TB3016

TB3016
Using the PIC® MCU CTMU for Temperature Measurement
Author:
Padmaraja Yedamale
Microchip Technology Inc.
The Charge Time Measurement Unit (CTMU), introduced on the latest generation of PIC24F and PIC18F
devices, uses a constant current source to calculate
both capacitance changes and the time difference
between events. The same current source can also be
used to measure temperature by exploiting a basic
principle of semiconductor physics. This allows the use
of a common and inexpensive diode, in the place of a
relatively more expensive thermistor or other temperature sensor. This brief describes the basic concepts of
temperature measurement using the CTMU.
IMPLEMENTATION
To implement this theory, all that is needed is to connect a regular junction diode to one of the microcontroller’s A/D pins (Figure 1). The A/D channel
multiplexer is shared by the CTMU and the ADC.
To perform a measurement, the multiplexer is configured to select the pin connected to the diode. The
CTMU current source is then turned on, and an A/D
conversion is performed on the channel. As shown in
the equivalent circuit diagram, the diode is driven by
the CTMU at IF. The resulting VF across the diode is
measured by the ADC.
FIGURE 1:
CTMU TEMPERATURE
MEASUREMENT CIRCUIT
BASIC PRINCIPLE
We can show that the forward voltage (VF) of a P-N
junction, such as a diode, is an extension of the
equation for the junction’s thermal voltage:
Simplified Block Diagram
PIC® microcontroller
Current Source
I
kT
V F = ------ ln ⎛ 1 – ----F-⎞
⎝
q
IS ⎠
where k is the Boltzmann constant (1.38 x 10-23 J K-1),
T is the absolute junction temperature in kelvin, q is the
electron charge (1.6 x 10-19 C), IF is the forward current
applied to the diode, and IS is the diode’s characteristic
saturation current.
Since k and q are physical constants, and IS is a constant for the device, this only leaves T and IF as independent variables. If IF is held constant, it follows from
the equation that VF will vary as a function of T. As the
natural log term of the equation will always be negative,
the temperature will be negatively proportional to VF. In
other words, as temperature increases, VF decreases.
CTMU
A/D Converter
ANx
A/D
MUX
VF
Equivalent Circuit
By using the CTMU’s current source to provide a
constant IF, it becomes possible to calculate the
temperature by measuring the VF across the diode.
CTMU
ADC
IF
VF
© 2009 Microchip Technology Inc.
DS93016A-page 1
TB3016
EXPERIMENTAL VALIDATION
Resolution (expressed as temperature per ADC
counts) is calculated as:
To test the theory, several devices with simple P-N
junctions were tested in a controlled temperature environment while measuring VF as previously described.
Included in the testing were three common silicon
diodes, two common bipolar transistors, and two LEDs.
An additional trial was run with two diodes (1N914)
connected in parallel as a single unit. Each device was
evaluated using an ADC voltage reference (VREF) of
3.3V.
Temperature was varied from 0°C to 105°C inclusive,
with 256 conversions being taken at roughly 5°C intervals over this range. The ADC readings (proportional to
voltage) were recorded for each temperature point.
These readings were used to directly calculate resolution, and converted to voltage to calculate line slope.
TABLE 1:
Number of samples × Temperature range
-------------------------------------------------------------------------------------------------------------------------------------Counts at min temperature – Counts at max tempertaure
The results of the trial are summarized in Table 1, and
presented in graphic form in Figure 2. As can be seen,
the correlation between temperature and VF is negative. Also as expected, the relationship between temperature and the forward voltage on the junction is
essentially linear. This makes it possible for any readily
available diode – or for that matter, any inexpensive
semiconductor – to function as a low-resolution
temperature sensor in conjunction with the CTMU.
EXPERIMENTAL VF VALUES (AS ADC COUNTS) FOR DIFFERENT DEVICES AS A
FUNCTION OF TEMPERATURE
ADC Readings (256 samples)
Min. Temp
Max. Temp
Resolution
(°C/Count)
1N4007
36,890
12,815
1.12
1N4148
32,500
8,980
1.15
-2.8 mV/°C
1N914
31,500
6,417
1.08
-2.98 mV/°C
2N3904 (NPN)
45,100
23,560
1.26
-2.56 mV/°C
2N3906 (PNP)
43,860
22,020
1.24
-2.60 mV/°C
SML-LXT0805GW-TR (Green LED)
52,500
27,150
1.07
-3.01 mV/°C
CML 5311F (Red LED)
54,280
32,265
1.23
-2.62 mV/°C
Two 1N914 (parallel)
31,500
6,660
1.09
-2.95 mV/°C
Component
DS93016A-page 2
Slope
-2.88 mV/°C
© 2009 Microchip Technology Inc.
TB3016
FIGURE 2:
ADC VALUES AS A FUNCTION OF TEMPERATURE FOR TESTED DEVICES
60000
1N4007
1N4148
1N914
50000
LED-D22
2N3904(NPN)
ADC count (x256)
LED-D27
40000
2N3906(PNP)
2|| 1N914
30000
20000
10000
0
0
20
40
60
80
100
120
Temperature (C)
INCREASING TEMPERATURE
MEASUREMENT RESOLUTION
The method described here is adequate for resolution
of about 1°C. In most cases, this represents an A/D
channel voltage change of about 3 mV. At this scale,
attempts to get better resolution will run into the limitations of the A/D converter. To achieve higher temperature resolutions, some minor changes to the
conversion method are needed. These include:
• Using a lower reference voltage for the ADC. One
significant determining factor in temperature resolution is the selection of VREF. Smaller values of
VREF tend to produce a larger difference voltage
to be converted; this produces a larger incremental reading per degree, and thus higher resolution.
Table 2 shows the expected temperature resolution for the same experiment, assuming an ADC
VREF of 2.0V.
• Using two diodes in series. Although this does not
increase resolution per se, the resulting doubling
of the change in the measured voltage per unit of
temperature will result in increased accuracy.
• Adding a single stage of voltage amplification with
an op amp. By increasing the voltage to the ADC
and matching it to the ADC voltage reference, resolution is increased. Although this adds several
external components and some cost to the solution, this may be desirable in applications where a
more precise determination of temperature is
required.
© 2009 Microchip Technology Inc.
TABLE 2:
EXPECTED TEMPERATURE
RESOLUTION FOR VREF OF
2.0V
Resolution (°C/Count)
Component
VREF = 3.3V
(observed)
VREF = 2.0V
(predicted)
1N4007
1.12
0.68
1N4148
1.15
0.69
1N914
1.08
0.66
2N3904
1.26
0.76
2N3906
1.24
0.75
SML-LXT0805GW-TR
1.07
0.64
CML 5311F
1.23
0.74
Two 1N914 (parallel)
1.09
0.66
DS93016A-page 3
TB3016
CONCLUSION
For applications using a PIC18F or PIC24F microcontroller with the CTMU, adding a temperature function
does not depend on the use of a special temperature
sensor; it can be done using a commodity diode and a
small addition of code to the application firmware. This
makes the incremental cost of adding the additional
feature very small indeed.
REFERENCES
PIC24F Family Reference Manual, Section 11, “Charge
Time Measurement Unit (CTMU)” (DS39724).
Microchip Technology Inc., 2008.
DS93016A-page 4
© 2009 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
devices in life support and/or safety applications is entirely at
the buyer’s risk, and the buyer agrees to defend, indemnify and
hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights.
Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
dsPIC, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro,
PICSTART, rfPIC, SmartShunt and UNI/O are registered
trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
FilterLab, Linear Active Thermistor, MXDEV, MXLAB,
SEEVAL, SmartSensor and The Embedded Control Solutions
Company are registered trademarks of Microchip Technology
Incorporated in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, CodeGuard,
dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN,
ECONOMONITOR, FanSense, In-Circuit Serial
Programming, ICSP, ICEPIC, Mindi, MiWi, MPASM, MPLAB
Certified logo, MPLIB, MPLINK, mTouch, PICkit, PICDEM,
PICDEM.net, PICtail, PIC32 logo, PowerCal, PowerInfo,
PowerMate, PowerTool, REAL ICE, rfLAB, Select Mode, Total
Endurance, WiperLock and ZENA are trademarks of
Microchip Technology Incorporated in the U.S.A. and other
countries.
SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2009, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received ISO/TS-16949:2002 certification for its worldwide
headquarters, design and wafer fabrication facilities in Chandler and
Tempe, Arizona; Gresham, Oregon and design centers in California
and India. The Company’s quality system processes and procedures
are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping
devices, Serial EEPROMs, microperipherals, nonvolatile memory and
analog products. In addition, Microchip’s quality system for the design
and manufacture of development systems is ISO 9001:2000 certified.
© 2009 Microchip Technology Inc.
DS93016A-page 5
WORLDWIDE SALES AND SERVICE
AMERICAS
ASIA/PACIFIC
ASIA/PACIFIC
EUROPE
Corporate Office
2355 West Chandler Blvd.
Chandler, AZ 85224-6199
Tel: 480-792-7200
Fax: 480-792-7277
Technical Support:
http://support.microchip.com
Web Address:
www.microchip.com
Asia Pacific Office
Suites 3707-14, 37th Floor
Tower 6, The Gateway
Harbour City, Kowloon
Hong Kong
Tel: 852-2401-1200
Fax: 852-2401-3431
India - Bangalore
Tel: 91-80-3090-4444
Fax: 91-80-3090-4080
India - New Delhi
Tel: 91-11-4160-8631
Fax: 91-11-4160-8632
Austria - Wels
Tel: 43-7242-2244-39
Fax: 43-7242-2244-393
Denmark - Copenhagen
Tel: 45-4450-2828
Fax: 45-4485-2829
India - Pune
Tel: 91-20-2566-1512
Fax: 91-20-2566-1513
France - Paris
Tel: 33-1-69-53-63-20
Fax: 33-1-69-30-90-79
Japan - Yokohama
Tel: 81-45-471- 6166
Fax: 81-45-471-6122
Germany - Munich
Tel: 49-89-627-144-0
Fax: 49-89-627-144-44
Atlanta
Duluth, GA
Tel: 678-957-9614
Fax: 678-957-1455
Boston
Westborough, MA
Tel: 774-760-0087
Fax: 774-760-0088
Chicago
Itasca, IL
Tel: 630-285-0071
Fax: 630-285-0075
Cleveland
Independence, OH
Tel: 216-447-0464
Fax: 216-447-0643
Dallas
Addison, TX
Tel: 972-818-7423
Fax: 972-818-2924
Detroit
Farmington Hills, MI
Tel: 248-538-2250
Fax: 248-538-2260
Kokomo
Kokomo, IN
Tel: 765-864-8360
Fax: 765-864-8387
Los Angeles
Mission Viejo, CA
Tel: 949-462-9523
Fax: 949-462-9608
Santa Clara
Santa Clara, CA
Tel: 408-961-6444
Fax: 408-961-6445
Toronto
Mississauga, Ontario,
Canada
Tel: 905-673-0699
Fax: 905-673-6509
Australia - Sydney
Tel: 61-2-9868-6733
Fax: 61-2-9868-6755
China - Beijing
Tel: 86-10-8528-2100
Fax: 86-10-8528-2104
China - Chengdu
Tel: 86-28-8665-5511
Fax: 86-28-8665-7889
Korea - Daegu
Tel: 82-53-744-4301
Fax: 82-53-744-4302
China - Hong Kong SAR
Tel: 852-2401-1200
Fax: 852-2401-3431
Korea - Seoul
Tel: 82-2-554-7200
Fax: 82-2-558-5932 or
82-2-558-5934
China - Nanjing
Tel: 86-25-8473-2460
Fax: 86-25-8473-2470
Malaysia - Kuala Lumpur
Tel: 60-3-6201-9857
Fax: 60-3-6201-9859
China - Qingdao
Tel: 86-532-8502-7355
Fax: 86-532-8502-7205
Malaysia - Penang
Tel: 60-4-227-8870
Fax: 60-4-227-4068
China - Shanghai
Tel: 86-21-5407-5533
Fax: 86-21-5407-5066
Philippines - Manila
Tel: 63-2-634-9065
Fax: 63-2-634-9069
China - Shenyang
Tel: 86-24-2334-2829
Fax: 86-24-2334-2393
Singapore
Tel: 65-6334-8870
Fax: 65-6334-8850
China - Shenzhen
Tel: 86-755-8203-2660
Fax: 86-755-8203-1760
Taiwan - Hsin Chu
Tel: 886-3-572-9526
Fax: 886-3-572-6459
China - Wuhan
Tel: 86-27-5980-5300
Fax: 86-27-5980-5118
Taiwan - Kaohsiung
Tel: 886-7-536-4818
Fax: 886-7-536-4803
China - Xiamen
Tel: 86-592-2388138
Fax: 86-592-2388130
Taiwan - Taipei
Tel: 886-2-2500-6610
Fax: 886-2-2508-0102
China - Xian
Tel: 86-29-8833-7252
Fax: 86-29-8833-7256
Thailand - Bangkok
Tel: 66-2-694-1351
Fax: 66-2-694-1350
Italy - Milan
Tel: 39-0331-742611
Fax: 39-0331-466781
Netherlands - Drunen
Tel: 31-416-690399
Fax: 31-416-690340
Spain - Madrid
Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
UK - Wokingham
Tel: 44-118-921-5869
Fax: 44-118-921-5820
China - Zhuhai
Tel: 86-756-3210040
Fax: 86-756-3210049
02/04/09
DS93016A-page 6
© 2009 Microchip Technology Inc.