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Digital integrated circuits II
The clocked SR-type NAND bistable circuit here functions
basically as before, except that the outputs can only change
states while the clock input CLK is at logic 1. When the clock
input CLK is at logic 0, it doesn’t matter what logic levels are
applied to the S and R inputs, there will be no effect on the
bistable.
Thus, with this simple addition, we have created a bistable
with a controlling input. Unless the controlling input is at logic
1, nothing else can happen. This can be a very useful thing in
electronics where, say, a number of things need to be counted
over a period of time. For example, by clocking the circuit
for, say, 1 hour, the number of cars passing a checkpoint in
that time could be counted electronically.
The D-type bistable
A variation on the basic clocked SR-type bistable is the ‘data’
bistable, or D-type bistable. A circuit is shown in Figure
11.9.
Figure 11.9
A D-type bistable
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