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Logic - 5

COMP109 Lectures

Sequential Circuits

Rules for a Sequential Circuit

  • Never combine two input wires.
  • A singe input wire can be split partway and used as input for two separate gates.
  • An output wire can be used as input.
  • An output of a gate can eventually feed back into that gate.

The only difference here between a combinatorial and a sequential circuit is the final rule.

What Happens Here?

  1. For the following circuit we know that the input would be $P$, making the output $\neg P$.

    A NOT gate connected to itself.

    This would not make sense as the wire says that they are equivalent.

    In real life this circuit would oscillate due to the time delay in switching. This would continue until it depleted the power in the circuit.

  2. Two NOT gates connected in a loop.

    This circuit will stay high or low depending on the initial value of the circuit.

    This is the same as this circuit with NOR gates:

    Two OR gates with their outputs connected to one input of the other. The other inputs are floating.

    For this circuit one of the inputs are fixed at zero. This follows on to the next subtopic.

Set/Reset Flip-Flop Circuit

This circuit is also known as a latch.

Two or gates connected as above. The floating input of the first is labelled S and the output is -Q. The floating input of the second is labelled R while the output is Q.

The two input signals are:

  • $S$(et)
  • $R$(eset)

This needs constant refreshing as the output will go when the power is removed.