UNIT III -SYNCHRONUS SEQUENTIAL CIRCUITS

PART-A

1. What is a sequential circuit and List the types of sequential circuit.

The sequential circuit consists of a combinational circuit to which memory
elements are connected to form a feed back path. The output at any instant of time
depends on the present inputs and past output. Synchronous Sequential Circuit and
Asynchronous Sequential Circuit.

2. What do you mean by synchronous sequential circuit & clocked

sequentialcircuits?
A synchronous sequential circuit is a system whose behavior can be defined from
the knowledge of its signal at discrete instants of time. Synchronous sequential
circuit that uses clock pulses in the inputs of memory elements is called clocked
sequential circuit. The change of internal state occurs in response to the
synchronized Clock pulses.

3. What is called a latch?

Unclocked flip-flop that responds to pulse duration is called a latch.

4. What is a flip-flop & List the different types of flip-flops.

It is a basic memory element used to store one bit of information. It is also called
as a bistable multivibrator. A flip-flop circuit has two outputs, one for the normal
value and one for the complement of the bit stored in it. It can store the binary
value indefinitely until directed by an input signal to switch states. RS, D, T, J K
and J K master slave flip-flop.

5. Name the different types of triggering employed in a flip-

flop?Level triggering and Edge triggering

6. What are the differences between RS latch and edge triggered RS flip-
flop?

7. RS LATCH: 1.No clock pulse input is present

2. Output changes state on the application of inputs

Edge Triggered RS FLIPFLOP:

1. Clock pulse input is present
2.Output changes state only on the transition of clock pulses depending on the
input.
 7. Give the truth table of RS flip-flop and JK flip-flop.

R S Qn+1 Qn+1
0 0 Qn Qn‟ No change
0 1 1 0 Set
1 0 0 1 Reset
1 1 Forbidden
J K Qn+1 Qn+1
0 0 Qn Qn‟ No change
0 1 0 1 Reset
1 0 1 0 Set
1 1 Qn‟ Qn Toggle

8. What do you mean by race around condition in a flip-flop?

When both J = K = 1 and clock pulse is „1‟ it causes the output to complement
again and repeat complementing until the pulse goes back to „0‟(i.e.) the output
toggles continuously. This Race condition arises when the width of the clock pulse
is greater than the propagation delay time of the flip-flop.

9. What is a master slave flip-flop?

A master slave flip-flop is constructed from two separate flipflops. One circuit
serves as a master and the other as a slave, the overall circuit is referred to as a
master slave flip-flop. Both the flipflops are positive level triggered but the
presence of inverter at the clock input of the slave flip-flop forces it to trigger at
the negative level.

10. What is direct preset and clear inputs?

• Preset -used to set the flip-flop in the initial stage.
• Clear- used to clear the flip-flop in the initial stage
11. What is the difference between truth table and excitation table?
A truth table is a table indicating the output of a combinational circuit for all input
states.An excitation table is a list of Flip-flop input conditions that will cause the
required state transitions.

12. What is a counter?

A Sequential circuit that goes through a prescribed sequence of states upon the
application of input pulses is called a counter.

13. What is a presettable counter & ripple counter?

Normally the count starts from zero in a counter. A presettable counter is used
to start the count from any number other than zero. A ripple counter is an
asynchronous counter, in which the output of the flip-flops change state like a
ripple in water and hence the name ripple counter.

14. Mention the applications of counter?

They are used for counting the number of occurrences of an event and are
useful for generating timing sequences to control operations in a digital
systems.They are used as frequency dividers in digital time pieces, such as,
electronic digital clocks, Automobile digital clock and wrist watches, frequency
counters, and oscilloscope and television receivers.
15. What is a modulo counter, ring and Johnson counter?
Modulo „n‟ counter have „n‟ different states that counts from 0 to n-1 by making
small changes in a counter circuit.A ring counter is a circular shift register with
only one flipflop being set at any particular time, all others are cleared. A johnson
/switched tail ring counter is a circular shift register with the complement output of
the last flip-flop connected to the input of the first flip-flop.

16. What is a BCD counter?

A BCD counter counts in binary coded decimal from 0000 to 1001 and back to
0000.It is also a decade counter since it counts from 0 to 9.

17. What is a register & a shift register??

A register is group of binary cells suitable for holding binary information. A group
of flipflops constitutes a register, since each flip-flop is a binary cell capable of
storing one bit of information.A register capable of shifting the binary information
either to the right or in the left is called a shift register.

18. What are the types of shift registers?

• Serial in Serial out(SISO) Serial in Parallel out(SIPO)
• Parallel in Parallel out(PIPO) Parallel in Serial out(PISO)
19. Mention the applications of shift registers?
To introduce time delay, Serial to Parallel converter, Parallel to serial
converter, Sequence generator, Ring counter.

20. What is sequence generator?

A circuit which generates a prescribed sequence of bits in synchronism with a
clock is referred to as a sequence generator.

21. What is a state table, state diagram& state equation?

The table which lists the time sequence of inputs, outputs and flip-flop states. A
state diagram is a graphical representation of the information available in a state
table. In the diagram, a state is denoted by a circle and the transitions between the
states are indicated by directing lines connecting the circles.A state equation
(transition equation) specifies the condition for a flip-flop state transition. It
denotes the next state as a function of the present state and inputs.
22. Explain mealy model and Moore model.
Moore Model:
Its output is a function of present state only.
Input changes does not affect the output
It requires more number of states for implementing same function.
Mealy model:
Its output is a function of present state as well as present input
Input changes may affect the output of the circuit
It requires less number of states for implementing same function.

23. What is the need for debounce circuit?

It is the one that removes the series of pulses that result from a contact
bounce and produces a single smooth transition of the binary signal from 0 to 1 or
from 1 to 0.
PART B