R S-Flip Flop Circuit
- two logic or digital circuits for an R S flip flop, one using NAND
gates and the other using NOR gates
R-S flip flops find uses in many applications in logic or
digital electronic circuitry. They provide a simple switching function whereby a
pulse on one input line of the flip flop sets the circuit in one state. Further
pulses on this line have no effect until the R-S flip flop is reset. This is
accomplished by a pulse on the other input line. In this way the R S flip flop
is toggled between two states by pulses on different lines.
Although chips are available with R-S functions in them, it
is often easier to create an R-S flip flop from spare gates that may already be
available on the board, or on a breadboard circuit using a chip that may be to
hand. To make an R S flip flop, it simple requires either two NAND gates or two
NOR gates.
Using two NAND gates and active low R S flip flop is
produced. In other words low going pulses active the flip flop. As it can be
seen from the circuit below, the two incoming lines are applied, one to each
gate. The other inputs to each of the NAND gates are taken from the output of
the other NAND gate.
It can be seen from the waveform diagram that a low going
pulse on input A of the flip flop forces the outputs to change, C, going high
and D going low. A low going pulse on input B then changes the state, with C
going low and D going high.
An R S flip flop using two NAND gates
The waveforms for an R S flip flop
The circuit for the NOR version of the circuit is exceedingly
similar and performs the same basic function. However using the NOR logic gate
version of the R S flip flop, the circuit is an active high variant. In other
words the input signals need to go high to produce a change on the output. This
may determine the choice of integrated circuit that is used. Although the NAND
gate version is probably more widely used, there are many instances where the
NOR gate circuit is of value.
An R S flip flop using two NOR gates
The waveforms for the NOR gate R S flip flop
These circuits are widely used in many electronic logic
circuit applications. There are also contained within many integrated circuits
where they are a basic building block. As such the R S flip flop is an
exceedingly popular circuit.
One useful application for a simple R S flip flop is as a
switch de-bounce circuit. When any mechanical switch makes or breaks contact,
the connection will make and break several times before the full connection is
made or broken. While for many applications this may not be a problem, it is
when the switch interfaces to logic circuitry. Here a series of pulses will pass
into the circuit, each one being captured and forming a pulse. Dependent upon
the circuit this may appear as a series of pulses, and falsely triggering
circuits ahead of time.
An R S flip flop used as a de-bounce circuit
It is possible to overcome this problem using a simple R S
flip flop. By connecting the swich as shown below, the flip flop will change on
the first sign of contact being made. Further pulses will not alter the output
of the circuit. Only when the switch is turned over to the other position will
the circuit revert to the other state. In this way a simple two gate circuit can
save the problems of de-bouncing the switch in other ways.
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