时序逻辑设计原则 (Sequential Logic Design principles):A sequential logic circuit is one whose outputs depend not only on itscurrent inputs, but also on the past sequence of inputs, possibly arbitrarilyfar back in time. The circuit controlled by the channel-up and channel-downpushbuttons on a TV or VCR is a sequential circuit—the channel selectiondepends on the past sequence of up/down pushes, at least since when youstarted viewing 10 hours before, and perhaps as far back as when you firstplugged the device into the wall.So it is inconvenient, and often impossible, to describe the behavior ofa sequential circuit by means of a table that lists outputs as a function of theinput sequence that has been received up until the current time. To knowwhere you’re going next, you need to know where you are now. With theTV channel selector, it is impossible to determine what channel is currentlyselected by looking only at the preceding sequence of presses on the up anddown pushbuttons, whether we look at the preceding 10 presses or thepreceding 1,000. More information, the current “state” of the channelselector, is needed. Probably the best definition of “state” that I’ve seen.
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