A Microprocessor-Based Digital Thermometer

Michael Holley

EEL 411

Seattle University

January 26, 1981

------------

This project was conducted without any outside assistance.

- -~----

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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• •

TEMPERATURE TRANSDUCERS

• OFFSET • CODE • IN

• PROBE

+2V

-25llA

FIGURE 2

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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

APPENDIX

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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