Questions: Finite State Machines (FSMs)

Question 1

In a Moore machine that controls a traffic light, the light color (red, green, yellow) is determined by:

• A. The current input signal from a car sensor
• B. Both the current state and the current input
• C. The current state alone
• D. The previous state and the previous input

Answer: (C)

In a Moore machine, outputs are a function of the current state only — not the current inputs. Each state has a fixed output associated with it. This means the light color is determined entirely by which state the FSM is currently in, regardless of what the sensor is doing right now. A Mealy machine would instead allow the output to vary based on both state and input.

Question 2

A Mealy machine usually requires more states than an equivalent Moore machine to recognize the same behavior.

• T. True
• F. False

Answer: False

It is actually the opposite: a Mealy machine often requires *fewer* states than the equivalent Moore machine. In a Mealy FSM, outputs are associated with transitions (state + input), so a single state can produce different outputs depending on the input. A Moore FSM must create separate states to produce different outputs, which can increase the state count. Both models are equally expressive — any FSM in one form can be converted to the other.

Question 3

Why does a finite state machine require flip-flops, while a combinational circuit does not?

Model answer: An FSM must remember its current state across clock cycles, but combinational circuits have no memory — their output is a pure function of current inputs. Flip-flops are edge-triggered memory elements that store the current state register and update it once per clock cycle, giving the FSM its ability to exhibit time-dependent (sequential) behavior.

This connects to the prerequisite topic: combinational circuits compute a function with no stored state; adding flip-flops creates a sequential circuit. The FSM architecture is exactly this: combinational logic computes the next state and the output, while flip-flops hold the current state. Without the flip-flops, the 'state' would immediately change as inputs changed, making the machine combinational rather than sequential.