Sequential Circuits

A Basic sequential circuit is nothing but a combinational circuit with some feedback paths between its output and input terminals.

Basic Sequential Circuit

Combinational Circuit

Feedback Circuit

Input Output

Feedback Path

Sequential Circuits

Basics of Sequential Circuits The direct inputs in to the

combinational circuit which are externally controlled are called primary inputs.

And the feedback inputs are called secondary inputs.

Basics of Sequential Circuits

Combinational Circuit

Feedback Circuit

Input Output

Feedback Path

Sequential Circuits

Primary Inputs

Secondary Inputs

“Latch” the base of sequential circuits.Latch is a combination of logic

gates that implements a feedback path.

e.g. A SR (NOR) latch.

R

S

Q

Q’

SR (NOR) LatchThe SR Latch Operations can be

illustrate with the help of following table

S R Action

0 0 Keep State

0 1 Q= 0 (Reset)

1 0 Q=1 (Set)

1 1 Unstable combination

Does it properly work like this?Can we prove it?How?Lets try....

SR (NOR) LatchLets study the previous table in detail.

Inputs P-Outputs N-Outputs Action

S R S R S R

0 0 0 1 0 1 Keep State

1 0 1 0

0 1 0 1 1 0 Q=0 (Reset)1 0 1 0

1 0 0 1 0 1 Q=1 (Set)

1 0 0 1

1 1 0 1 0 0 Unstable combinatio

n1 0 0 0

Types of Sequential CircuitsThere are two types of Sequential

Circuits:1. Asynchronous2. Synchronous

Before starting this topic lets understand the behaviour of sequential circuits.

Behaviour of Sequential Circuits Any practical logic gate cannot

operate instantaneously.It means it will have a time delay

between the application of a change in input and the appearance of the corresponding output.

And this delay is unpredictable.

Asynchronous Sequential Circuits

Asynchronous sequential circuits are those sequential circuits in which the inputs and outputs do not change at pre-assigned times.

The inherent delays are not rigidly controlled.

This leads to problems known as race conditions.

Race Conditions.It is a problem in which the

behaviour of the circuit is different depending upon the relative magnitude of delays.

Types of Race ConditionUncritical Race conditionCritical Race condition

Uncritical Race condition: Some times the ultimate destination is the same stable state regardless of which variable wins the race. This kind of race condition is known as uncritical race condition.

Types of Race Condition

Critical Race condition : When the stable destination state critically depends upon which of the variables wins the race. This condition is known as critical race condition.

Synchronous Sequential CircuitsSynchronous Sequential Circuits do

not get affected by unknown delays in the feedback path, instead the memory elements (logic gates) change their state only at pre-assigned discrete intervals of time.

This synchronization can be achieved by a timing device called a clock pulse generator that produces a periodic train of clock pulses.

Synchronous Sequential CircuitsSynchronous sequential circuits which use

clock pulses at the input of memory elements are called clocked sequential circuits.

These sequential circuits are the most commonly used circuits in practice and do not exhibit instability problems like race conditions etc.

Its design is simple and they operate reliably.

They are slower than asynchronous Sequential Circuits.

Clock Inputlets familiarise ourselves with

clock input.

Clock Cycle

Time

0

1 Rising edge Falling edge

Flip FlopsThe storage elements employed

in clocked sequential circuits are called flip-flops.

A flip-flop is a binary cell capable of storing one bit of information.

Flip-flops allow sequential circuits to have state (i.e., memory), which is something that combinational logic circuits do not have.

Basics of Flip-FlopsA flip-flop has two outputs, one

for the normal value and one for the complement value of the bit stored in it.

There are mainly 4 types of Flip-flops:1. SR 2. JK3. D4. T

Basics of Flip-FlopsThe SR & JK Flip-Flops takes two

inputs where as D & T Flip-Flops takes only one input.

Block diagram of Flip-Flop

Flip-FlopInputs Output

s

Q

Q’

Is there any logic behind flip-flops name? Yes/ No?Yes there is meaning behind the

name of every flip-flops.So lets study one by one.SR: In this S stand for set and R

stand for reset. JK: The logical meaning is same

as SR flip-flop i.e. J stand for set and K stand for reset. So why its name JK rather than SR?

logic behind flip-flops nameOne thing is clear and that is, we

have SR flip-flops already exist. So why it is JK not AB?Because this name is dedicated

to the person who discover IC that is integrated circuit.

The name is Jack Kalbay.

Registers A register is a group of flip-flop

with each flip-flop capable of storing one bit of information. An n-bit register has a group of n flip-flops and is capable of storing any binary information of n bits.

In addition to the flip-flops, a register may have combinational gates that perform certain data-processing tasks.

RegistersNow which Flip-Flop we can use

in order to make registers?RS,JK,D or T?The answer is D.Because D flip-flop can store the

information as it is.

Let’s construct a 4-bit register with D flip-flopSo what we need for this. 4-bit register means it can store

4-bits of information. For this we need as least 4 flip

flops. It is a sequential circuit so it also

need a clock. So we need a clock also.Moreover we need 4 bits of input.

Let’s start our constructionD

>C

Q

D

>C

Q

D

>C

Q

D

>C

Q

I0

I1

I2

I3

Clock

A0

A1

A2

A3

Shift RegistersRegisters that are capable of moving

information position wise upon the occurrence of a clock signal are called shift registers.

They are a group of flip-flops connected in a chain so that the output from one flip-flop becomes the input of the next flip-flop.

All the flip-flops are driven by a common clock, and all are set of reset simultaneously.

Shift RegistersShift registers can be categorized

based on the manner in which information is entered into and outputted from a register.

The basic ways are Serial or Parallel.When the information is transferred

in a parallel manner, all the bits that comprise the information are handled simultaneously as a entity in a single unit of time.

Shift RegistersThe serial handling of information

involves the bit by bit availability of the information in a time sequence.

So finally we have four possible ways by which registers can transfer information: ◦Serial-in serial-out.◦Serial-in parallel-out.◦Parallel-in Serial-out.◦Parallel-in Parallel-out.

Shift RegistersRegisters are also classified by

whether they can move the information in one or two directions i.e. Unidirectional or bidirectional.

J>K

Q

J>K

Q

J>K

Q

Clock

Increment

Load

Clear

I0

I1

I2

A0

A1

A2

Output carry