The Need for Clocked Circuits

Over the last two sessions, we have explained the basics of logic circuits. In our previous session, we looked at combinational circuits, and this time we’ve looked at sequential circuits. To conclude this session, we’d like to point out that there are many considerations that must be taken into account when designing logic circuitry. One of the more important issues relates to the use of clocked circuits.
In particular, it sometimes happens that a combinational circuit will output a very short unwanted signal as a result of a slight delay in signal transmission. This unwanted output, which is appropriately called a glitch, can produce errors in the logic circuitry. The problem can be prevented by building in clocked circuitry, as illustrated below.

Figure shows how clocked circuitry is included. As you can see, the combinational circuits are sandwiched between flip-flops (FF).
A glitch survives only during the short interval before the combinational circuit’s output stabilizes. To prevent a glitch: Wait for the output to stabilize, and then change the clock, causing the flip-flop to capture the intended output value.

With this session, we conclude our introduction to digital circuitry. We began the series by defining what we mean by " digital," and then proceeded quickly through a discussion of basic logic circuits, digital ICs, combinational circuits, and—finally—sequential circuits.
Of course, we’ve only touched the surface, and there’s still a tremendous amount to be learned about the practice of circuit design. We encourage you to make time to continue studying this area on your own. And we look forward to seeing you at the next session, where we will begin with an introduction to microcontrollers.