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Shutting the ’stable gate
Digital integrated circuits II
All the digital electronic circuits we’ve looked at so far function
more-or-less instantly. If certain inputs are present, a
circuit will produce an output which is defined by the Boolean
statement for the circuit — it doesn’t matter how complex
the circuit is, a combination of inputs will produce a certain
output.
For this reason, such circuits are known as combinational.
While they may be used in quite complex digital circuits they
are really no more than electronic switches — the output of
which depends on the correct combination of inputs.
Put another way, they display no intelligence of any kind,
simply doing what is demanded of them — instantly.
One of the first aspects of intelligence — whether in the
animal world (including humans) or in the electronics world
— is memory (that is, the ability to decide a course of action
dependent not only on applied inputs but also on a knowledge
of what has previously taken place).
Digital circuits can easily be made that remember logic states.
We say they can store binary information. The family of devices
that do that in digital electronics are known as bistable
circuits (called this, because they remain stable in either of
two states), sometimes also known as latches. It’s this ability
to remain stable in either of the two binary states that allows
them to form the basis of digital memory.
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