Page 19 - DSP Integrated Circuits

4 Chapter 1 DSP Integrated Circuits

Early standard digital signal processors were based on the Harvard architec-
ture that has two buses and separate memories for data and instructions. Gener-
ally, standard digital signal processors are provided with MACs—multiplier-
accumulators—in order to perform sum-of-product computations efficiently. The
high performance of these processors is achieved by using a high degree of paral-
lelism. Typically, a multiply-and-add, data fetch, instruction fetch and decode, and
memory pointer increment or decrement can be done simultaneously. Typical
drawbacks are the limited on-chip memory size and the relatively low I/O band-
width. The architectures used in modern standard digital signal processors will be
further discussed in Chapter 8.
Early signal processors used fixed-point arithmetic and often had too short
internal data word length (16 bits) and too small on-chip memory to be really effi-
cient. Recent processors use floating-point arithmetic which is much more expensive
than fixed-point arithmetic in terms of power consumption, execution time, and chip
area. In fact, these processors are not exclusively aimed at DSP applications. Appli-
cations that typically require floating-point arithmetic are SD-graphics, multimedia,
and mechanical CAD applications. Fixed-point arithmetic is better suited for DSP
applications than floating-point arithmetic since good DSP algorithms require high
accuracy (long mantissa), but not the large dynamic signal range provided by float-
ing-point arithmetic. Further, problems due to nonlinearities (rounding of products)
are less severe in fixed-point arithmetic. Hence, we conclude that the current gener-
ation of standard signal processors is not efficient for many DSP applications.

1.4 APPLICATION-SPECIFIC ICs FOR DSP

The development effort for a large integrated circuit typically ranges between 1
and 10 man-years, depending on the uniqueness of the function, performance con-
straints, and the availability and performance of design tools. The combined
advances in system design capability and VLSI technology have made it possible
to economically design unique integrated circuits for use in dedicated applications,
so-called application-specific integrated circuits (ASICs) [14]. This option makes
new innovative system solutions practical.
The possibility of incorporating a whole signal processing system into one chip
has a multitude of effects. It will dramatically increase the processing capacity and
simultaneously reduce the size of the system, the power consumption, and the pin-
restriction problem, which may be severe when a system has to be implemented
using several chips. Reliability will also increase when the number of pins and the
working temperature of the chips are reduced. Although VLSI technology solves or
circumvents many problems inherent in older technologies, new limits and draw-
backs surface. The main problems originate from the facts that the systems to be
designed tend to be very complex and are often implemented in the most advanced
VLSI process available. The latter has the adverse effect that the system often must
be designed by using untested building blocks and incomplete and unproved CAD
tools. Because of the innovative and dynamic nature of DSP techniques, the design
team often lacks experience, since a similar system may not have been designed
before. These factors make it difficult to estimate accurately the time it will take for
the whole design process up to the manufacture of working chips.