Sequential Circuits A Basic sequential circuit is nothing but a combinational circuit with some feedback paths between its output and input terminals.
Sequential Circuits
Feb 24, 2016
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. Input. Output. Feedback Path. Feedback Circuit. Sequential Circuits. - PowerPoint PPT Presentation
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Sequential CircuitsA 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 Action0 0 Keep State0 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 R0 0 0 1 0 1 Keep State
1 0 1 00 1 0 1 1 0 Q=0
(Reset)1 0 1 01 0 0 1 0 1 Q=1 (Set)
1 0 0 11 1 0 1 0 0 Unstable
combination1 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 ConditionCritical 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 Outputs
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
IncrementLoad
Clear
I0
I1
I2
A0
A1
A2
Output carry
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