a D flip-flop copies D to Q on the rising edge of the clock, and remembers its state at all other times.
Asynchronous design in theory is more general than synchronous design, because the timing of the system is not limited by clocked registers. Just as analog circuits are more general than digital circuits because analog circuits can use any voltage, asynchronous circuits are more general than synchronous circuits because they can use any kind of feedback. However, synchronous circuits have proved to be easier to design and use than asynchronous circuits, just as digital are easier than analog circuits. Despite decades of research on asynchronous circuits, virtually all digital systems are essentially synchronous.
Of course, asynchronous circuits are occasionally necessary when communicating
between systems with different clocks or when receiving inputs at arbitrary times, just as analog circuits are necessary when communicating with the real world of continuous voltages. Furthermore, research in asynchronous circuits continues to generate interesting insights, some of which can improve synchronous circuits too.引自第108页
I really like his clear explain and the knowledge I had learned in 2011 has been revised and be more clear for me.
The example he used are very classical and the mind flow is very detail. Memorize them! Follow the master's mind.
He used some easy understanding analogue such that: Asyn and Syn are just like Analog and Digital. Both are useful but Syn/Digital are essential and Asyn/Analog are hard to design but critical in some conditions.
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