Sequential Circuits

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

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 Input Output Primary Inputs Feedback Path Feedback Circuit Secondary Inputs Sequential Circuits

“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 Q Q’ S

SR (NOR) Latch • The SR Latch Operations can be illustrate with the help of following table

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

SR (NOR) Latch • Lets study the previous table in detail.

Types of Sequential Circuits There are two types of Sequential Circuits: • Asynchronous • 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 Condition • Uncritical Race condition • Critical 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 Circuits Synchronous 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 Circuits • Synchronous 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 Input • lets familiarise ourselves with clock input. Falling edge Rising edge 1 0 Time Clock Cycle

Flip Flops • The 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-Flops • A 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: • SR • JK • D • T

Basics of Flip-Flops • The SR & JK Flip-Flops takes two inputs where as D & T Flip-Flops takes only one input. Block diagram of Flip-Flop Flip-Flop Q Inputs Outputs 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 name • One 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.

Registers • Now 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-flop • So 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 construction D Q I0 A0 >C D Q I1 A1 >C D Q I2 A2 >C D Q I3 A3 >C Clock

Shift Registers • Registers 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 Registers • Shift 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 Registers • The 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 Registers • Registers are also classified by whether they can move the information in one or two directions i.e. Unidirectional or bidirectional.

Clear Load Increment Q J > K A0 I0 Q J > K A1 I1 Q J > K A2 I2 Output carry Clock

Sequential Circuits