1445326735900-0

Digital integrated circuits II
gate, which makes the output of the other gate go to logic 1.
This, in turn, is fed back to the second input of the first gate,
forcing its output to remain at logic 0 — even (and this is the
important point) after the original input is removed. This is
an important point!
Applying another logic 1 input to the first gate has no further
effect on the circuit — it remains stable, or latched, in this
state.
On the other hand, a logic 1 input applied to the other gate’s
input causes the same set of circumstances to occur but in
the other direction, causing the circuit to become stable, or
latch, in the other way.
This sort of switching backwards and forwards in two alternate
but stable states accounts for yet another term that is
often applied to bistable circuits — they are commonly called
‘flip-flops’.
The SR-type NOR bistable operation can be summarised in
a truth table, in a similar way that non-bistable or combinational
logic gates can be summarised. However, because we
are trying to tabulate inputs and outputs that change with
time, then strictly speaking we should call this a ‘function
table’, and such a function table for the SR-type NOR bistable
is shown in Figure 11.4.
Account has been taken in the function table for changing
from one logic state to the other, by the symbol , which
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