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Digital integrated circuits II
Switch SW1 in the circuit of Figure 11.7 is a simple single-pole,
double-throw (SPDT) switch, and should have break-beforemake
contacts (in other words, when switching from one
connection to the other, its switch contacts will disconnect
from one connection before it connects to the other — so
there is a point in the switch motion when all three contacts
are disconnected from each other). The switch can be either
a push-button switch, or a conventional flick-type switch.
Operation is fairly simple. At rest, one of the NAND gate inputs
connects through the switch to logic 0, while the other NAND
gate’s input is connected through a resistor to logic 1. As the
switch is switched, the second NAND gate’s input is connected
to logic 0 though the switch, and the circuit operates as previously
described like a standard SR-type NAND bistable. It
doesn’t matter whether there is contact bounce or not at this
time, as repeated pulses do not affect the bistable.
When the switch is operated again, so the first NAND gate’s
input is connected to logic 0 and the bistable jumps to its
other stable state. Again, contact bounce will make no difference
to it.
One or both of the outputs of the simple de-bouncing circuit
can be applied to the inputs of following circuitry, safe in
the knowledge that contact bounce will have no effect. The
requirement for no contact bounce is true for any sequential
circuit, actually. So this very simple solution can be used not
just in the simple circuits we see here in this chapter, but for
all sequential circuits. Whenever a sequential logic circuit
requires a manually pulsed input, this circuit can be used.
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