Digital Design
Module 3
Introduction to Sequential Circuit
Amit Kumar
AP
SCSE, GU Greater Noida
OUTLINE
Introduction to Sequential Circuit
• Types of Sequential Circuit
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Synchronous Sequential Circuit
Asynchronous Sequential Circuit
Latches
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SR Latch
Two Types of Switching Circuits
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Combinational Circuits
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Combinational circuits have only input and output.
Output depends on input.
Example: AND,OR,NAND,NOR,XOR etc
Sequential Circuits
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Sequential circuits have input, present state, next
state and output. Next state depends upon
present state and input. Output depends upon
present state and input
Example: Flip-Flops etc
Sequential Circuit: Introduction
• Every digital system is likely to have combinational circuits, most
systems encountered in practice also include storage elements, which
require that the system be described in term of sequential logic.
• A sequential circuit consists of a feedback path, and employs some
memory elements.
Sequential circuit = Combinational logic + Memory Elements
Cont…
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Most digital systems like digital watches, digital
phones, digital computers, digital traffic light
controllers and so on require memory elements
Memory elements are digital circuits that can
store and retrieve data in the form of 1's and
0's.
The output of the systems with memory
depends not only on present inputs but also on
what has happened in the past
Types of Sequential Circuits
There are two types of sequential
circuits:
synchronous: outputs change only
at specific time
asynchronous: outputs change at
any time
Synchronous Sequential Circuit
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The behavior of a synchronous sequential circuit
depends upon the any input signal at any instant
of time and order of input change.
This synchronization is achieved by clock
generators that provides clock pulses (see the
next slide).
The storage elements used in clocked sequential
circuits are called flip-flops.
Asynchronous Sequential Circuit
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In asynchronous sequential circuits the storage
elements are time delay devices (i.e., storage is
because of their propagation delay).
They implemented by feedback that may cause
instability in Asynchronous circuits.
Synchronous Clocked Sequential Circuit
A sequential circuit may use many flip-flops to store as many bits as necessary.
The outputs can come either from the combinational circuit or from the flip-flops or
both.
Latches
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Store one bit of information.
Storage elements that operate with signal
levels are referred to as Latches.
Level sensitive devices.
Latches are the basic circuits from which
all flip flop are constructed.
Clock Response in Latch
In Fig (a) a positive level response in the control input allows changes, in the
output when the D input changes while the clock pulse stays at logic 1.
SR Latch
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2.
3.
4.
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If SR=10 then Q=1 and the latch is storing a 1,
We call this setting the Latch.
If SR =10 and we change to SR=00 then the
latch will remain set with Q= 1. In other words
it "remembers" to stay set
If SR=01 then Q=0 and the latch is storing a 0.
We call this resetting or Clearing the latch
If SR =01 and we change to SR=00 then the
latch will remain set with Q= 0.
We call the value of Q at any given time the
state of the latch
SR Latch
The SR latch is a circuit with two cross-coupled NOR gates or two cross-coupled
NAND gates. It has two inputs labeled S for set and R for reset.
SR Latch with NAND Gates