1.4. Design Methodologies
Digital design depends on the type of problem, the work already completed, the strategic direction of the
organization and the skills/resources available to the project team. Having said that, in general, there are
three approaches available to the designers under traditional Hierarchical-Oriented Designs:
Top-down Design Methodology
Start with larger block of design and then work out the detail of each block. Bottom-up Design Methodology
Start with components and figure out how to interconnect them to design the system. Middle-out Design Methodology
A combination of the bottom-up and top-down. Most designs are done this way: start with the
top-down design, then modify the design to take advantage of the available components (based
on cost, availability, and reliability).Another way of thinking about the problem of design that has a strong following in the software
development community and is being used in the hardware community under the module design concept
is Object-Oriented Design (OOD).
Designers commonly agree that there are four main properties or benefits associated with object-oriented
design:
Encapsulation
As the name implies, the internals of the design are hidden from the user and only the interface
definition (input/output) are available to the user. Users benefit since they have a limited amount
of information to learn. Designers benefit since they are able to upgrade the module without
involving the user as long as the new interface is a superset of an existing interface.
Inheritance
This simply means that an object may be built on the features available in the base object
property. Of course, the benefit is that the designers only have to work on the additional feature
and simply reuse the existing functionality. Polymorphism
OOD allows the designer to create objects that behave differently based on the attributes of input. Composition (One object can be built using many others.)
A new object may be developed based on the composition of multiple existing objects.Hopefully, at this point you are thinking “why wouldn’t everyone use OOD?” The main drawback of
OOD is the high level of planning required for each module, and discipline needed to follow the four
properties in design.