1. SEQUENTIAL CIRCUIT
DESIGN
By : Bipin Thapa

2. Sequential Circuit
 It is a type of logic circuit whose output
depends not only on the present value of its
input signals but on the past history of its
inputs.
 It is in contrast to combinational logic, whose
output is a function of only the present input.
 It has state (memory) while combinational
logic does not. Or, in other words, sequential
logic is combinational logic with memory.
 It is used to construct:-
 basic building block in all digital circuitry
 memory circuits and other devices

3.  Digital sequential logic circuits are divided
into synchronous and asynchronous types.
 In synchronous sequential circuits, the state of
the device changes only at discrete times in
response to a clock signal.
 In asynchronous circuits the state of the device
can change at any time in response to
changing inputs.

4. Control of an alarm system
 the simplest case of a sequential circuit
 Alarm is on when the sensor generates the “Set”
signal in response to some undesirable events
 Once the alarm is on, it can only be turned off
manually through a reset button
 Memory is needed to remember that the
alarm has to be active until the reset signal
arrives

5. Synchronous Sequential
Circuits
 All sequential logic today
is clocked or synchronous logic.
 In a synchronous circuit, a clock (or clock
generator) generates a sequence of the clock
signal which is distributed to all the memory
elements in the circuit.
 The basic memory element in sequential logic
is the flip-flop.
 The output of all the storage elements (flip-
flops) in the circuit at any given time, the
binary data they contain, is called the state of
the circuit.

6. fig. Synchronous circuit

7. Asynchronous Sequential
Circuits
 Asynchronous sequential logic is not
synchronized by a clock signal; the outputs of
the circuit change directly in response to
changes in inputs.
 The advantage of asynchronous logic is that it
can be faster than synchronous logic, because
the circuit doesn't have to wait for a clock
signal to process inputs.
 The speed of the device is potentially limited
only by the propagation delays of the logic
gates used.
 However, asynchronous logic is more difficult

8.  The main problem is that digital memory
elements are sensitive to the order that their
input signals arrive; if two signals arrive at a
logic gate at almost the same time, which state
the circuit goes into can depend on which
signal gets to the gate first.
 Therefore the circuit can go into the wrong
state, depending on small differences in
the propagation delays of the logic gates. This
is called a race condition.

9. fig. asynchronous
circuits

10. Counter
 In digital logic and computing, 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.
 Counters can be implemented quite easily using register-type
circuits such as the flip-flop, and a wide variety of
classifications exist:
 Asynchronous (ripple) counter – changing state bits are used as
clocks to subsequent state flip-flops
 Synchronous counter – all state bits change under control of a
single clock
 Decade counter – counts through ten states per stage
 Up/down counter – counts both up and down, under command of
a control input
 Ring counter – formed by a shift register with feedback
connection in a ring
 Johnson counter – a twisted ring counter
 Cascaded counter

11. 1) Asynchronous (ripple) counter:- An
asynchronous (ripple) counter is a single d-type
flip-flop, with its J (data) input fed from its own
inverted output. This circuit can store one bit,
and hence can count from zero to one before it
overflows (starts over from 0).
2) Synchronous counter:- In synchronous counters,
the clock inputs of all the flip-flops are connected
together and are triggered by the input pulses.
Thus, all the flip-flops change state
simultaneously (in parallel).

12. Register
 Registers are groups of flip-flops, where each
flip-flop is capable of storing one bit of
information.
 An n-bit register is a group of n flip-flops.
 The basic function of a register is to hold
information in a digital system and make it
available to the logic elements for the
computing process.
 Since each flip-flop is capable of storing either
a "0" or a "1", there is a finite number of 0-1
combinations that can be stored into a register.

13.  Each of those combinations is known
as state or content of the register.
 With flip-flops we can store data bitwise but
usually data does not appear as single bits.
 Instead it is common to store data words of n
bit with typical word lengths of 4, 8, 16, 32 or
64 bit.

14. Shift Registers :-
 A shift register is an n-bit register with a
provision for shifting stored data by one bit
position at each tick of the clock.

15. Shift Registers – Serial-in, Parallel-
out

16. Sequential Circuit Design and
Procedure
1. Problem Statement
2. State Table
3. The number of States May be reduced
4. Assign binary variable to each state
5. Determine number of flip-flop and assign a letter
symbol to each
6. Choose the type of flip-flop to be used
7. From the state table, derived the circuit excitation
and output tables
8. Simplify
9. Draw the Logic diagram

17. Thank!!
You!!