時序邏輯設計原則 (Sequential Logic Design principles):A sequential logic circuit is one whose outputs depend not only on its
current inputs, but also on the past sequence of inputs, possibly arbitrarily
far back in time. The circuit controlled by the channel-up and channel-down
pushbuttons on a TV or VCR is a sequential circuit—the channel selection
depends on the past sequence of up/down pushes, at least since when you
started viewing 10 hours before, and perhaps as far back as when you first
plugged the device into the wall.
So it is inconvenient, and often impossible, to describe the behavior of
a sequential circuit by means of a table that lists outputs as a function of the
input sequence that has been received up until the current time. To know
where you’re going next, you need to know where you are now. With the
TV channel selector, it is impossible to determine what channel is currently
selected by looking only at the preceding sequence of presses on the up and
down pushbuttons, whether we look at the preceding 10 presses or the
preceding 1,000. More information, the current “state” of the channel
selector, is needed. Probably the best definition of “state” that I’ve seen.