Important Questions and Answers: Synchronous Sequential Logic

1.     What are the classifications of sequential circuits?

The sequential circuits are classified on the basis of timing of their signals into two types. They are, 1) Synchronous sequential circuit. 2) Asynchronous sequential circuit.

2.     Define Flip flop.

The basic unit for storage is flip flop. A flip-flop maintains its output state either at 1 or 0 until directed by an input signal to change its state.

3.     What are the different types of flip-flop?

There are various types of flip flops. Some of them are mentioned below they are,RS flipflop SR flip-flopD flip-flopJK flip-flopT flip-flop

4.     What is the operation of D flip-flop?

In D flip-flop during the occurrence of clock pulse if D=1, the output Q is set and if D=0, the output is reset.

5.     What is the operation of JK flip-flop?

·  When K input is low and J input is high the Q output of flip-flop is set. ·  When K input is high and J input is low the Q output of flip-flop is reset ·  When both the inputs K and J are low the output does not change

·  When both the inputs K and J are high it is possible to set or reset the flip-flop (ie) the output toggle on the next positive clock edge.

6.     What is the operation of T flip-flop?

T flip-flop is also known as Toggle flip-flop. •When T=0 thereis no change in the output.

•When T=1 the output switch to the complement state (ie) the output toggles.

7.     Define race around condition.

In JK flip-flop output is fed back to the input. Therefore change in the output results change in

the input. Due to this in the positive half of the clock pulse if both J and K are high then output toggles continuously. This condition is called ‘race around condition’.

8.     What is edge-triggered flip-flop?

The problem of race around condition can solved by edge triggering flip flop. The term edge triggering means that the flip-flop changes state either at the positive edge or negative edge of the clock pulse and it is sensitive to its inputs only at this transition of the clock.

9.     What is a master-slave flip-flop?

A master-slave flip-flop consists of two flip-flops where one circuit serves as a master and the other as a slave.

10.                        Define rise time.

The time required to change the voltage level from 10% to 90% is known as rise time(tr).

11.                        Define fall time.

The time required to change the voltage level from 90% to 10% is known as fall time(t_f).

12.                        Define skew and clock skew.

The phase shift between the rectangular clock waveforms is referred to as skew and the time delay between the two clock pulses is called clock skew.

13.                        Define setup time.

The setup time is the minimum time required to maintain a constant voltage levels at the excitation inputs of the flip-flop device prior to the triggering edge of the clock pulse in order for the levels to be reliably clocked into the flip flop. It is denoted as setup.

14.                        Define hold time.

The hold time is the minimum time for which the voltage levels at the excitation inputs must

remain constant after the triggering edge of the clock pulse in order for the levels to be reliably clocked into the flip flop. It is denoted as t_hold .

15.                        Define propagation delay.

A propagation delay is the time required to change the output after the application of the input.

16.                        Define registers.

A register is a group of flip-flops flip-flop can store one bit information. So an n-bit register has a group of n flip-flops and is capable of storing any binary information/number containing n-bits.

17.                        Define shift registers.

The binary information in a register can be moved from stage to stage within the register or into or out of the register upon application of clock pulses. This type of bit movement or shifting is essential for certain arithmetic and logic operations used in microprocessors. This gives rise to group of registers called shift registers.

18.                        What are the different types of shift type?

There are five types. They are, Serial In Serial Out Shift Register Serial In Parallel Out Shift Register Parallel In Serial Out Shift Register

Parallel In Parallel Out Shift Register & Bidirectional Shift Register

19.                        Explain the flip-flop excitation tables for RS FF. RS flip-flop

In_→RS flip-flop there are four possible transitions from the present state to the next state. They_→ are

,0_→ 0 transition: This can happen either when R=S=0 or when R=1 and S=0.

0 _→ 1 transition: This can happen only when S=1 and R=0.

1        0 transition: This can happen only when S=0 and R=1.

1        1 transition: This can happen either when S=1 and R=0 or S=0 and R=0.

20.                        Define sequential circuit?

In sequential circuits the output variables dependent not only on the present input variables but they also depend up on the past history of these input variables.

21. Give the comparison between combinational circuits and sequential circuits.

22.                    What do you mean by present state?

The information stored in the memory elements at any given time defines the present state of the sequential circuit.

23.                    What do you mean by next state?

The present state and the external inputs determine the outputs and the next state of the sequential circuit.

24.                    State the types of sequential circuits?

1.Synchronous sequential circuits 2.Asynchronous sequential circuits

25.                    Define synchronous sequential circuit

In synchronous sequential circuits, signals can affect the memory elements only at discrete instant of time.

26.                    Give the comparison between synchronous & Asynchronous counters.

27.                    The t_pd for each flip-flop is 50 ns. Determine the maximum operating frequency for MOD - 32 ripple counter

f max (ripple) = 5 x 50 ns = 4 MHZ

28.                    What are secondary variables?

Present state variables in asynchronous sequential circuits

29.                    What are excitation variables?

Next state variables in asynchronous sequential circuits

GLOSSARY TERMS

1.                 Sequential circuit-In sequential circuits the output variables dependent not only on the present input variables but they also depend up on the past history of these input variables.

2.                 Counters- a Counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal.

3.                 State diagram- The state diagram is constructed using all the states of the sequential circuit in question. It builds up the relationship between various states and also shows how inputs affect the states.

4.                 Flip-f1ops- The basic unit for storage is flip flop. A flip-flop maintains its output state either at

1 or 0 until directed by an input signal to change its state.

5.                 Master-slave flip-flop -A master-slave flip-flop consists of two flip-flops where one circuit serves as a master and the other as a slave.

6.                 Latches- They has level sensitive control signal input.

7.                 Johnson counter- A Johnson counter is a special case of shift register, where the output from the last stage is inverted and fed back as input to the first stage.

8.                 Register-A register is a group of flip-flops flip-flop can store one bit information.

9.                 Shift registers- The binary information in a register can be moved from stage to stage within the register or into or out of the register upon application of clock pulses.

10.            Universal shift register- If the register has both shifts and parallel-loads, it is referred as universal shift register.