Important Short Questions and Answers: Synchronous and Asynchronous Sequential Circuits

SYNCHRONOUS AND ASYNCHRONOUS SEQUENTIAL CIRCUITS

1.    Draw the block diagram for Moore model.

2.         What are hazard free digital circuits?

A circuit which has no hazard like static-0-hazard and static-1-hazard is called hazard free digital circuit.

3.         What are the basic building blocks of a algorithmic state machine chart?

4.         What are the two types of asynchronous sequential circuits?

·              Fundamental mode circuit

·              Pulse mode circuit

5.         What is state table?

The state table representation of a sequential circuit consists of three sections labelled present state, next state and output. The present state designates the state of flip-flops before the occurrence of a clock pulse. The next state shows the states of flip-flops after the clock pulse, and the output section lists the value of the output variables during the present state.

6.           What are Hazards?

The unwanted switching transients (glitches) that may appear at the output of a circuit are called Hazards.

7.           Distinguish between a flowchart and an ASM chart.

·              A conventional flow chart describes the square of procedural steps and decision paths for an algorithm without concern for their time relationship.

·              The ASM chart describes the sequence of event as well as timing relationship between the states of a sequential controller and the events that occur while going from one state to the next.

8.           What is a state diagram? Give an example.

A state diagram is a type of diagram used in computer science and related fields to describe the behaviour of systems. State diagrams require that the system described is composed of a finite number of states; sometimes, this is indeed the case, while at other times this is a reasonable abstraction. Many forms of state diagrams exist, which differ slightly and have different semantics.

9.           Write the VHDL code for a half adder.

HALF ADDER –

Entity entity

HALFADD is port ( A,B : in bit; S,C : out bit );

end HALFADD; --

Architecture

Architecture struct of HALFADD is

begin S <= A xor B;

C <= A and B;

end struct;

10.      Write a verilog model of a full subtractor circuit.

 module full_subtractor ( a ,b ,c ,diff ,borrow );

output diff ; output borrow ; input a ;

input b ; input c ;

assign diff = a ^ b ^ c;

assign borrow = ((~a) & b) | (b & c) | (c & (~a)); end module

11.      Under what circumstances asynchronous circuits are prepared.

(i)          Fundamental mode asynchronous circuits

(ii)       Pulse mode asynchronous circuits

12.      Differentiate fundamental mode and pulse mode asynchronous sequential circuits.

13.    Design a 3 input AND gate using verilog.

module and ( a ,b ,c ,f);

output diff ; input a ; input b ; input c ;

assign f = a &b & c; endmodule

14.    What is synchronous sequential circuit?

·              In synchronous circuits the input are pulses (or levels and pulses) with certain restrictions on pulse width and circuit propagation delay. Therefore synchronous circuits can be divided into clocked sequential circuits and uncklocked or pulsed sequential circuits.

·              In a clocked sequential circuit which has flip-flops or, in some instances, gated latches, for its memory elements there is a (synchronizing) periodic clock connected to the clock inputs of all the memory elements of the circuit, to synchronize all internal changes of state

15.    Write short notes on Hazards.

The unwanted switching transients (glitches) that may appear at the output of a circuit are called Hazards.

·              Static-0-Hazard

·              Static-1-Hazard