1445326735900-0

Digital integrated circuits II
In other words (and, yes, I know that it was an awful lot of
words), we have constructed a form of electronic memory,
one of the outputs of which can be set to a high logic state by
application of a positive-going pulse to one of its inputs. It’s a
small, but, very significant advance — changing combinational
logic gates into bistable circuits that can be used together
to create sequences of logic level variations. In effect, what
we have now done is to cross over from combinational logic
circuits to a new type of logic circuits known as sequential
logic circuits, with this simple but highly significant and very
important addition of memory. Whatever we want to call them:
bistables; latches; or flip-flops, they are sequential, and they
can be used to create even more complex logic circuits.
SR-type NAND bistable
In Figure 11.3 the gates are both NOR gates, but they can be
just as simply NAND gates, as shown in Figure 11.4. This gives
us an SR-type bistable made from NAND gates. Easy enough
— but there are some important considerations.
Figure 11.5 An SR-type bistable circuit comprising two cross-coupled NAND
gates — note the inverted inputs when compared with Figure 11.3
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