Page 32 - DSP Integrated Circuits

1.6 DSP System Design 17

hierarchy of abstractions upon the design. In this way, the system is systemati-
cally decomposed into regular and modular blocks. In practice, however, a meet-in-
the-middle approach is often used. In this approach, which is illustrated in Figure
1.17, the specification-synthesis process is carried out in essentially a top-down
fashion, but the actual design of the building blocks is performed in a bottom-up
fashion. The design process is therefore divided into two almost independent parts
that meet in the middle. The circuit design phase can be shortened by using effi-
cient circuit design tools or even automatic logic synthesis tools. Often, some of the
building blocks are already available in a circuit library.

SYSTEM DESIGN

Figure 1.17 Meet-in-the-middle design approach

1.6.3 Design Transformations

Note that Figure 1.15 not only shows how the subsystem design process is parti-
tioned into several phases or levels of abstraction, it also shows that each level has
a specification and a synthesis phase. In fact, the whole design process consists of
alternating specification and synthesis operations. The result of the synthesis at a
particular level, i.e., an implementation at that level, acts as a specification for the
next lower level.
At each level in the design process, the representation of the system is refined
and transformed into a more detailed, lower-level representation, as illustrated in
Figure 1.18. The transformation of a representation from one design level to a lower
level is called synthesis. Generally, the downward transition between two levels is a
one-to-many mapping, since the synthesis process adds information to the design.