Page 40 - DSP Integrated Circuits

1.7 Integrated Circuit Design 25

begin --Connect the ports of the components
SQ: Generator
port(x, y, z, u, v);
Si: Adder
port(x, y, z, u, v);
end Behavior_desc;

Box 1.3. VHDL description of a test bench for the full-adder

1.7 INTEGRATED CIRCUIT DESIGN

Integrated circuit technology has made it possible to produce chips with several
millions of transistors. However, complex circuits are difficult to design and call for
special design methodologies. For example, debugging a flawed VLSI chip is both
difficult and time consuming, since the turnaround time for design changes ranges
from several weeks to many months. Long design times may lead to lost opportu-
nities of marketing the chip ahead of the competition and recouping the invest-
ment. This forces batching of design changes and the use of design methods that
enforce perfect designs. Thus, the correctness of the design is of paramount impor-
tance for a successful project.
Generally, the degree of flexibility in the circuit design is very high. The
designer specifies the logic and circuit realization, physical placement, and even
the details of the individual gates and transistors. Despite this flexibility there are
limits in the ability, at one level of the design, to compensate for shortcomings at
higher levels. These limits come from inherent constraints in the technology (size,
power, and throughput), including the need to limit the addition of new complexity
at lower levels of the design. Performance is therefore an issue that must be
addressed at all levels of the design. However, experience indicates that design
decisions and optimizations at the higher levels in the design process are more
important than low-level optimizations.
The VLSI circuit design phase can begin when the system design phase has
been completed. In this phase the transistors and their interconnections are laid out
on the chip surface. The integrated circuit design phase can be more or less sophisti-
cated, from fully automatic chip generation, down to hand-crafted design of every
transistor. We stress that the system solution should be frozen and not allowed to be
changed during the layout phase. This is because changes in the design may intro-
duce errors. We will later discuss various design approaches in more detail.
VLSI circuit and system design processes are in many ways similar to tradi-
tional software design, but with the more stringent requirement of a "first-time-
right" design. Typically, the software design starts with an initial description that
is gradually refined until the desired goal is reached. CASE tools that are com-
monly used for the design of large and complex software may also be used for the
system design phase. The basic incremental design philosophy is quite general,
but there is one important difference—as the VLSI and system design move down-
ward in the hierarchy, qualitative changes occur in the models. For example, an
electrical-level model may not be obtained just by an incremental expansion of a
behavioral model of the corresponding logic module. Conversely, in software all of