Sample Design Report




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This web page presents a sample design report [Herwald, 1999] written in a microprocessor design course at Virginia Tech. Note that instructors of other design courses may have different expectations as far as the format (layout and typography) and style (structure, language, and illustration) of design reports in their classes. Note that in this report, carets (>) are given to reveal the line spacings in the report's format (in an actual report, these carets would not appear). Moreover, in this report, the actual appendices are not complete. In an actual report, these appendices would be complete, and each would begin on a separate page. In the actual report, the following typeface sizes were used: 14-point type for the title, 12-point type for the subheadings and text, and 10-point type for the figure captions.

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Design of a Temperature Measurement and Display System
Using the 68HC11 Microcontroller
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Executive Summary
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This report presents the design of a temperature measurement and display system that uses the Motorolla HC11 microcontroller. This design makes use of the HC11 analog-to-digital converter and the serial subsystems. Temperature measurement and display circuits were built and control software was written to use the added hardware. In this design, the overall objectives were met. By keeping track of the measured temperature, the HC11 is able to control a temperature display that uses light emitting diodes. Also, if the temperature becomes very cold or hot, an alarm message is sent to a host PC terminal. This design has many potential applications, including temperature control and factory automation.
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Introduction
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This semester, our design team is creating a design using the Motorolla 68HC11 microcontroller, referred to here as the HC11. Before beginning this design project, we are performing a series of preliminary designs and tests on this microcontroller. This report presents one of these preliminary designs: the design of a temperature measurement and display system.

This preliminary design makes use of the HC11 analog-to-digital (A/D) converter and the serial subsystems. These components are shown in Figure 1. The hardware added for the preliminary design includes a temperature measurement circuit connected to one of the HC11 A/D input pins on Port E, and light emitting diodes (LEDs) connected to each pin of Port B. These LEDs act as temperature indicators. Additionally, the design includes a connection between the HC11 and a remote personal computer (PC) using an RS-232 connection. This connection serves to send messages to the PC regarding temperature. An assembly software program developed for this design performs various functions for using the added hardware.

This report breaks down the preliminary design into two sections. In the first section, separate hardware and software designs are presented. The hardware designed for section one includes the temperature measurement circuit and the connection of LEDs to Port B. The software designed for section one initializes the A/D converter and serial subsystems, controls the measurement and storage of the temperature in a RAM variable called TEMP, and controls the display of temperature on the LED outputs.

The second section of the report presents the connection of the HC11 to a remote PC terminal through an RS-232 connection. After adding the software required to initialize the serial subsystem, subroutines, which are added to the software program developed for the report's first section, allow the HC11 to send a message to the PC if the measured temperature falls below 20 degrees or above 90 degrees.

Figure 1. Temperature measurement and display system developed for the Motorolla 68HC11 microcontroller, which is attached to a universal evaluation board (EVBU).
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Discussion of Design and Testing
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This section presents a description of the design for this first preliminary design. This description is broken into sections: (1) connecting a temperature measurement circuit to the HC11 microcontroller; and (2) adding a serial output to the HC11 microcontroller.
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Connecting a Temperature Measurement Circuit to the HC11. Connecting a temperature measurement circuit to the HC11 microcontroller involved both hardware and software. Hardware was added to control the measurement and display of the temperature. This hardware included a temperature measurement circuit and LEDs attached to Port B (the temperature measurement circuit is shown in Figure A-1 in Appendix A). The circuit was designed according to the specifications obtained from the Computer Engineering Laboratories website for ECPE 4535 [Lineberry, 1998]. Within the circuit is an LM3911 temperature controller integrated circuit (IC), the output of which is connected into a non-inverting opamp. The output of the opamp connects to the HC11 A/D input pin E2 through a 1000-ohm resistor. The circuitry is scaled so that 0 volts out corresponds to 0 degrees and 5 volts out corresponds to 110 degrees.

To each of the output pins of Port B, LEDs are connected using a 74HC244 buffer IC and 330-ohm current limiting resistors as shown in Appendix A. The LEDs are located in the breadboard area of the trainer kits.

To control this added hardware, the HC11 was programmed following the pseudo code, flow chart, and program listing given in Appendices B, C, and D, respectively. The program shown in Appendix D consists of three subroutines that are called from MAIN. The three subroutines are named STARTUP, GETTEMP, and SETDISP.

The STARTUP subroutine is used to enable the A/D converter subsystem, which is connected to Port E. First the A/D is powered up by setting bit 7 of the OPTION register. After a 100 microsecond delay, the control word $22 is written to the ADCTL register to start continuous, single-scan conversions on Port E pin E2.

The subroutine GETTEMP is used to input and scale the analog voltage from the temperature sensor circuit. The register ADR3 holds the result of the A/D conversions. This result is loaded into accumulator A and then multiplied by a scale factor contained in accumulator B by using the MUL instruction. The result contained in accumulator A is then right shifted once giving the temperature in degrees Fahrenheit, which is then stored in the RAM variable TEMP. The equation for finding TEMP is shown in (1).

TEMP = ADR3 ( Scale Factor >> 1) (1)

The subroutine SETDISP controls the lighting of the LEDs connected to Port B based on the present value of TEMP. First, TEMP is loaded into accumulator A and compared with the value 20, the designated cut-off for low temperature. Accumulator B is cleared to zero and represents the initial count value for the number of LEDs to turn on. If the value in accumulator A is greater than or equal to 20, then the count in accumulator B is incremented and 10 is subtracted from accumulator A. The process then repeats as long as the value in accumulator A is greater than or equal to 20. This process is shown in flowchart form in Figure 2 (the complete process appears in Figure C-1 of Appendix C). After the number of LEDs to turn on have been determined as shown in Figure 2, the number of bits indicated by the count value in accumulator B are set high on Port B beginning with bit 0.

Figure 2. Flowchart illustrating the determination of the number of Port B bits to enable for the LED display.
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To test the operation the GETTEMP and SETDISP subroutines, the actual temperature was measured with a temperature probe and compared with the measured value represented by the LED display indicators at several different temperature settings. Table 1 shows the results of the measurement comparison, where the actual temperatures measured are shown on the left, and the temperatures represented by the number of LEDs lit are shown on the right. From Table 1, it is verified that the developed hardware and software for this part of the lab were functioning properly.

Table 1. Comparison of temperature measurements.
Actual Temperature Number of LEDs Lit
15*F 0
28*F 1
33*F 2
56*F 4
110*F 8

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Adding Serial Output to the HC11. This section presents the addition of three subroutines to the existing software developed in the previous section. The added subroutines, listed in Appendix D, are called OUTSCI, MESSOUT, and j-&j-&.. j-&j-&(c)ABREP~1TXT t-&t-&LABORA~1HTM D-&D-&TPTMODELDOC (r)L-&(r)L-&-OTES4~1DOC NRD&NRD&-ut2.htmlSCANQADATOTEXTUm6D؜D؜LABHAN~1HTM1Trash TEXTdosa@F$PF$PTRASH labreport2.htmlH.VTEXTMSWD>D"D"(tm)O:ABREP~1HTMfailure IBMBIO COIBMDOS COMUdata byte to be sent is contained in accumulator A. The subroutine checks that the TDRE bit is set in the SCSR register, and if it is, the data byte in accumulator A is written to the SCDR register. If the TDRE bit is not set, the program continually reads the SCSR register until bit TDRE is set before writing accumulator A to the SCDR register.

The subroutine MESSOUT utilizes the OUTSCI subroutine to write character strings to the remote PC terminal. Before calling MESSOUT, the X index register is set to point to the beginning of the character string to be sent. The MESSOUT subroutine then sends out the string by calling OUTSCI for each character until the NULL character is reached, which marks the end of a string.

The third and final subroutine TEMPCHK was added to the existing software program to check the temperature range. The subroutine TEMPCHK calls MESSOUT to print the message "Temperature is very low.\n" if TEMP is less than 20 degrees Fahrenheit. If TEMP is greater that 90 degrees Fahrenheit, TEMPCHK calls MESSOUT to print the message "Temperature is very high.\n" A flag variable called FLG ensures that the messages are not repeatedly sent for each entry into the very hot or very cold temperature regions. FLG is set to zero if TEMP is between 20 and 90 degrees and to one otherwise.
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Conclusions
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This preliminary design report has discussed the development of a temperature measurement and display system. Temperature measurement and display circuits were built and control software was written to use the added hardware. By keeping track of the measured temperature, the HC11 was able to control an LED temperature display. Also, if the temperature became very cold or hot, an alarm message was sent to a host PC terminal. In this preliminary design, the overall objectives were met.

Although the objectives were met, several mistakes and difficulties were encountered while performing the preliminary design presented. The initial setup of the serial subsystem of the 68HC11 involved some troubleshooting. We also had problems with sending the alarm messages more than one time because a flag variable was not set. The diagnosis and solutions to these problems are discussed in this section.

Initially, the serial writes from the 68HC11 to the host PC did not work properly because the SP was not initialized and the data string was not terminated properly. With an incorrect SP, the function calls to the OUTSCI subroutine did not work as expected, and in fact caused the program to crash. By correctly loading the SP, the OUTSCI subroutine worked as expected. We also had a problem sending out messages using MESSOUT because we did not terminate the message strings correctly with the NULL zero. By adding the NULL zero to the end of the strings, the sending of messages worked as expected.

A final problem was the output rate of the alarm messages. At first, we did not set a flag to indicate to the program that a message had already been sent to the PC. This failure caused messages to be continually sent to the PC terminal when the temperature was outside of the normal operating region. This problem was fixed by making a variable called FLG that was set as soon as the alarm message was sent and then cleared when the temperature returned to the normal operating region.

This preliminary design has introduced us to the important topics of A/D conversion and serial communications. The use of the A/D converter has allowed access to analog inputs from within the computer. Besides temperature measurement, the use of an A/D converter has many potential applications in automatic control systems and factory automation. For example, in an electric motor drive, the phase currents and flux are continually measured by using scaling circuitry and an A/D converter input to a microprocessor or DSP. We also expect to use serial communications in our final project to send data to a host PC monitor program that can modify and display important variables in the 68HC11.
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References
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Lineberry, Bob, "Computer Engineering Laboratories Website at Virginia Tech," http://www.ee.vt.edu/cel (Blacksburg, VA: ECE Department, 1998).
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Spasov, Peter, Microcontroller Technology: The 68HC11, 2nd ed. (Englewood Cliffs, NJ: Prentice Hall, 1996).

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Appendix A: Hardware Schematic
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Figure A-1. Hardware schematic for the temperature measurement circuit designed for this lab. In an actual report, all the connections, pin numbers, and pin labels should be shown.


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Appendix B: Pseudo Code for the Software Developed
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XXXXXXXXXXXXXXXXXX*
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*In an actual report, the pseudo code would appear here.


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Appendix C: Program Flow Chart
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Figure C-1. Flow chart for the lab program. In an actual report, you would fill in the blocks.


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Appendix D: Program Listing
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Program Header
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include statements
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Equate Statements
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RAM var defs
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ORG $B600
LDS #$1FF
JSR STARTUP ;Initialize A/D and SCI, init RAM vars
MAIN JSR GETTEMP ;Set RAM var TEMP to current temperature
JSR SETDISP ;Update LED display
JSR TEMPCHK ;Check to see if hot or cold message should be sent
BRA MAIN ;Repeat

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Subroutine Listings
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Interrupt Service Routines
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