Important Short Questions and Answers: Asynchronous Sequential Circuits

1.                 Define Asynchronous sequential circuit?

In asynchronous sequential circuits change in input signals can affect memory element at any instant of time.

2.                 Give the comparison between synchronous & Asynchronous sequential circuits?

Synchronous sequential circuits

•        Memory elements are clocked flip-flops

•        Easier to design.

Asynchronous sequential circuits.

•        Memory elements are either unlocked flip - flops or time delay elements.

•        More difficult to design.

3.                 What is race around condition?

In the JK latch, the output is feedback to the input, and therefore changes in the output results change in the input. Due to this in the positive half of the clock pulse if J and K are both high then output toggles continuously. This condition is known as race around condition.

4.                 What is fundamental mode sequential circuit?

Input variables changes if the circuit is stable -inputs are levels, not pulses -only one input can change at a given time

5.                 What are pulse mode circuits?

Inputs are pulses -widths of pulses are long for circuit to respond to the input -pulse width must not be so long that it is still present after the new state is reached

6.                 What are the significance of state assignment?

In synchronous circuits-state assignments are made with the objective of circuit reduction Asynchronous circuits-its objective is to avoid critical races.

7.                 What are the different techniques used in state assignment?

Shared row state assignment -one hot state assignment

8.                 What are the steps for the design of asynchronous sequential circuit?

Construction of primitive flow table

Reduction of flow table -state assignment is made Realization of primitive flow table

9.                 What is hazard?

Unwanted switching transients

10.            What is static 1 hazard?

Output goes momentarily 0 when it should remain at 1

11.            What is static 0 hazard?

Output goes momentarily 1 when it should remain at 0

12.            What is dynamic hazard?

Output changes 3 or more times when it changes from 1 to 0 or 0 to 1

13.            What is the cause for essential hazards?

Unequal delays along 2 or more path from same input

14.            What is flow table?

State table of an synchronous sequential network

15.            What is primitive flow chart?

One stable state per row

16.            What is combinational circuit?

Output depends on the given input. It has no storage element.

17.            Define merger graph.

The merger graph is defined as follows. It contains the same number of vertices as the state table contains states. A line drawn between the two state vertices indicates each compatible state pair. It two states are incompatible no connecting line is drawn.

18.            Define closed covering

A Set of compatibles is said to be closed if, for every compatible contained in the set, all its implied compatibles are also contained in the set. A closed set of compatibles, which contains all the states of M, is called a closed covering.

19.            Define state table.

For the design of sequential counters we have to relate present states and next states. The table, which represents the relationship between present states and next states, is called state table.

20.            Define total state

The combination of level signals that appear at the inputs and the outputs of the delays define what is called the total state of the circuit.

21.            What are the steps for the design of asynchronous sequential circuit?

1. Construction of a primitive flow table from the problem statement.

2. Primitive flow table is reduced by eliminating redundant states using the state reduction

3. State assignment is made

4. The primitive flow table is realized using appropriate logic elements.

22.            Define primitive flow table

It is defined as a flow table which has exactly one stable state for each row in the table. The design process begins with the construction of primitive flow table.

23.    What are the types of asynchronous circuits?

1. Fundamental mode circuits

2. Pulse mode circuits

24.            Give the comparison between state Assignment Synchronous circuit and state assignment asynchronous circuit.

In synchronous circuit, the state assignments are made with the objective of circuit reduction. In asynchronous circuits, the objective of state assignment is to avoid critical races.

25.            What are races?

When 2 or more binary state variables change their value in response to a change in an input variable, race condition occurs in an asynchronous sequential circuit. In case of unequal delays, a race condition may cause the state variables to change in an unpredictable manner.

26.            Define non critical race.

If the final stable state that the circuit reaches does not depend on the order in which the state variable changes, the race condition is not harmful and it is called a non critical race.

27.            Define critical race?

If the final stable state depends on the order in which the state variable changes, the race condition is harmful and it is called a critical race.

28.            What is a cycle?

A cycle occurs when an asynchronous circuit makes a transition through a series of unstable states. If a cycle does not contain a stable state, the circuit will go from one unstable to stable to another, until the inputs are changed.

29.            Define secondary variables

The delay elements provide a short term memory for the sequential circuit. The present state and next state variables in asynchronous sequential circuits are called secondary variables.

30.            Define flow table in asynchronous sequential circuit.In asynchronous sequential circuit state table is known as flow table because of the behavior of the asynchronous sequential circuit. The stage changes occur in independent of a clock, based on the logic propagation delay, and cause the states to

.flow. from one to another.

31. A pulse mode asynchronous machine has two inputs. If produces an output whenever two consecutive pulses occur on one input line only. The output remains at 1 until a pulse has occurred on the other input line. Write down the state table for the machine.

32.            Write short note on shared row state assignment.

Races can be avoided by making a proper binary assignment to the state variables. Here, the state variables are assigned with binary numbers in such a way that only one state variable can change at any one state variable can change at any one time when a state transition occurs. To accomplish this, it is necessary that states between which transitions occur be given adjacent assignments. Two binary are said to be adjacent if they differ in only one variable.

33.            Write short note on one hot state assignment.

The one hot state assignment is another method for finding a race free state assignment. In this method, only one variable is active or hot for each row in the original flow table, ie, it requires one state variable for each row of the flow table. Additional row are introduced to provide single variable changes between internal state transitions.

GLOSSARY TERMS

1.                 Fundamental mode-A transition from one stable state to another occurs only in response to a change in the input state.

2.                 Pulse mode-Inputs are pulses -widths of pulses are long for circuit to respond to the input -pulse width must not be so long that it is still present after the new state is reached

3.                 State assignment- In synchronous circuits-state assignments are made with the objective of circuit reduction Asynchronous circuits-its objective is to avoid critical races.

4.                 Primitive flow table-It is defined as a flow table which has exactly one stable state for each row in the table. The design process begins with the construction of primitive flow table.

5.                 Hazards- Unwanted switching transients at the output

6.                 Essential hazards-Unequal delays along 2 or more path from same input

7.                 Critical race- If the final stable state that the circuit reaches does not depend on the order in which the state variable changes, the race condition are not harmful and it is called a non critical race.

8.                 Non critical race-If the final stable state that the circuit reaches does not depend on the order in which the state variable changes, the race condition is not harmful and it is called a non critical race

9.                 Cycles- A cycle occurs when an asynchronous circuit makes a transition through a series of unstable states. If a cycle does not contain a stable state, the circuit will go from one unstable to stable to another, until the inputs are changed.

10.            Merger graph-The merger graph is defined as follows. It contains the same number of vertices as the state table contains states. A line drawn between the two state vertices indicates each compatible state pair. It two states are incompatible no connecting line is drawn.