Page 383 - DSP Integrated Circuits

368 Chapter 8 DSP Architectures

Many DSP algorithms have an inherent hierarchy that can be mirrored
directly by the control structure. For example, the nested loops in the FFT have
such a hierarchical structure. It is often efficient to map a hierarchy of signals to a
hierarchical control structure.

8.4.5 Synchronous and Asynchronous Systems
There was no "One, two, three, and away," but they began running
when they liked and left off when they liked, so that it was not easy to
know when the race was over.
Lewis Carroll

Timing philosophy; is one of the most important attributes characterizing a com-
puting system. There are two types of possible timing schemes—synchronous and
asynchronous.
Synchronous timing techniques, which are the most commonly used tech-
niques, are characterized by the existence of a global clock that defines the basic
unit of time. This time reference is used to order the activities into a proper
sequence. The whole system is an isosynchronous domain. This is in practice
accomplished by a central, global clock that broadcasts clock signals to all units in
the system so that the entire system operates in a lock-step fashion.
Asynchronous timing techniques operate without a global clock. Communica-
tion among operational units is performed by means of interlock handshaking
schemes. Asynchronous systems are difficult to design, and communication between
synchronous and asynchronous systems is difficult and may cause malfunction.

8.4.6 Self-Timed Systems

A man with a watch knows the time.
A man with two watches is not sure.
Asynchronous, or self-timed, circuitry is often used in CMOS circuits because of its
performance and area efficiency [8]. Synchronization between self-timed systems
or modules is obtained by causal handshaking operations instead of by using a glo-
bal clock. Communication between two modules can be established using two sig-
nals and an SR latch, as shown in Figure 8.11. The Enable signal sets the latch,

Figure 8.11 Principle of self-timed systems