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684 CHAPTER 14/ASYNCHRONOUS STATE MACHINE DESIGN AND ANALYSIS
Digital Machines
Synchronous Asynchronous
Sequential Machines Sequential Machines
(Chapters 10-13) (Chapters 14-16)
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FIGURE 14.1
Breakdown of the various classes of digital machines showing chapters in this text pertinent to each
class with emphasis on asynchronous sequential machines, the subject of this and subsequent chapters.
14.1.1 Features of Asynchronous FSMs
All sequential machines have certain characteristics in common. However, there are features
owned more or less exclusively by asynchronous FSMs:
• The presence of memory in the absence of the familiar clocked flip-flop
• The appearance of the asynchronous machine as a combinational logic circuit
with feedback
Other more subtle features distinguish asynchronous FSMs from those that are syn-
chronous. These features include the possible existence of certain timing defects such as
endless cycles (oscillations), critical races (races that can produce error transitions), static
hazards in both the NS and output logic, and essential hazards. Static hazards that are
generated in the NS-forming logic of asynchronous FSMs can cause the malfunction of
these machines. Static hazards that form in the NS logic of synchronous FSMs are of no
consequence since they are filtered out by the action of clock in the memory flip-flops.
Also, in synchronous FSMs static hazards that are produced in the output-forming logic
can be filtered out by using an output holding register. This advantage is also not shared
by asynchronous FSMs, since there is no system clock with which to trigger a register. In
short, the benefits of clock, which are taken for granted in synchronous machine design,
do not exist in asynchronous FSMs. The reality is that endless cycles, critical races, and
hazards can occur in asynchronous FSMs and, if present, can and do cause the machines to
malfunction. A detailed study of these and other timing defects and the actions required to
eliminate them constitute a significant portion of this chapter.
