DIGITAL ELECTRONICS (331102)
PRACTICAL: 11 TO STUDY SEQUENTIAL CIRCUITS 1.0
2.0
AIM : To study different flip flops.
PRIOR CONCEPTS :
Knowledge of working of AND, OR, NOT N OT gate.
Knowledge of working of NAND and Ex - OR gates.
3.0
4.0
INTRODUCTION All the instructions that direct a computer's ope ration exist as a sequence of binary digits or bits (0s and 1s) – the instructions and the data are rep resented this way. The logic gates can be arranged in groups that cause these binary numbers to either act as adders, substractor, multipliers, dividers or l ogical comparators. Other groups of gates can act as storage for the instructions and data. These ‘groups’ are, in hardware design terms, latches and flip flops.
SEQUENTIAL CIRCUITS :
Unlike Combinational Logic circuits that change state depending upon the actual signals being applied to their inputs at that time, Sequential Logic circuits have some form of inherent "Memory" built in to them and they are able to take into account their previous input state as well as those actually present, a sort of "before" and "after" is involved. They are generally termed as Two State or Bistable devices which can have their output set in either of two basic states, a logic level "1" or a logic level "0" and will remain "Latched" indefinitely in this current state or condition until some other input trigger pulse or signal is applied which will change its state once again.
44 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
DIGITAL ELECTRONICS (331102)
5.0
The word "Sequential" means that things happen i n a "sequence", one after another and in Sequential Logic circuits, the actual clock signal determines when things will happen next. Simple sequential logic circuits can be constructed from standard Bistable circuits such as Flip-flops, Latches or Counters and which themselves can be made by simply connecting together NAND Gates and/or NOR Gates in a particular combinational way to produce t he required sequential circuit. Sequential Logic circuits can be divided into 3 main categories: 1. Clock Driven : Synchronous circuits that are synchronized to a specific clock signal. 2. Event Driven : Asynchronous Circuits that react or change state when an external event occurs. 3. Pulse Driven : which is a combination of Synchronous and Asynchronous circuits.
LATCH : An asynchronous latch is an electronic sequential logic circuit used to store information in an asynchronous arrangement. (Asynchronous: they have no Clock input.)
One latch can store one bit.
They change output state only in response to data input.
Essentially, they hold a bit value and it remains constant until new inputs force it to change. A type of single-bit single -bit stable storage.
45 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
DIGITAL ELECTRONICS (331102)
6.0
FLIP FLOPS : As with latches, flip-flops are another example of a circuit employing sequential logic.
A flip-flop can also be called a BISTABLE GATE.
A type of single-bit storage but not as stable as a latch.
7.0
A basic flip-flop has two stable states. The flip-flop maintains its states indefinitely until an input pulse (a trigger from the clock) is received. If a trigger is received, the t he flip-flop outputs change their states according to defined rules, and remain in those states until another trigger is received. There are several different kinds of flip-flop circuits, with designators such as : R – S Flip Flop o J – K Flip Flop o o D Flip flop T Flip flop (a variation on J – K Flip Flop) o
DIFFERENCE BETWEEN LATCH AND FLIP FLOP : Latches are level-sensitive while flip-flops are edge-sensitive. Both might require the use of a clock signal and are used in sequential logic. (The clock on the latch is for synchronisation whereas the clock on the flip -flop may trigger a change in output.) For a latch, the output tracks the input when the clock signal is high, so as long as the clock is logic 1 the output can change if the input also changes. (Logic 1 + new data = new output). Flip-flops, in comparison, will store the input only when there is a rising/falling edge of the clock. (Edge- triggered, tri ggered, so they may flip on clock pulses.)
8.0
EXERCISE :
Define sequential circuits.
Ans :
Define Combinational Circuits.
Ans :
Define Latch. Can we say that flip flop is a latch?
Ans : 46 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
DIGITAL ELECTRONICS (331102)
9.0
ASSIGNMENT :
Differentiate between Sequential circuits and combinational circuits.
Classify the sequential circuits.
Draw block diagram of Sequential circuits and explain the same.
Grades for Exercise: .................................................
Grades for Assignment: .................................................
Signature of Lab Co-ordinators: .................................................
47 | P a g e SOHIL VOHRA (LECTURER - SHRI K.J. POLYTECHNIC COLLEGE, BHARUCH (C.E. DEPTT))
