
D
ALERT
STBY
SDATA
SCLK
ADD0
ADD1
GND
NC = NO CONNECT
Low-Cost Microprocessor
System Temperature Monitor*
FEATURES
Alternative to the ADM1021
On-Chip and Remote Temperature Sensing
No Calibration Necessary
1 C Accuracy for On-Chip Sensor
3 C Accuracy for Remote Sensor
Programmable Over/Under Temperature Limits
Programmable Conversion Rate
2-Wire SMBus Serial Interface
Supports System Management Bus (SMBus) Alert
200 A Max Operating Current
1 A Standby Current
3 V to 5.5 V Supply
Small 16-Lead QSOP Package
APPLICATIONS
Desktop Computers
Notebook Computers
Smart Batteries
Industrial Controllers
Telecomms Equipment
Instrumentation
PRODUCT DESCRIPTION
The ADM1021A is a two-channel digital thermometer and under/
over temperature alarm, intended for use in personal computers
and other systems requiring thermal monitoring and management.
The device can measure the temperature of a microprocessor
using a diode-connected PNP transistor, which may be provided
on-chip in the case of the Pentium
®
III or similar processors,
or can be a low-cost discrete NPN/PNP device such as the
2N3904/2N3906. A novel measurement technique cancels out
the absolute value of the transistor's base emitter voltage, so
that no calibration is required. The second measurement chan-
nel measures the output of an on-chip temperature sensor, to
monitor the temperature of the device and its environment.
The ADM1021A communicates over a two-wire serial interface
compatible with SMBus
standards. Under and over temperature
limits can be programmed into the devices over the serial bus,
and an
ALERT output signals when the on-chip or remote
temperature is out of range. This output can be used as an inter-
rupt, or as an SMBus alert.
*Patents Pending.
Pentium is a registered trademark of Intel Corporation.
2
REV. A
ADM1021ASPECIFICATIONS
(T
A
= T
MIN
to T
MAX
1
, V
DD
= 3.0 V to 3.6 V, unless otherwise noted.)
Parameter
Min
Typ
Max
Unit
Test Conditions/Comments
POWER SUPPLY AND ADC
Temperature Resolution
1
°C
Guaranteed No Missed Codes
Temperature Error, Local Sensor
±1
°C
3
+3
°C
Temperature Error, Remote Sensor
3
+3
°C
T
A
= 60
°C to 100°C
5
+5
°C
Supply Voltage Range
3
3.6
V
Note 2
Undervoltage Lockout Threshold
2.5
2.7
2.95
V
V
DD
Input, Disables ADC, Rising Edge
Undervoltage Lockout Hysteresis
25
mV
Power-On Reset Threshold
0.9
1.7
2.2
V
V
DD
, Falling Edge
3
POR Threshold Hysteresis
50
mV
Standby Supply Current
1
5
µA
V
DD
= 3.3 V, No SMBus Activity
4
µA
SCLK at 10 kHz
Average Operating Supply Current
130
200
µA
0.25 Conversions/Sec Rate
Autoconvert Mode, Averaged Over 4 Seconds
225
330
µA
Two Conversions/Sec Rate
Conversion Time
65
115
170
ms
From Stop Bit to Conversion Complete
(Both Channels)
D+ Forced to D + 0.65 V
Remote Sensor Source Current
120
205
300
µA
High Level (Note 3)
7
12
16
µA
Low Level (Note 3)
D-Source Voltage
0.7
V
Address Pin Bias Current (ADD0, ADD1)
50
µA
Momentary at Power-On Reset
SMBUS INTERFACE
Logic Input High Voltage, V
IH
2.2
V
V
DD
= 3 V to 5.5 V
STBY, SCLK, SDATA
Logic Input Low Voltage, V
IL
0.8
V
V
DD
= 3 V to 5.5 V
STBY, SCLK, SDATA
SMBus Output Low Sink Current
6
mA
SDATA Forced to 0.6 V
ALERT Output Low Sink Current
1
mA
ALERT Forced to 0.4 V
Logic Input Current, I
IH
, I
IL
1
+1
µA
SMBus Input Capacitance, SCLK, SDATA
5
pF
SMBus Clock Frequency
100
kHz
SMBus Clock Low Time, t
LOW
4.7
µs
t
LOW
Between 10% Points
SMBus Clock High Time, t
HIGH
4
µs
t
HIGH
Between 90% Points
SMBus Start Condition Setup Time, t
SU:STA
4.7
µs
SMBus Repeat Start Condition
250
ns
Between 90% and 90% Points
Setup Time, t
SU:STA
SMBus Start Condition Hold Time, t
HD:STA
4
µs
Time from 10% of SDATA to 90% of SCLK
SMBus Stop Condition Setup Time, t
SU:STO
4
µs
Time from 90% of SCLK to 10% of SDATA
SMBus Data Valid to SCLK
250
ns
Time from 10% or 90% of SDATA to 10%
Rising Edge Time, t
SU:DAT
of SCLK
SMBus Data Hold Time, t
HD:DAT
0
µs
SMBus Bus Free Time, t
BUF
4.7
µs
Between Start/Stop Condition
SCLK Falling Edge to SDATA
1
µs
Master Clocking in Data
Valid Time, t
VD,DAT
NOTES
1
T
MAX
= 100
°C; T
MIN
= 0
°C.
2
Operation at V
DD
= 5 V guaranteed by design, not production tested.
3
Guaranteed by design, not production tested.
Specifications subject to change without notice.
ADM1021A
3
REV. A
ABSOLUTE MAXIMUM RATINGS
*
Positive Supply Voltage (V
DD
) to GND . . . . . . 0.3 V to +6 V
D+, ADD0, ADD1 . . . . . . . . . . . . . . . 0.3 V to V
DD
+ 0.3 V
D to GND . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to +0.6 V
SCLK, SDATA,
ALERT, STBY . . . . . . . . . . . 0.3 V to +6 V
Input Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
±50 mA
Input Current, D . . . . . . . . . . . . . . . . . . . . . . . . . . . .
±1 mA
ESD Rating, All Pins (Human Body Model) . . . . . . . . 2000 V
Continuous Power Dissipation
Up to 70
°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650 mW
Derating above 70
°C . . . . . . . . . . . . . . . . . . . . . 6.7 mW/°C
Operating Temperature Range . . . . . . . . . . 55
°C to +125°C
Maximum Junction Temperature (T
J
max) . . . . . . . . . . 150
°C
Storage Temperature Range . . . . . . . . . . . . 65
°C to +150°C
Lead Temperature, (Soldering 10 sec) . . . . . . . . . . . . . 300
°C
IR Reflow Peak Temperature . . . . . . . . . . . . . . . . . . . . . 220
°C
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent 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 CHARACTERISTICS
16-Lead QSOP Package:
JA
= 150
°C/W.
ORDERING GUIDE
Temperature
Package
Package
Model
Range
Description
Option
ADM1021AARQ
0
°C to 100°C
16-Lead QSOP RQ-16
PIN FUNCTION DESCRIPTIONS
Pin No.
Mnemonic Description
1, 5, 9, 13, 16 NC
No Connect
2
V
DD
Positive Supply, 3 V to 5.5 V.
3
D+
Positive Connection to Remote
Temperature Sensor.
4
D
Negative Connection to Remote
Temperature Sensor.
6
ADD1
Three-State Logic Input, Higher
Bit of Device Address.
7, 8
GND
Supply 0 V Connection.
10
ADD0
Three-State Logic Input, Lower
Bit of Device Address.
11
ALERT
Open-Drain Logic Output Used as
Interrupt or SMBus Alert.
12
SDATA
Logic Input/Output, SMBus Serial
Data. Open-Drain Output.
14
SCLK
Logic Input, SMBus Serial Clock.
15
STBY
Logic Input Selecting Normal
Operation (High) or Standby Mode
(Low).
PIN CONFIGURATION
TOP VIEW
(Not to Scale)
NC = NO CONNECT
NC
V
DD
D+
D
NC
ADD1
GND
GND
NC
STBY
SCLK
NC
SDATA
ALERT
ADD0
NC
ADM1021A
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
P
S
P
t
HD;STA
t
SU;STA
t
SU;DAT
t
HIGH
t
F
t
HD;DAT
t
R
t
LOW
t
HD;STA
t
BUF
S
SCL
SDA
t
SU;STO
Figure 1. Diagram for Serial Bus Timing
LEAKAGE RESISTANCE M
20
15
25
100
TEMPERATURE ERROR
C
10
1
0
10
15
20
10
5
5
30
D+ TO GND
D+ TO V
DD
Figure 2. Temperature Error vs. PC Board Track Resistance
3
1
0
2
FREQUENCY Hz
100
TEMPERATURE ERROR
C
4
5
100M
1k
10k
100k
1M
10M
250mV p-p REMOTE
100mV p-p REMOTE
Figure 3. Temperature Error vs. Power Supply Noise
Frequency
5
4
3
1
0
2
FREQUENCY Hz
1
TEMPERATURE ERROR
C
10
1k
10k
10M
100M
6
7
8
9
100
100k
1M
50mV p-p
100mV p-p
25mV p-p
Figure 4. Temperature Error vs. Common-Mode Noise
Frequency
TEMPERATURE C
50
ERROR
C
60
0
70
80
100
120
3
1
1
2
2
90
110
DEV10
LOWER SPEC LEVEL
UPPER SPEC LEVEL
Figure 5. Temperature Error of ADM1021A vs.
Pentium III Temperature
CAPACITANCE nF
1
2
TEMPERATURE ERROR
C
12
14
4
6
8
10
12
14
16
18
20
22
24
0
2
4
6
8
10
Figure 6. Temperature Error vs. Capacitance Between
D+ and D
SCLK FREQUENCY kHz
1
SUPPLY CURRENT
A
20
0
V
DD
= 3.3V
5
10
25
50
75
100
1000
250
500
750
40
60
70
50
30
10
V
DD
= 5V
Figure 7. Standby Supply Current vs. Clock Frequency
ADM1021ATypical Performance Characteristics
4
REV. A
ADM1021A
5
REV. A
4
0
2
FREQUENCY Hz
TEMPERATURE ERROR
C
10mV p-p
100k
1M
10M
100M
1G
1
3
Figure 8. Temperature Error vs. Differential-Mode Noise
Frequency
CONVERSION RATE Hz
250
0.125
SUPPLY CURRENT
A
0.25
0.5
8
300
350
400
550
4
0.0625
450
500
200
150
100
50
5 VOLTS
3.3 VOLTS
2
1
Figure 9. Operating Supply Current vs. Conversion Rate
0
20
SUPPLY VOLTAGE V
0
SUPPLY CURRENT
A
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
40
60
80
100
20
Figure 10. Standby Supply Current vs. Supply Voltage
TIME Seconds
TEMPERATURE
C
0
25
50
75
100
125
REMOTE
TEMPERATURE
INT
TEMPERATURE
0
2
3
4
5
6
7
8
9
10
1
Figure 11. Response to Thermal Shock
FUNCTIONAL DESCRIPTION
The ADM1021A contains a two-channel A-to-D converter with
special input-signal conditioning to enable operation with remote
and on-chip diode temperature sensors. When the ADM1021A
is operating normally, the A-to-D converter operates in a free-
running mode. The analog input multiplexer alternately selects
either the on-chip temperature sensor to measure its local tem-
perature, or the remote temperature sensor. These signals are
digitized by the ADC and the results stored in the Local and
Remote Temperature Value Registers as 8-bit, two's comple-
ment words.
The measurement results are compared with Local and Remote,
High and Low Temperature Limits, stored in four on-chip regis-
ters. Out-of-limit comparisons generate flags that are stored in
the status register, and one or more out-of-limit results will
cause the
ALERT output to pull low.
The limit registers can be programmed, and the device con-
trolled and configured, via the serial System Management Bus.
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.
· Masking or enabling the
ALERT output.
· Selecting the conversion rate.
On initial power-up, the Remote and Local Temperature values
default to 128
°C. Since the device normally powers up converting,
a measurement of local and remote temperature is made and these
values are then stored before a comparison with the stored limits
is made. However, if the part is powered up in standby mode
(STBY pin pulled low), no new values are written to the register
before a comparison is made. As a result, both RLOW and LLOW
are tripped in the Status Register thus generating an ALERT out-
put. This may be cleared in one of two ways:
1. Change both the local and remote lower limits to 128
°C
and read the status register (which in turn clears the
ALERT output).
2. Take the part out of standby and read the status register
(which in turn clears the ALERT output). This will work
only if the measured values are within the limit values.
MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, or the base-emitter
voltage of a transistor, operated at constant current. Unfortu-
nately, this technique requires calibration to null out the effect
of the absolute value of V
BE
, which varies from device to device.
ADM1021A
6
REV. A
C1*
D+
D
REMOTE
SENSING
TRANSISTOR
I
N I
I
BIAS
V
DD
V
OUT+
TO ADC
V
OUT
BIAS
DIODE
LOWPASS FILTER
f
C
= 65kHz
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 2.2nF TYPICAL, 3nF MAX.
*
Figure 12. Input Signal Conditioning
The technique used in the ADM1021A is to measure the
change in V
BE
when the device is operated at two different
currents.
This is given by:
V
BE
= KT/q
× ln (N)
where:
K is Boltzmann's constant,
q is charge on the electron (1.6
× 10
19
Coulombs),
T is absolute temperature in Kelvins,
N is ratio of the two currents.
Figure 12 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, provided for tem-
perature monitoring on some microprocessors, but it could
equally well be a discrete transistor. If a discrete transistor is
used, the collector will not be grounded and should be linked to
the base. To prevent ground noise interfering with the measure-
ment, 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. If the sensor is operating in a noisy environment,
C1 may optionally be added as a noise filter. Its value is typically
2200 pF, but should be no more than 3000 pF. See the section
on layout considerations for more information on C1.
To measure
V
BE
, the sensor is switched between operating
currents of I and N
× I. The resulting waveform is passed through a
65 kHz low-pass filter to remove noise, then to a chopper-
stabilized amplifier that performs the functions of amplification
and rectification of the waveform to produce a dc voltage propor-
tional to
V
BE
. This voltage is measured by the ADC to give a
temperature output in 8-bit two's complement format. To fur-
ther reduce the effects of noise, digital filtering is performed by
averaging the results of 16 measurement cycles.
Signal conditioning and measurement of the internal temperature
sensor is performed in a similar manner.
DIFFERENCES BETWEEN THE ADM1021 AND THE
ADM1021A
Although the ADM1021A is pin-for-pin compatible with the
ADM1021, there are some differences between the two devices.
Below is a summary of these differences and reasons for the changes.
1. The ADM1021A forces a larger current through the remote
temperature sensing diode, typically 205
µA versus 90 µA
for the ADM1021. The main reason for this is to improve
the noise immunity of the part.
2. As a result of the greater Remote Sensor Source Current the
operating current of the ADM1021A is higher than that of
the ADM1021, typically 205 mA versus 160 mA.
3. The temperature measurement range of the ADM1021A is
0
°C to 127°C, compared with 128°C to +127°C for the
ADM1021. As a result, the ADM1021 should be used if
negative temperature measurement is required.
4
The power-on-reset values of the remote and local tempera-
ture values are 128
°C in the ADM1021A as compared with
0
°C in the ADM1021. As the part is powered up converting
(except when the part is in standby mode, i.e., Pin 15 is
pulled low) the part will measure the actual values of remote
and local temperature and write these to the registers.
5. The four MSBs of the Revision Register may be used to
identify the part. The ADM1021 Revision Register reads
0xh and the ADM1021A reads 3xh.
6. The power-on default value of the Address Pointer Register
is undefined in the ADM1021A and is equal to 00h in the
ADM1021. As a result, a value must be written to the Address
Pointer Register before a read is done in the ADM1021A.
The ADM1021 is capable of reading back local temperature
without writing to the Address Pointer Register as it defaulted
to the local temperature measurement register at power-up.
7. Setting the mask bit (Bit 7 Config Reg) on the ADM1021A
will mask current and future ALERTs. On the ADM1021
the mask bit will only mask future ALERTs. Any current
ALERT will have to be cleared using an ARA.
TEMPERATURE DATA FORMAT
One LSB of the ADC corresponds to 1
°C, so the ADC can
theoretically measure from 128
°C to +127°C, although the
device does not measure temperatures below 0
°C so the actual
range is 0
°C to 127°C. The temperature data format is shown in
Table I.
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.
ADM1021A
7
REV. A
Table III. List of ADM1021A Registers
READ Address (Hex)
WRITE Address (Hex)
Name
Power-On Default
Not Applicable
Not Applicable
Address Pointer
Undefined
00
Not Applicable
Local Temp. Value
1000 0000 (80h) (128
°C)
01
Not Applicable
Remote Temp. Value
1000 0000 (80h) (128
°C)
02
Not Applicable
Status
Undefined
03
09
Configuration
0000 0000 (00h)
04
0A
Conversion Rate
0000 0010 (02h)
05
0B
Local Temp. High Limit
0111 1111 (7Fh) (+127
°C)
06
0C
Local Temp. Low Limit
1100 1001 (C9h) (55
°C)
07
0D
Remote Temp. High Limit
0111 1111 (7Fh) (+127
°C)
08
0E
Remote Temp. Low Limit
1100 1001 (C9h) (55
°C)
Not Applicable
0F
1
One-Shot
10
Not Applicable
Reserved
Undefined
2
11
11
Reserved
Undefined
2
12
12
Reserved
Undefined
2
13
13
Reserved
Undefined
2
14
14
Reserved
Undefined
2
15
16
Reserved
Undefined
2
17
18
Reserved
Undefined
2
19
Not Applicable
Reserved
Undefined
2
20
21
Reserved
Undefined
FE
Not Applicable
Manufacturer Device ID
0100 0001 (41h)
FF
Not Applicable
Die Revision Code
0011 xxxx
(3xh)
NOTES
1
Writing to address 0F causes the ADM1021A to perform a single measurement. It is not a data register as such and it does not matter what data is written to it.
2
These registers are reserved for future versions of the device.
Table I. Temperature Data Format
Temperature
Digital Output
0
°C
0 000 0000
1
°C
0 000 0001
10
°C
0 000 1010
25
°C
0 001 1001
50
°C
0 011 0010
75
°C
0 100 1011
100
°C
0 110 0100
125
°C
0 111 1101
127
°C
0 111 1111
REGISTERS
The ADM1021A contains nine registers that 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, and further
details are given in Tables II to IV. It should be noted that the
ADM1021A's registers are dual port, and have different addresses
for read and write operations. Attempting to write to a read address,
or to read from a write address, will produce an invalid result.
Register addresses above 0Fh are reserved for future use or used
for factory test purposes and should not be written to.
Address Pointer Register
The Address Pointer Register itself does not have, nor does it
require, an address, as it is the register to which the first data
byte of every Write operation is written automatically. This data
byte is an address pointer that sets up one of the other registers
for the second byte of the Write operation, or for a subsequent
Read operation.
Value Registers
The ADM1021A has two registers to store the results of Local
and Remote temperature measurements. These registers are
written to by the ADC and can only be read over the SMBus.
Status Register
Bit 7 of the Status Register indicates that the ADC is busy con-
verting when it is high. Bits 5 to 3 are flags that indicate the
results of the limit comparisons.
If the local and/or remote temperature measurement is above the
corresponding high temperature limit or below the corresponding
low temperature limit, then one or more of these flags will be set.
Bit 2 is a flag that is set if the remote temperature sensor is open-
circuit. These five flags are NOR'd together, so that if any of them
is high, the
ALERT interrupt latch will be set and the ALERT
output will go low. Reading the Status Register will clear the five
flag bits, provided the error conditions that caused the flags to be
set have gone away. While a limit comparator is tripped due to a
value register containing an out-of-limit measurement, or the sen-
sor is open-circuit, the corresponding flag bit cannot be reset. A
flag bit can only be reset if the corresponding value register con-
tains an in-limit measurement, or the sensor is good.
Table II. Status Register Bit Assignments
Bit
Name
Function
7
BUSY
1 When ADC Converting.
6
LHIGH
*
1 When Local High Temp Limit Tripped.
5
LLOW
*
1 When Local Low Temp Limit Tripped.
4
RHIGH
*
1 When Remote High Temp Limit Tripped.
3
RLOW
*
1 When Remote Low Temp Limit Tripped.
2
OPEN
*
1 When Remote Sensor Open-Circuit.
10
Reserved.
*These flags stay high until the status register is read or they are reset by POR.
ADM1021A
8
REV. A
The
ALERT interrupt latch is not reset by reading the Status
Register, but 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.
Configuration Register
Two bits of the configuration register are used. If Bit 6 is 0, which
is the power-on default, the device is in operating mode with the
ADC converting. If Bit 6 is set to 1, the device is in standby
mode and the ADC does not convert. Standby mode can also
be selected by taking the
STBY pin low. In standby mode the val-
ues stored in the Remote and Local Temperature Registers remain
at the value they were when the part was placed in standby.
Bit 7 of the configuration register is used to mask the
ALERT out-
put. If Bit 7 is 0, which is the power-on default, the
ALERT output
is enabled. If Bit 7 is set to 1, the
ALERT output is disabled.
Table IV. Configuration Register Bit Assignments
Power-On
Bit
Name
Function
Default
7
MASK1
0 =
ALERT Enabled
0
1 =
ALERT Masked
6
RUN/STOP
0 = Run
0
1 = Standby
50
Reserved
0
Conversion Rate Register
The lowest three bits of this register are used to program the con-
version rate by dividing the ADC clock by 1, 2, 4, 8, 16, 32, 64
or 128, to give conversion times from 125 ms (Code 07h) to 16
seconds (Code 00h). This register can be written to and read back
over the SMBus. The higher five bits of this register are unused
and must be set to zero. Use of slower conversion times greatly
reduces the device power consumption, as shown in Table V.
Table V. Conversion Rate Register Codes
Average Supply Current
Data
Conversion/sec
µ
A Typ at V
CC
= 3.3 V
00h
0.0625
150
01h
0.125
150
02h
0.25
150
03h
0.5
150
04h
1
150
05h
2
150
06h
4
160
07h
8
180
08h to FFh
Reserved
Limit Registers
The ADM1021A has four limit registers to store local and remote,
high and low temperature limits. These registers can be written
to and read back, over the SMBus. The high limit registers
perform a > comparison while the low limit registers perform a
< comparison. For example, if the high limit register is programmed
as a limit of 80
°C, measuring 81°C will result in an alarm condi-
tion. Even though the temperature measurement range is from 0 to
127
°C, it is possible to program the limit register with negative
values. This is for backwards-compatibility with the ADM1021.
One-Shot Register
The one-shot register is used to initiate a single conversion and
comparison cycle when the ADM1021A is in standby mode,
after which the device returns to standby. This is not a data
register as such and it is the write operation that causes the one-
shot conversion. The data written to this address is irrelevant and
is not stored.
SERIAL BUS INTERFACE
Control of the ADM1021A is carried out via the serial bus. The
ADM1021A is connected to this bus as a slave device, under the
control of a master device.
ADDRESS PINS
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 will respond. The ADM1021A has two
address pins, ADD0 and ADD1, to allow selection of the device
address, so that several ADM1021A's can be used on the same
bus, and/or to avoid conflict with other devices. Although only
two address pins are provided, these are three-state, and can be
grounded, left unconnected, or tied to V
DD
, so that a total of
nine different addresses are possible, as shown in Table VI.
It should be noted that the state of the address pins is only sampled
at power-up, so changing them after power-up will have no effect.
Table VI. Device Addresses
ADD0
ADD1
Device Address
0
0
0011 000
0
NC
0011 001
0
1
0011 010
NC
0
0101 001
NC
NC
0101 010
NC
1
0101 011
1
0
1001 100
1
NC
1001 101
1
1
1001 110
ADD0, ADD1 sampled at power-up only.
The serial bus protocol operates as follows:
1. 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 will follow.
All slave peripherals connected to the serial bus respond to
the START condition and shift in the next eight bits, con-
sisting of a 7-bit address (MSB first) plus an R/
W bit, which
determines the direction of the data transfer, i.e., whether
data will 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 Acknowl-
edge 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 will write to the slave
device. If the R/
W bit is a 1, the master will read from the
slave device.
ADM1021A
9
REV. A
R/
W
A6
SCLK
SDATA
A5
A4
A3
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADM1021A
START BY
MASTER
1
9
1
9
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADM1021A
STOP BY
MASTER
1
9
SCL (CONTINUED)
SDA (CONTINUED)
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 3
DATA BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
ACK. BY
ADM1021A
Figure 13. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register
R/
W
A6
SCLK
SDATA
A5
A4
A3
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADM1021A
STOP BY
MASTER
START BY
MASTER
1
9
1
ACK. BY
ADM1021A
9
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2
ADDRESS POINTER REGISTER BYTE
Figure 14. Writing to the Address Pointer Register Only
R/
W
A6
SCLK
SDATA
A5
A4
A3
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
NO ACK.
BY MASTER
STOP BY
MASTER
START BY
MASTER
1
9
1
ACK. BY
ADM1021A
9
FRAME 1
SERIAL BUS ADDRESS BYTE
FRAME 2 DATA BYTE FROM ADM1021A
Figure 15. Reading Data from a Previously Selected Register
ADM1021A
10
REV. A
2. Data is sent over the serial bus in sequences 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, as a low-to-high transition when the
clock is high may be interpreted as a STOP signal. The num-
ber 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.
3. 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 10th clock pulse to assert a STOP condi-
tion. 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 10th clock pulse, then high
during the 10th 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 changed without
starting a new operation.
In the case of the ADM1021A, write operations contain either
one or two bytes, while read operations contain one byte and
perform the following functions:
To write data to one of the device data registers or read data
from it, the Address Pointer Register must be set so that the
correct data register is addressed, data can then be written into
that register or read from it. 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 reg-
ister selected by the address pointer register.
This is illustrated in Figure 13. The device address is sent over
the bus followed by R/
W set to 0. This is followed by two data
bytes. The first data byte is the address of the internal data reg-
ister to be written to, which is stored in the Address Pointer
Register. The second data byte is the data to be written to the
internal data register.
When reading data from a register there are two possibilities:
1. If the ADM1021A's Address Pointer Register value is unknown
or not the desired value, it is first necessary to set it to the
correct value before data can be read from the desired data
register. This is done by performing a write to the ADM1021A
as before, but only the data byte containing the register read
address is sent, as data is not to be written to the register.
This is shown in Figure 14.
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 15.
2. If the Address Pointer Register is known to be already at the
desired address, data can be read from the corresponding
data register without first writing to the Address Pointer
Register, so Figure 14 can be omitted.
NOTES
1. Although it is possible to read a data byte from a data register
without first writing to the Address Pointer Register, if the
Address Pointer Register is already at the correct value, it is
not possible to write data to a register without writing to the
Address Pointer Register, because the first data byte of a
write is always written to the Address Pointer Register.
2. Remember that the ADM1021A registers have different
addresses for read and write operations. The write address of
a register must be written to the Address Pointer if data is to
be written to that register, but it is not possible to read data
from that address. The read address of a register must be
written to the Address Pointer before data can be read from
that register.
ALERT OUTPUT
The
ALERT output goes low whenever an out-of limit mea-
surement is detected, or if the remote temperature sensor is
open-circuit. It is an open-drain and requires a 10 k
pull-up to
V
DD
. Several
ALERT outputs can be wire-ANDED together, so
that the common line will go low if one or more of the
ALERT
outputs goes low.
The
ALERT output can be used as an interrupt signal to a pro-
cessor, or it may be used as an
SMBALERT. Slave devices on
the SMBus can normally not signal to the master that they want
to talk, but the
SMBALERT function allows them to do so.
One or more
ALERT outputs are connected to a common
SMBALERT line connected to the master. When the SMBALERT
line is pulled low by one of the devices, the following procedure
occurs as illustrated in Figure 16.
MASTER
RECEIVES
SMBALERT
MASTER SENDS
ARA AND READ
COMMAND
DEVICE SENDS
ITS ADDRESS
NO
ACK
START
ALERT RESPONSE ADDRESS
RD
ACK
DEVICE ADDRESS
STOP
Figure 16. Use of
SMBALERT
1.
SMBALERT pulled low.
2. Master initiates a read operation and sends the Alert Response
Address (ARA = 0001 100). This is a general call address
that must not be used as a specific device address.
3. The device whose
ALERT output is low responds to the
Alert Response Address and the master reads its device
address. The address of the device is now known and it can
be interrogated in the usual way.
4. If more than one device's
ALERT output is low, the one with
the lowest device address, will have priority, in accordance with
normal SMBus arbitration.
5. Once the ADM1021A has responded to the Alert Response
Address, it will reset its
ALERT output, provided that the
error condition that caused the
ALERT no longer exists. If
the
SMBALERT line remains low, the master will send ARA
again, and so on until all devices whose
ALERT outputs were
low have responded.
ADM1021A
11
REV. A
LOW POWER STANDBY MODES
The ADM1021A can be put into a low power standby mode
using hardware or software, that is, by taking the
STBY input
low, or by setting Bit 6 of the Configuration Register. When
STBY is high, or Bit 6 is low, the ADM1021A operates nor-
mally. When
STBY is pulled low or Bit 6 is high, the ADC is
inhibited, so any conversion in progress is terminated without
writing the result to the corresponding value register.
The SMBus is still enabled. Power consumption in the standby
mode is reduced to less than 10
µA if there is no SMBus activ-
ity, or 100
µA if there are clock and data signals on the bus.
These two modes are similar but not identical. When
STBY is
low, conversions are completely inhibited. When Bit 6 is set but
STBY is high, a one-shot conversion of both channels can be initi-
ated by writing XXh to the One-Shot Register (address 0Fh).
SENSOR FAULT DETECTION
The ADM1021A has a fault detector at the D+ input that
detects if the external sensor diode is open-circuit. This is a
simple voltage comparator that trips if the voltage at D+ exceeds
V
CC
1 V (typical). The output of this comparator is checked
when a conversion is initiated, and sets Bit 2 of the Status Reg-
ister if a fault is detected.
If the remote sensor voltage falls below the normal measuring
range, for example due to the diode being short-circuited, the
ADC will output 128
°C (1000 0000). Since the normal operat-
ing temperature range of the device only extends down to 0
°C,
this output code will never be seen in normal operation, so it
can be interpreted as a fault condition.
In this respect, the ADM1021A differs from and improves upon
competitive devices that output zero if the external sensor goes
short-circuit. These devices can misinterpret a genuine 0
°C
measurement as a fault condition.
If the external diode channel is not being used and is shorted
out, the resulting
ALERT may be cleared by writing 80h (128
°C)
to the low limit register.
APPLICATIONS INFORMATION
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1021A is designed to work with substrate transistors
built into processors, or with discrete transistors. Substrate tran-
sistors will generally be PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN,
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+.
The user has no choice in the case of substrate transistors, but if
a discrete transistor is used, the best accuracy will be obtained
by choosing devices according to the following criteria:
1. Base-emitter voltage greater than 0.25 V at 6
µA, at the high-
est operating temperature.
2. Base-emitter voltage less than 0.95 V at 100
µA, at the lowest
operating temperature.
3. Base resistance less than 100
.
4. Small variation in h
FE
(say 50 to 150), which indicates tight
control of V
BE
characteristics.
Transistors such as 2N3904, 2N3906 or equivalents in SOT-23
package are suitable devices to use.
Thermal Inertia and Self-Heating
Accuracy depends on the temperature of the remote-sensing
diode and/or the internal temperature sensor being at the same
temperature as that being measured, and a number of factors
can affect this. Ideally, the sensor should be in good thermal
contact with the part of the system being measured, for example
the processor. If it is not, the thermal inertia caused by the mass
of the sensor will cause 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, as it will be 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, will often be remote from the
processor and will only be monitoring the general ambient tem-
perature around the package. The thermal time constant of the
QSOP-16 package is about 10 seconds.
In practice, the package will have electrical, and hence thermal,
connection to the printed circuit board, so the temperature rise
due to self-heating will be negligible.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and because
the ADM1021A is measuring very small voltages from the
remote sensor, care must be taken to minimize noise induced at
the sensor inputs. The following precautions should be taken:
1. Place the ADM1021A 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 be four to eight inches.
2. Route the D+ and D tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground plane
under the tracks if possible.
3. Use wide tracks to minimize inductance and reduce
noise pickup. 10 mil track minimum width and spacing
is recommended.
GND
D+
D
GND
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
Figure 17. Arrangement of Signal Tracks
4. Try to minimize the number of copper/solder joints, which
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 240
µV, and thermocouple voltages are
about 3
µV/°C of temperature difference. Unless there are
two thermocouples with a big temperature differential between
them, thermocouple voltages should be much less than 240
µV.
12
C0005605/01(A)
PRINTED IN U.S.A.
ADM1021A
REV. A
5. Place a 0.1
µF bypass capacitor close to the V
DD
pin and
2200 pF input filter capacitors across D+, D close to the
ADM1021A.
6. If the distance to the remote sensor is more than eight inches,
the use of twisted pair cable is recommended. This will work
up to about 6 to 12 feet.
7. For really long distances (up to 100 feet), use shielded twisted
pair such as Belden #8451 microphone cable. Connect the
twisted pair to D+ and D and the shield to GND close to
the ADM1021A. Leave the remote end of the shield uncon-
nected to avoid ground loops.
Because the measurement technique uses switched current sources,
excessive cable and/or filter capacitance can affect the measure-
ment. When using long cables, the filter capacitor may be
reduced or removed.
Cable resistance can also introduce errors. 1
series resistance
introduces about 1
°C error.
APPLICATION CIRCUITS
Figure 18 shows a typical application circuit for the ADM1021A,
using a discrete sensor transistor connected via a shielded, twisted
pair cable. The pull-ups 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 ADM1021A can be inter-
faced directly to the SMBus of an I/O chip. Figure 19 shows
how the ADM1021A might be integrated into a system using
this type of I/O controller.
V
DD
STBY
SCLK
SDATA
ALERT
ADD0
ADD1
GND
D+
D
0.1 F
ALL 10k
3.3V
TO CONTROL
CHIP
SET TO REQUIRED
ADDRESS
IN
OUT
I/O
C1*
SHIELD
2N3904
*C1 IS OPTIONAL
ADM1021A
Figure 18. Typical ADM1021A Application Circuit
USB USB
PROCESSOR
DISPLAY
SYSTEM BUS
DISPLAY
CACHE
ICH
I/O CONTROLLER
HUB
SYSTEM
MEMORY
GMCH
FWH
(FIRMWARE HUB)
ADM1021A
D+
D
ALERT
SCLK
SDATA
SUPER
I/O
SMBUS
PCI BUS
PCI SLOTS
2 USB PORTS
CD ROM
HARD
DISK
2 IDE PORTS
Figure 19. Typical System Using ADM1021A
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Lead Shrink Small Outline Package
(RQ-16)
16
9
8
1
0.197 (5.00)
0.189 (4.80)
0.244 (6.20)
0.228 (5.79)
PIN 1
0.157 (3.99)
0.150 (3.81)
SEATING
PLANE
0.010 (0.25)
0.004 (0.10)
0.012 (0.30)
0.008 (0.20)
0.025
(0.64)
BSC
0.059 (1.50)
MAX
0.069 (1.75)
0.053 (1.35)
0.010 (0.20)
0.007 (0.18)
0.050 (1.27)
0.016 (0.41)
8
0
PDF 
ZIP