Sequential Circuit Design Questions and Answers

Answer: A

In an FSM diagram, a circle represents a state.

In a state diagram of a finite state machine, each circle (node) stands for one state of the machine, labelled with its name or code. Arrows between circles are transitions, labelled with the input conditions that cause them and, in a Mealy machine, the outputs. In a Moore machine the output is written inside the state circle.

A change of state is shown by an arrow, not a circle. Output values are written on arrows or inside circles but are not what the circle represents. The initial state is one particular state, usually marked by an arrow from nowhere or a double circle, so 'initial state' is too narrow.

Answer: C

A Mealy machine's output depends on the current input and the present state.

In a Mealy machine, outputs are attached to transitions, so the output is a function of both the state the machine is in and the input it is receiving. A change in input can change the output at once, without waiting for a state change.

Output that depends only on the present state defines a Moore machine, the usual contrast. Output from the input alone would make it a combinational circuit, not a state machine, and the clock only times the state changes; it does not determine the output value.

Answer: D

The minimum number of flip-flops is 3.

The sequence 0-1-0-2-0-3 has six steps, and the state 0 occurs three times with three different successors (1, 2 and 3). A synchronous counter's next state depends only on its present state, so the three 0s must be three distinct internal states that happen to give output 0.

That makes six states in all. Two flip-flops give only four states, so at least three are needed (\(2^{3} = 8 \ge 6\)). Options 0, 1 and 2 cannot hold six distinct states.

Answer: C

The sequential circuit is the REGISTER.

A sequential circuit's output depends on its inputs and on stored past state, so it needs memory elements. A register is a group of flip-flops that stores a binary word, clocked to load and hold data, so it is sequential.

Parity generators, adders and multiplexers are combinational circuits: their outputs depend only on the present inputs, and they contain no memory elements.

Answer: B

The RS latch is a sequential logic circuit, the second option. Its outputs depend on the present inputs and on the stored state, because of the feedback between its cross-coupled gates, so it has memory.

A multiplexer and a full adder are combinational circuits, whose outputs depend only on the present inputs. A ROM gives a fixed output for each address and has no feedback in its logic, so it is also classed as a combinational device.