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A Microprocessor-Based Digital Thermometer
Michael Holley
EEL 411
Seattle University
January 26, 1981
------------
ABSTRACT
This independent study project was to build a
temperature measurement system that incorporated a
microprocessor for enhanced performance. Before the project
was approved, a list of performance specifications and a
completion schedule were defined. Design and construction
required 80 hours with an additional 160 hours to complete
the software. The time schedule was met and the project
completed in December 1980.
The temperature measurement system is an 8 channel
digital thermometer that may run unattended, making readings
at a selected rate. It might be used to monitor the
temperatures at various points on a piece of equipment or in
laboratory process. The system could send the measured
temperatures once an hour to a printer or remote computer.
A Motorola MC6802 microprocessor was used because a
development system was available. The writing of ~he
operating software (30 pages of assembly language) without a
development system would have taken much more than 160
hours. Hardware - software debug is also faster with a
development system.
TABLE OF CONTENTS
I INTRODUCTION •..••....•....•..•....•.•• 1
II SYSTEM REQUIREMENTS ..•..•............• 2
III DESIGN OVERVIEW... • • • • • • • • • • • • • • . • • . . • 3
IV TEMPERATURE TRANSDUCER •...•••••....•.. 5
V ANALOG TO DIGITAL SYSTEM ..•.••.••..•.. 7
VI MICROPROCESSOR CONTROLLER ••••••••••••• 10
VII SYSTEM SOFTWARE •••.•.••••••••••••.•.•• 12
VIII RESULTS AND SU~~RY •.••••••••••••.•••• 15
REFERENCES • • • • • • • • • • • • • • • • • • • • • • • • • • • • 16
BIBLIOGRAPHY. . . . . . . . • • • • • • • . • • . . • • • • . • 17
LIST OF FIGURES
1 DIGITAL THERMOMETER BLOCK DIAGRAM ••.•• 4
2 ANALOG TO DIGITAL SYSTEM •••••••.•••.•• 8
APPENDIX
SUMMARY OF PROGRAMS ••••••••••••••••••• A1
SUMHARY OF COM.MANDS .....•...........•. A2
SYSTEM OPERATION EXAMPLE ..•....•..••.. A3
CONTROLLER SCHEMATICS ••••••••••••••••. AS
ANALOG SCHEMATICS ••.••••.•••.••...•.•. A8
CH..:Z\SSIS SCHEMATIC •••••.••..•••••..... A 10
I. INTRODUCTION
One of the major oversights in new product design is
thermal dissipation. A package the size of a shoe box may
hold ten large scale integrated circuits, a battery with
charger, a printer, and a display; and consume 40 watts of
power. Without proper design, the internal package
temperature may rise 40°C over ambient temperature. An
often omitted design check is thermal analysis of the
product over the specified temperature range.
Testing the product's operation is a time consuming
task. In the past, a technician would manually record the
temperature and other parameters; today, costs prohibit this
practice. Without some automated method of testing, the new
product testing is abbreviated, and the product is sent into
the field with a hope and a prayer.
To accomplish this testing, a microprocessor-controlled
digital thermometer was developed. The new product testing
may be done, unattended, using this digital thermometer and
other recording instruments. The number of testing hours in
a week increases from 40 to 168.
1
-~~~--~------~-- ----
II. SYSTE~ REQUIREMENTS
OVERVIEW: The device proposed is a 8 channel temperature
monitor. The device must be able to read selected channels
with selected delays between readings and shall be
controlled by a remote computer or a remote terminal. The
device shall make readings upon demand or run unattended,
making readings at the selected rate.
OPERATION: The Temperature Monitoring System shall be
programmed with the following commands.
1. Read a single channel upon demand.
2. Set period of delay for use in continuous read mode.
3. Select channels to be read in the continuous read mode.
4. Select Fahrenheit or Celsius scale.
5. Enter the continuous read mode.
6. Reset or exit continuous read mode.
The communication with the remote computer or terminal
shall be with serial ASCII using a RS-232 interface. The
bit rate shall include 110, 300, and 1200 baud. The
operator shall be able to install the temperature
transducers and the transducers shall be interchangeable
between channels without recalibration.
2
,.- ·.
III. DESIGN OVERVIEW
A controller is required to meet the operational
specifications. This could be done with a small desk-top
computer or with a small microprocessor system. The
microprocessor has a lower hardware cost, but a higher
software cost than the desk-top computer. The software
development is a one time cost, so if the thermometer were
to be put into production the microprocessor would be the
choice.
There are three types of low cost temperature
transducers; thermocouples, thermistors, and semiconductor
junctions. The thermocouples are the only transducer that
will operate at high temperatures (2000°C), but this
thermometer doesn't require that range. All three are
nonlinear, but the semiconductor can be linearized with
circuitry on the same piece of silicon. With the signal
conditioning performed at the transducer high quality relays
are not required for multiplexing the transducers.
The block diagram of the digital thermometer is shown
in Figure 1. A microprocessor is interfaced to the
operator's terminal, an analog to digital converter (A/D),
and the probe selector. Each block gives a description, a
part number, and the address on the microprocessor bus.
3
MICROPROCESSOR HC6802
RAH 128 Byte part of 6802 0000 - 007F
ROM 2K Byte MCM2716 F800 - FFFF
TD1.ER MC6840 6000 - 6007
SERIAL I/0 MC6850 8004 - 8006
DIGITAL THERMOMETER BLOCK DIAGRAM
TEIDUNAL .. ...
PARALLEL I/0 MC6821 "~~------~ COOO - C001
A TO D MC14433
PARALLEL I/O
C002 - C003
TEMPERATURE PROBES Analog Devices AD-590
Figure 1 Digital Thermometer
-_f
1 OF 8 SWITCH
4
IV. TEMPERATURE TRANSDUCER
A two-terminal integrated circuit (AD-590) that acts as
a temperature-dependent current source was chosen for the
system temperature transducer [1]. This transducer passes
1pA/•K over a range of 218°K to 423°K (-55°C to 150°C) with
a supply voltage of 4 to 30 volts. The fact that the output
is a current, and the transducer is insensitive to the
terminal voltage, makes the transducer easy to use, even
over hundreds of feet of wire.
The transducer uses a fundamental property of the
silicon transistor to produce a voltage proportional to
temperature. That is, the difference between the
base-emitter voltage of two transistors operating at a
constant ratio of collector current densities will be
directly proportional to absolute temperature. The voltage
is equal to (kT/q) (ln r); where k, Boltzman's constant, q,
the charge of an electron, and r, the ratio of current
densities, are constants [2].
The temperature dependent voltage is converted to a
current by low temperature coefficient thin film resistors.
The transducer's output current is a multiple of this
0 0 current. The scale factor of 1 pA/ K and the offset at 25 C
are adjusted by a laser trim process of the resistors while
the IC is still in wafer form. After final assembly, 100
percent of the devices are tested at -55°C, 25°C, and 125°C;
5
over one-half of the units have calibration errors of less
the 1° C [ 3] •
The voltage compliance (+4V to +30V) and the reverse
blocking charactertic {-20V) of the transducer allows it to
be powered from +SV CMOS logic. This eliminates the need
for expensive low-thermal relays; low cost CMOS analog
multiplexers may be used to switch the transducer's output.
The transducer is available in unpackaged chip form, in
two transistor style packages, or in a stainless steel
tubular probe. A transducer in a T0-52 transistor case with
an accuracy of 2.0°C, a repeatability of 0.1°C, and a
nonlinearity of 0.8°C costs about $5. The accuracy
specification is after the user corrects the error at a
single temperature {25°C).
6
V. ANALOG TO DIGITAL SYSTEM
The analog to digital system consist of a probe
selector, a current to voltage converter, and an analog to
diqital converter. Figure 2 is a block diagram of the
analog to diaital system
The temperature transducers have a reverse blocking
characteristic (similiar to a diode) that permits easy
multiplexing. All of the positive terminals are connected to
the voltage-to-current converter while the negative
terminals are connected to a -5V source through a CMOS
1-of-8 analog switch. Only one transducer has power
applied, and the other seven are effectively out of the
circuit.
Selection of the transducer is controlled by three data
lines from the system controller interface.
Each temperature transducer has an unique calibration
offset that must be corrected. Using diode encoding, the
offset error is stored in the probe's connector. A probe
may be connected to any channel and the system controller
will read that probe's correct offset error. The range of
errors that may be coded is +/- 8.5 degrees with 0.5 degree
resolution.
The "Kelvin" current is changed to a "Celsius" current
by summing a 273.2 microamp current of opposite polarity.
7
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. --- - . -sv 1
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298.2 llA I
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• •
TEMPERATURE TRANSDUCERS
• OFFSET • CODE • IN
• PROBE
+2V
-25llA
FIGURE 2
)
+o.2sv
CURRENT TO VOLTAGE
------------
EOC
A/D • • 8 •
3 LINES
CONTROLLER INTERFA CE
... ...
... ... 5 LINES • • ... ....
~
ANALOG TO DIGITAL SYSTEM
)
cD
This reduces the magnitude of the analog signal that must be
digitized. (ie 298.4 degrees Kelvin equals 25.2 degrees
Celsius.) An operational amplifier (op-amp) is used to
convert the output current of the temperature transducer to
a voltage. The output of the op-amp is 10 mV per degree
Celsius so the -55 to +150 degree input yields a -0.55 to
+1.50 volt output.
The system performance specifications require an A/D
with 10 or more bits resolution. Either successive
approximation or dual-slope integrating A/D converters would
provide this 10 bit resolution. The dual-slope converter is
inherently noise-immune because the input signal is
conditioned by an integrator. The integration period slows
the converter but speed isn't important in a temperature
measurement.
A 3 1/2 digit (12 bit) dual-slope converter was
selected (MC14433) [4]. A+/- 2.000 volt full scale input
range and a 4 conversions per second rate was chosen. The
converter's output consist of 4 BCD data lines, 4 digit
select lines and a end of conversion strobe.
9
------------------------------'--- ----~-~-------~ ---~--------
VI. MICROPROCESSOR
The heart of the system controller is a 8-bit
microprocessor, a MC6802 [5]. Upon command, the
microprocessor will select the desired transducer, read the
analog to digital converter, process the digitized reading,
and send the temperature reading to the operator's terminal.
A block diagram of the entire system is shown in Figure 1.
The controller was designed for low volume production,
with the option of using a single chip microcomputer,
MC6801, if high volume production was desired [6]. At
present, the controller requires five major IC's that could
be replaced by a single custom programmed microcomputer IC.
Although the MC6800 series microprocessor was used, any of
the popular 8-bit microprocessors would meet the system
requirements.
For data storage, the microprocessor has 128 bytes of
read/write Random Access Memory (RAM). The control program
is stored in a 2K byte Erasable Programmable Read Only
Memory (EPROM) that allows easy program modification. If
the future program requirements exceed the 2K byte EPROM
(MCM2716), a 4K byte EPROM (MCM2732) may be used [7] [8].
The programmable timer (MC6840) has three 16-bit binary
counters, that may be used for interval measuring, event
counting, or generating output signals [9]. Counter 1
generates a 4800 Hertz square wave for the 300 baud serial
--------~--~ --~--
1 0
-
interface. To use a 110 baud or 1200 baud terminal, the
counter may be reprogrammed by a simple software command.
Counter 3 utilizes a selectable divide by eight prescaler to
produce a 1 Hertz output by counting the 1 MHz system clock.
The 1 Hertz signal is used by the system for timekeeping.
The delay, in seconds, between temperature readings is
controlled by counter 2.
An Asynchronous Communications Interface Adaptor
(ACIA), MC6850, provides the data formatting and control for
the serial interface [10]. The 8-bit parallel data of the
microprocessor system bus is serially transmitted and
received by the ACIA with proper formatting and error
checking. The TTL levels of the ACIA are translated to
RS-232 levels by external buffers.
A Peripheral Interface Adapter (PIA), MC6821, is used
to interface the analog to digital converter (A/D) and the
probe selector to the microprocessor [11]. The PIA has two
8-bit bidirectional data ports (A and B) and four control
lines. Each line of the data port may be programmed as an
input or an output. The A/D converter's "end of conversion"
strobe is connected to a PIA control input, while all 8
lines of port A are used to input the A/D's readings. Three
output lines of Port B select the temperature probe while
the other 5 lines input the probe offset code.
1 1
VII. SOFTWARE
The key to a successful microprocessor system is the
operating software. The interchangeability of the
temperature probes, the transducer error correction and
other system enhancements are due to software, not
additional hardware. The entire operating system was
written in assembly language, and requires only 1.5K bytes
of ROM and 100 bytes of RAM. The program consists of three
modules; a monitor, the thermometer program, and a math
library.
The monitor module handles system initialization, the
input-output routines, and system debug. On a power-up or
system reset, the peripheral interface IC's must be
initialized. For example, one section of the timer needs to
be programmed to produce the correct baud rate for the
serial interface. The other software modules route all
input and output through the monitor routines. This way the
input could be changed from the terminal to a local keypad
with one software change to the monitor. If each
sub-routine handled input-output, many software changes
would be required. The monitor also has memory examine and
change functions that aid in hardware and software
troubleshooting.
The digital thermometer module contains all the command
functions and a routine to call the functions. When the 11 T 11
~~ ~----- ----
12
command is entered, the "Read Single Temperature"
sub-routine is executed, then the program waits for the next
command. The commands are explained in the System Operation
Appendix.
The math routines simulate a simple 5-digit calculator.
The calculator has a 3 register stack, addition and
subtraction, multiplication and division by 10, and
binary-decimal conversions. The math operations use 6-byte
Binary Coded Decimal (BCD) registers, but storage is done
with a 3-byte packed BCD form. Negative decimal numbers are
represented in the ten's complement form [12]. The 16-bit
binary conversions are needed to communicate with some
peripheral interfaces, such as the programmable timer.
13
The reading from the analog to digital converter is the
"Celsius" voltage without the offset correction. This
reading is entered on the stack along with the probe offset.
An addition operation yields the corrected Celsius
temperature. Conversion to Fahrenheit requires a
multiplication and an addition, equation 1. To reduce the
program length, full multiplication was omitted; only
decimal shifting is used, equation 2.
F = C X 1.8 + 32 (1)
F = ( ( C+C) X 1 0 - ( C+C) + 3 2 0) I 1 0 ( 2 )
If future applications require full multiplication, the
routines can be added; presently this limited form meets all
requirements.
14
VIII. RESULTS AND SUMI4ARY
All the design goals and specifications given in
section II were met. The conversion rate of the A/D was
adjusted to 4 readings per second. With a 18 readings per
second rate, the system didn't meet the repeatability
specification. Since the slower rate met all requirements,
no futher rates were tried. As a performance check, four
probes were compared with a laboratory grade glass
thermometer (ASTM 63-C) in a water bath. Over a range of
0 0 0 C to 30 C the probes tracked each other and the
thermometer to within 1°C. In still air the probe's
self-heating caused about a 1 degree temperature rise.
This digital thermometer demonstrates the improvements
in temperature logging that are possible with solid state
temperature transducers and microprocessor controllers.
With automated measurement equipment, testing may be
conducted unattended; greatly reducing the test cost.
The next project for this thermometer will be the
monitoring of the room temperatures in a house. A real-time
clock with calender is being added to the system along with
an input to monitor the furnace activity. The information
collected will be used to design a
microprocessor-controlled thermostat for the house.
15
16
REFERENCES
[1] Data Acquisition Products Catalog. Norwood, MA: Analog
Devices, Inc., 1979, pp. 85S-92S.
[2] M. P. Timko, "A Two-Terminal IC Temperature Transducer,"
IEEE J. Solid-State Circuits, vol. SC-11, pp. 784-788,
Dec. 1976.
[3] Analog Devices, Inc., private correspondence, Dec. 15,
1978.
[4] Hotorola CMOS Data. Austin, Texas: Motorola, Inc.,
1978, pp. 7-244 to 7-255.
[5] The Complete Microputer Data Library. Phoenix:
Motorola, Inc., 1978, pp. 1-36 to 1-55.
[ 6] Ref. [ 5] , pp. 1-6 4 to 1-7 2.
[ 7] Ref • [ 5 ] , pp • 5-7 8 to 5- 8 3 •
[8] Component Data Catalog. Santa Clara, CA.: Intel Corp.,
1978, pp. 4-44 to 4-54.
[9] Ref. [5], pp. 1-107 to 1-118.
[10] Ref. [5], pp. 1-211 to 1-218.
[ 11 ] Ref • [ 5] , pp. 1-9 0 to 1-1 0 0 •
[12] M6800 Microprocessor Applications Manual. Phoenix:
Motorola, Inc., 1975, pp. 2-6 to 2-10.
------ ----- ~--·- ----·---
---· BIBLIOGRAPHY
R. c. Camp, T. A. Smay, and C. J. Triska, Microcomputer
Systems Principles. Portland, OR: Matrox Publishers, 1978.
Complete Microputer Data Library. Phoenix: Motorola, Inc.,
1978.
Component Data Catalog. Santa Clara, CA.: Intel Corp.,
1978.
Data Acquisition Products Catalog. Norwood, 11A: Analog
Devices, Inc., 1979.
N.otorola CMOS Data. Austin, Texas: Motorola, Inc., 1978,
M6800 Microprocessor Applications Manual. Phoenix:
Motorola, Inc., 1975.
A. Osborne, s. Jacobson, and J. Kane, An Introduction to
Microcomputers. Vol. II. Berkeley, CA: Osborne and
Associates, 1977.
M. P. Timko, "A Two-Terminal IC Temperature Transducer,"
IEEE J. Solid-State Circuits, vol. SC-11, pp. 784-788,
Dec. 1976.
17
SUMMARY OF PROGRAMS
DIGITAL THERMOMETER PROGRAM
FBOO to FAAD 685 Bytes
COMMAND LOOP Select Fahrenheit or Celsius Read single channel Set delay between readings Select channels to be read Start continuous reading
MATH ROUTINES
FCOO to FDB4 436 Bytes
BINARY CODED DECIMAL ROUTINES
Clear Registers Push nwnber on stack (ENTER) Add two numbers Multiply register by 10 Divide register by 10 Complement register (negative numbers)
Convert binary to BCD Convert BCD to binary
MONITOR ROUTINES
FEOO TO FF61 353 Bytes
Initialize system Input characters Output characters Print messages Examine and change Memory Jump to another program
RESTART VECTORS
------- --- ·---
A1
Digital Thermometer Commands
A - Enter the thermometer program
T N - N=0-7. Read a single channel.
D N - N=10-9999. Set delay in seconds.
S xxxxxxxx - x=1 or 0. Select channels to be read in continuous read mode. ie. S 11100001 will turn on channels 0,1,2,7, the other w~ll be skipped.
M x - x=C or F. Select Fahrenhiet or Celsius mode
C - Enter continuous read mode
Control C - Exit continuous read mode
X - Exit thermometer program
Monitor Commands
hhhh - a four digit hex address the user enters. HHHH - a four digit hex address the monitor prints. dd - a two digit hex data byte the user enters. DD - a two digit hex data byte the monitor prints.
M hhhh DD dd - Examine the data at location hhhh, If the data is to be changed enter two hex digits. If the data is correct just enter a space or any non hex character (ie 0-9 A-F).
N HHHH DD dd - This will examine the next memory location.
B HHHH DD dd - This will back up the memory pointer and examine the memory location before.
V HHHH DD dd - This examines the same memory location.
J hhhh - Jump to the address given.
A2
---------------
,__
-
The following section describes the op~ration of the digital thermometer. The temperature transducers may be clamped or epoxied in position on the unit under test. The probes are then connected to the rear panel of the controller. Any probe may be connected to any channel; the calibration correction is encoded in the probe's connector. The operator controls the digital thermometer from the keyboard of a video or printing terminal.
DIGITAL THERMOMETER OPERATION EXAMPLE
TERMINAL DISPLAY COMMENTS
M F Fahrenheit Mode
T 1 +073.7 Read channel 1
M c Celsius Mode
T 1 +023.2 Read channel 1
D 30 30 second delay between readings
s 11001000 Select channels 0,1, and 5
c Enter continuous reading
TEMPERATURE IN C 000000 Total time elapsed
0 +023.6 1 +023.3 5 +037.4
000030 Next set of readings 0 +023.5 1 +023.2 5 +038.1
000060 60 Seconds elapsed 0 +023.5 1 +023.3 5 +038.9
(Control C) Break reading cycle
T 0 +023.6 Read single channel
A3
PERFORMANCE:
1. Temperature measurement range -25 to +135 degrees C.
2. Temperature accuracy of +/- 3 degrees c.
3. Temperature repeatability of +/- 0.3 degrees C.
4. Calibration cycle of 6 months or greater.
5. Measurement time for 8 channels 10 seconds or less.
6. Delay range of 10 seconds to 1 hour.
7. Delay accuracy of +/- 0.025% per day.
8. Temperature transducers shall operate up to 100 feet
from control unit.
9. The temperature transducer shall operate in air or in
contact with materials at ground potential. The transducer
shall have a still ·air thermal time constant of 60 seconds
or less.
10. The temperature transducer shall have a replacement
cost of $50 or less.
11. The environment for the control unit shall be from 0 to
50 degrees C.
12. The power available is 120 volts, 60 cycle, 5 watts.
A4
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