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[8] T. Sticht, "Some relationships of mental aptitude, reading ability, and
listening ability using normal and time-compressed speech," J. Commun., vol. 18, pp. 243-258, Sept. 1968.
[9] W. De L'Aune et al., "Speech compression: Personality correlates of
successful use," Vis. Impair. Blind., vol. 71, pp. 66-70, Feb. 1977.
[10] J. Voor, "The effects of practice upon comprehension of timecompressed speech," thesis, University of Louisville, Louisville, KY,
1962.
Application of Top-Down Principles to Digital
System Design
DAVID J. COMER
Abstract-The concept of top-down design of programs is directly
transferable to digital system design. This approach is not only useful in
practical design, it is an excellent vehicle to use in teaching digital design
classes. Top-down design of digital systems is considered here along with an
example of this method.
I. INTRODUCTION
Since 1968 when the American Society for Engineering Education published the "Goals of Engineering Education" [1],
electrical engineering programs have attempted to offer more
design-oriented courses. This report, based on a study of engineering programs throughout the United States, recognized a serious
lack of design courses in the typical engineering curriculum.
Students were being trained to analyze a system or circuit rather
than develop the system or circuit to meet a set of specifications.
The "Goals of Engineering Education" report recommended that
this curriculum deficiency be corrected.
Since design methods encompass analysis methods, design is
more time consuming than analysis. Time is required to first
develop a good analysis capability in the student and then proceed
to the design problem. Furthermore, analysis methods generally
lead to a single answer, whereas design methods may have several
equally good answers. The conciseness of analysis appeals to an
instructor in an already overcrowded curriculum which continues
to absorb new topics.
While industry includes engineering jobs that depend almost
solely on analysis, a high precentage of jobs require the application
of design methods. If a program is to truly prepare a graduate for
industry, design principles must be treated in enough depth to be
useful to the student. The method described in this paper provides
a framework for teaching digital system design.
III. ANALYSIS VERSUS DESIGN
In elementary school we are given analysis problems such as the
following. Mother buys three cans of corn at 27 cents per can, two
loaves of bread at 58 cents per loaf, four quarts of milk at 30 cents
per quart, and 2 pounds of steak at $2.65 per pound. How much
money was spent on groceries?
This type of problem serves a useful purpose in teaching a child
how math can be used. It has a single answer that immediately
measures a person's ability to apply the required mathematics.
Furthermore, design methods are generally based on analysis;
thus, it is logical to teach analysis prior to design.
A design problem based on this same example might read as
follows. Mother has $10 with which to purchase groceries. She
must buy enough food to serve three people and she desires to
include items from the four basic food groups. Given the following
price list for food, construct an appropriate shopping list.
Obviously, this problem has many answers depending on the
length of the price list, the rate of sales tax, the appetites of the
people involved, and other factors. This design problem is more
closely related to a real world problem than is the analysis problem.
It is a situation that mother may face daily and its solution requires
an ability to analyze. Generally, it is solved by a trial-and-error
approach, and may require several trials to reach an acceptable
answer.
Analysis can be considered to be an important tool in solving a
design problem. A designer must fully understand analysis before
he can effectively apply design methods. However, the use of this
tool should not be emphasized over the design procedure in digital
system classes.
Trial-and-Error Methods in Design
Trial-and-error methods are often used in digital design just as
they might be used to solve the previous food purchase example. A
trial system is proposed, then this system is analyzed. By comparing the results of analysis to the specifications, an error can be
evaluated. If this error is too large, the trial system is modified to
decrease the error. Unfortunately, several trials may be required to
achieve acceptable results, making the time expended excessive.
While it is perhaps impossible to eliminate trial-and-error
methods, a good design procedure should minimize such methods
in producing a reliable system.
IV. TOP-DOWN DESIGN
The term "top-down design" has grown out of the programming area and relates closely to the concept of structured programming. Niklaus Wirth, the developer of the Pascal programming
language, provides the following definition of structured programming [2]. "Structured programming is the formulation of programs as hierarchical, nested structures of statements and objects
of computation." Implied in this definition is a decomposition or
II. DEFINITION OF DIGITAL DESIGN
breaking down of a large problem into component parts. These
The word "design" is defined by one dictionary to be ". an parts are then decomposed into smaller problems with this
underlying scheme or plan that governs the development of an successive refinement continuing until each remaining task can be
entity . . ." This definition can be expanded to apply to digital implemented by simple program statements or a series of statesystem design: a digital design is a plan, often expressed in ments. As each statement or structure is executed, a part of the
schematic or block diagram form, that governs the development of overall objective is accomplished.
The program is decomposed into units called modules. These
a digital system.
To be effective, digital design methods should allow sound modules are decomposed into control structures or series of
techniques to be applied to the development of digital systems. statements. The statement is the basic unit of the program.
A very significant point in applying top-down programming is
These techniques must allow the designer to produce a reliable
system and to minimize at least some resources such as design time, that the modules should be selected to result in minimum
interaction between these units. In general, complexity can be
component cost, production time, or production cost.
reduced with the weakest possible coupling between modules.
Manuscript received March 14, 1983; revised June 6, 1983.
Departmnent of Electrical Engineering, Brigham
The author is with the
Young University, Provo, UT 84602.
Minimizing connections between modules also minimizes the
paths along which changes and errors can propagate into other
parts of the system [3].
0018-9359/83/1100-0170$01.00 © 1983 IEEE
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IEEE TRANSACTIONS ON EDUCATION, VOL. E-26, NO. 4, NOVEMBER 1983
171
While complete independence is impossible since all modules
must be harnessed to perform the overall program objective,
interaction is minimal in a well-designed program.
V. TOP-DOWN DESIGN OF DIGITAL SYSTEMS
When applied to digital systems, top-down design proposes a
broad abstract unit to satisfy the given system function. This
function will be characterized by a set of specifications. The overall
system is then decomposed into modules. These modules are
partitioned in such a way to be as independent of each other as
possible, but working together will satisfy the overall system
function. Just as in programming where higher levels of structure
contain the major decision-making or control elements of the
program [3], one of the partitioned modules will be the control unit
of the system. This control module is not decomposed further at
this time. Successive refinement of all other modules leads to
decomposition into operational units. These units will often be
MSI circuits or groups of circuits. Refinement continues, with an
accompanying increase in detail, until all sections can be implemented by known devices or circuits.
After all modules except the control module have been reduced to
circuits, the time relationship of the required control signals can be
specified. A major difference between top-down programming and
top-down system design should be emphasized at this point. In a
von Neumann architecture, the control unit is forced to execute
instructions sequentially to complete a program. In a digital
system, several modules or operational units can be functioning
simultaneously. As long as the control module can provide
appropriate signals to all circuits, parallel operation is generally
preferred in order to achieve higher system efficiency.
Once the timing diagram has been generated, the control module
is designed. There are two major methods of contructing this
module. The first uses conventional state machine design techniques to produce a controlling state machine [4]. This results in an
all hardware system unless a ROM-controlled state machine is
used to create a firmware system. The second method consists of
using a microprocessor control unit. When this is done, the
microprocessor can use software to absorb some of the functions
performed by other operational units. In this case, the control unit
and other components of the system may be replaced by the
Fig. 1. Functional modules.
Fig. 2. Operational units.
DVM must perform the measurement function and display the
results. A control module is required to direct the operation of the
measurement and display modules.
Fig. I reflects a high level of abstraction. At this step of the
design, almost no detail appears. Modules indicate only broad
functions to be performed with no indication relative to how those
functions should be carried out. To proceed to the next step, the
designer must choose an approach. In this example, the designer
might choose to use a dual-slope technique for the measurement
module, three seven-segment LED displays for the display module,
and a state machine for the control module.
All modules except the control module are then decomposed
into operational units as shown in Fig. 2. It is at this point that the
microprocessor.
inexperienced designer or student experiences the most frustraThere are certain characteristics ofthe digital field that make top- tion. There is no plug-in procedure that can be used here. This step
down design an easy method to apply. For example, the large requires creativity along with trail-and-error methods to complete.
number of SSI and MSI circuits available allows operational units
Fig. 2 represents a more detailed picture of the system. Modules
and even modules to be realized directly with chips. These chips are have now been broken down into units that perform specific
generally designed to require a minimal number of control inputs operations, hence the name operational units. Each of these units
and thus, minimal interaction between modules is automatically can be implemented by a circuit or series of circuits using wellachieved, at least to an extent.
known design procedures or presently available IC chips. The
Another advantage of the top-down method in digital design is comparator, counter, code converter/driver, and seven-segment
that the control module can be realized as a state machine. This displays are off-the-shelf items. The electronic switch and integraallows well-known design procedures to be used in designing a tor may also be purchased or designed specifically to meet the given
reliable control unit.
specifications.
The major advantage, however, is that this method provides a
Fig. 3 shows some of the basic circuits that may be used to make
framework for carrying out a design problem. Although creativity up the counting, code converting, and display units. The system is
is required to apply this method, the structured approach directs now completely specified with the exception of the control unit.
the effort to the simplest units or procedures. Trial-and-error
A timing chart is constructed, showing the relationship between
techniques are minimized using top-down design.
all necessary control signals, inputs, and outputs. The controlling
state machine is then designed by conventional methods [4] to
VI. AN EXAMPLE
produce the control signals necessary to drive the basic circuits.
In order to illustrate the approach, we will consider the design of This completes the design process.
a 2-1 / 2 digit digital voltmeter (DVM). Obviously, this problem has
VII. EXTENSION OF METHOD TO MICROPROCESSOR DESIGN
been solved many times over by equipment manufacturers, but it
The method proposed in the preceding paragraphs can easily be
relates this well-known design to the use of the top-down
extended to a microprocessor-based system. Once the decomposiapproach.
The problem statement would begin with a set of specifications tion to operational units is completed, the diagram is checked to
that the DVM should meet. After a study of these specifications, find the number of operations that can be performed by the
the DVM can be decomposed into a set of modules as shown in Fig. microprocessor. Since the controlling state machine can always be
1. These modules are chosen according to function performed. The implemented by a microprocessor, we can implement the control
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IEEE TRANSACTIONS ON EDUCATION, VOL. E-26, NO. 4, NOVEMBER 1983
172
simple DVM, but we will consider this example for illustrative
purposes. If used for the DVM, the microprocessor could implement the control module, the counting operation, and the code
conversion required for display. These functions would be performed by software while the electronic switch, integrator, analog
comparator, and display would remain as hardware units.
7490 DECRDE COUNTER
7447R CODE CONVERTER
Fig. 3. Basic circuits.
unit and any other operations that the microprocessor can perform
in software. The remaining operational units that cannot be
absorbed by the microprocessor are decomposed into basic
circuits. Again, a timing chart is generated and the microprocessor
is programmed to accomplish the necessary internal operations
and provide control signals for the external circuitry.
Although it is beyond the scope of this article, a comparison of
cost effectiveness between the conventional and microprocessorbased systems must be carried out before the final configuration is
chosen. It would not be cost effective to use a microprocessor for a
VIII. CONCLUSIONS
The idea of top-down design can be carried over from the
programming area to the digital design area and provides a vehicle
for teaching design methods. The approach offers a set of
guidelines that directs the student's efforts toward specific realizable goals. The concepts of successive refinement and minimal
interaction between units can be visualized easily when chips are
used for the system implementation. The method can be extended
to microprocessor-based digital system design.
REFERENCES
[I] Goals Committee, "Goals of engineering education," J. Eng. Educ., vol.
58, p. 438, Jan. 1968.
[2] N. Wirth, "On the composition of well-structured programs," Comput.
Surveys, vol. 6, pp. 247-259, Dec. 1974.
[3] V. R. Basili and F. T. Baker, Structured Programmming: Integrated
Practices, L-os Alamitos, CA: IEEE Computer Society Press, 1981.
[4] W. I. Fletcher, An Engineering Approach to Digital Design. Englewood Cliffs, NJ: Prentice-Hall, 1980.
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