Page 20 - DSP Integrated Circuits

1.4 Application-Specific ICs for DSP 5

Characteristic for DSP is the short step from basic research and innovation to
practical applications. Therefore, a strong incentive exists to keep trade and
design secrets from the competitors. This is to some extent possible, at least for a
reasonably long time (months), if they are put into an application-specific inte-
grated circuit. The cumulative effect is that the total system cost tends to be low
and the performance gain provides an incentive to develop application-specific
integrated circuits, even for low-volume applications.

1.4.1 ASIC Digital Signal Processors

In order to overcome some of the drawbacks
discussed previously, considerable effort has
been invested in developing CAD tools for the
design of specialized digital signal processors.
Generally, these processors are designed (pre-
programmed) to execute only a fixed or limited
set of algorithms, and cannot be reprogrammed
after manufacturing. Typically only some
parameters in the algorithms can be set by the
user. These signal processors are called applica-
tion-specific signal processors. A signal proces-
sor that can only execute a single algorithm is
sometimes referred to as an algorithm-specific
signal processor. Typically these ASIC proces-
sors are used in applications where a standard
processor cannot meet the performance
requirements (e.g., throughput, power con-
sumption, chip area). High-throughput applica-
tions are found in, for example, high-definition
TV (HDTV) and communication systems. Low
power requirement is stringent in battery-pow-
Figure 1.2 Overview of the
ered applications. In high-volume applications
implementation
the lower unit cost, due to the smaller chip
process using the
area, may be another significant advantage. ASIC digital signal
The performance in terms of throughput, processor approach
power consumption, and chip area depends
strongly on the architecture and the imple-
mented instruction set. As illustrated in Figure 1.2, the processor can be matched to
the algorithm by implementing only those instructions that actually are used and by
providing several specialized data paths so that the required throughput is met.
Several co-operating processors are often required in high-throughput applications.
A major factor contributing to the overall performance of ASIC signal proces-
sors is that the data word length can be adjusted to the requirements. The amount
of on-chip memory can therefore be minimized. This is important since it is expen-
sive in terms of chip area to implement large on-chip memories. Note that the use
of external memories may result in reduced throughput since the practical data
rates are much lower than for internal memories.
A significant performance improvement in terms of throughput, power con-
sumption, and chip area over the standard processor approach is obtained at the