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**DIGITAL ELECTRONICS & it0203 Semester - 3**

P.Rajasekar & C.M.T.Karthigeyan Asst.Professor SRM University, Kattankulathur

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Disclaimer The contents of the slides are solely for the purpose of teaching students at SRM University. All copyrights and Trademarks of organizations/persons apply even if not specified explicitly.

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**Unit 4 - sequential circuit**

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**Overview Synchronous Sequential Logic Design Procedure**

Introduction to sequential circuits Types of sequential circuits Storage elements Latches Flip-flops Sequential circuit analysis State tables Design Procedure

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**Introduction to Sequential Circuits**

Inputs Outputs Combina-tional Logic A Sequential circuit contains: Storage elements: Latches or Flip-Flops Combinational Logic: Implements a multiple-output switching function Inputs are signals from the outside. Outputs are signals to the outside. Other inputs, State or Present State, are signals from storage elements. The remaining outputs, Next State are inputs to storage elements. Storage Elements State Next State

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**Introduction to Sequential Circuits**

Inputs Outputs Combina-tional Logic Combinatorial Logic Next state function Next State = f(Inputs, State) Output function (Mealy) Outputs = g(Inputs, State) Output function (Moore) Outputs = h(State) Output function type depends on specification and affects the design significantly Storage Elements State Next State

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**Types of Sequential Circuits**

Depends on the times at which: storage elements observe their inputs, and storage elements change their state Synchronous Behavior defined from knowledge of its signals at discrete instances of time Storage elements observe inputs and can change state only in relation to a timing signal (clock pulses from a clock) Asynchronous Behavior defined from knowledge of inputs an any instant of time and the order in continuous time in which inputs change If clock just regarded as another input, all circuits are asynchronous! Nevertheless, the synchronous abstraction makes complex designs tractable!

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Basic (NAND) S – R Latch “Cross-Coupling” two NAND gates gives the S -R Latch: Which has the time sequence behavior: S = 0, R = 0 is forbidden as input pattern S (set) Q Q R (reset) Time R S Q Comment 1 ? Stored state unknown “Set” Q to 1 Now Q “remembers” 1 “Reset” Q to 0 1 1 1 Now Q “remembers” 0 1 1 Both go high 1 1 ? ? Unstable!

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Basic (NOR) S – R Latch Cross-coupling two NOR gates gives the S – R Latch: Which has the time sequence behavior: S (set) R (reset) Q R S Q Comment ? Stored state unknown 1 “Set” Q to 1 Now Q “remembers” 1 “Reset” Q to 0 Now Q “remembers” 0 Both go low Unstable! Time

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Clocked S - R Latch Adding two NAND gates to the basic S - R NAND latch gives the clocked S – R latch: Has a time sequence behavior similar to the basic S- R latch except that the S and R inputs are only observed when the line C is high. C means “control” or “clock”. S R Q C

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**Clocked S-R Latch (continued)**

The Clocked S-R Latch can be described by a table: The table describes what happens after the clock [at time (t+1)] based on: current inputs (S,R) and current state Q(t). S R Q C

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**D Latch Adding an inverter to the S-R Latch, gives the D Latch:**

Note that there are no “indeterminate” states! D Q C Q D Q(t+1) Comment No change 1 Set Q Clear Q No Change C D Q

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**Flip-Flops The latch timing problem Master-slave flip-flop**

Edge-triggered flip-flop Standard symbols for storage elements Direct inputs to flip-flops

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**Storage Elements Flip Flops**

In a sequential circuit the changes should take place with clock transitions and not with levels.

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**S-R Master-Slave Flip-Flop**

Consists of two clocked S-R latches in series with the clock on the second latch inverted The input is observed by the first latch with C = 1 The output is changed by the second latch with C = 0 The path from input to output is broken by the difference in clocking values (C = 1 and C = 0). The behavior demonstrated by the example with D driven by Y given previously is prevented since the clock must change from 1 to 0 before a change in Y based on D can occur. C S R Q

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**Flip-Flop Solution Use edge-triggering instead of master- slave**

An edge-triggered flip-flop ignores the pulse while it is at a constant level and triggers only during a transition of the clock signal Edge-triggered flip-flops can be built directly at the electronic circuit level, or A master-slave D flip-flop which also exhibits edge-triggered behavior can be used.

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**Edge-Triggered D Flip-Flop**

The edge-triggered D flip-flop is the same as the master- slave D flip-flop It can be formed by: Replacing the first clocked S-R latch with a clocked D latch or Adding a D input and inverter to a master-slave S-R flip-flop The change of the D flip-flop output is associated with the negative edge at the end of the pulse It is called a negative-edge triggered flip-flop C S R Q D

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**Edge-Triggered D Flip-Flop**

The edge-triggered flip-flop consists of two latches: master and slave.

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JK Flip-Flop Other types of flip-flops may be constructed from D flip-flop. Two other types less commonly used are JK and T flip flops.

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T Flip-Flop

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**Flip Flop Characteristic Table**

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**Flip Flop Characteristic Equations**

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**Sequential Circuit Analysis**

General Model Current State at time (t) is stored in an array of flip-flops. Next State at time (t+1) is a Boolean function of State and Inputs. Outputs at time (t) are a Boolean function of State(t) &(sometimes)Inputs (t). Inputs Combina-tional Logic Outputs Storage Elements CLK State Next State

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**Example x y Input: x(t) Output: y(t) State: ((A(t), B(t))**

What is the Output Function? What is the Next State Function? A C D Q y x B CP

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**State Table Characteristics**

State table – a multiple variable table with the following four sections: Present State – the values of the state variables for each allowed state. Input – the input combinations allowed. Next-state – the value of the state at time (t+1) based on the present state and the input. Output – the value of the output as a function of the present state and (sometimes) the input. From the viewpoint of a truth table: the inputs are Input, Present State and the outputs are Output, Next State

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Example: State Table The state table can be filled in using the next state and output equations: A(t+1) = A(t)x(t) + B(t)x(t) B(t+1) =A (t)x(t) y(t) =x (t)(B(t) + A(t)) Present State Input Next State Output A(t) B(t) x(t) A(t+1) B(t+1) y(t) 1

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**Example: Alternate State Table**

2-dimensional table that matches well to a K- map. Present state rows and input columns in Gray code order. A(t+1) = A(t)x(t) + B(t)x(t) B(t+1) =A (t)x(t) y(t) =x (t)(B(t) + A(t)) Present State Next State x(t)= x(t)=1 Output x(t)=0 x(t)=1 A(t) B(t) A(t+1)B(t+1) A(t+1)B(t+1) y(t) y(t) 0 0 0 1 1 0 1 1

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State Diagram

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**Analysis with JK Flip Flops**

Determine flip-flop input equations in terms of present state and inputs. For each JKFF determine J=f(Q(t),x(t)) and K=g(Q(t),x(t)) Using the flip-flop characteristic table determine the next state values in the state table. Substitute equations for J and K into Q(t+1)=JQ’+K’Q to calculate the values of next-states

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Analysis with JK FF

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State Table

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State Diagram

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Excitation Tables Input equations for D-FF is obtained directly from next state For JK and T FFs FF input equations cannot be obtained easily from state table. A table that lists the inputs for a given change of state is needed. It is called excitation table

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Excitation Tables

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Synthesis Using JK FF

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Synthesis Using JK FF

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Synthesis Using JK FF

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Shift Register A register capable of shifting the binary information held in each cell to its neighboring cell, is called a shift register. Serial input is shifted to right with each clock pulse.After four clock pulses serial input appears at the Serial output.

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Serial Transfer A digital system operates at serial mode: information transferred and manipulated one bit at a time. Serial transfers take place with the system clock. In the next clock contents of shift register A is transferred to shift register B.

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Serial Transfer

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Mysterious Function

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Serial Adder Two binary numbers to be added are stored in two shift registers A and B. The shift registers are shifted right starting with least significant bit, the bits are added by the full adder and carry is stored in a D flip-flop. When the addition is completed the sum is in regiater A. Serial adder will require n clock pulses for adding n-bit numbers. A parallel adder can perform the n-bit addition in one clock pulse period. However a parallel adder will have n full adders.

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**Universal Shift Register**

Clear control Shift-right Shift left Parallel load Parallel output

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Counters A register that goes through a prescribed sequence of states upon application of input pulses is called a counter. A counter that follows binary number sequence is called a binary counter n-bit binary counter consists of n flip-flops and count from 0 to 2^n-1.

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Ripple Counters Two categories: ripple counters and synchronous counters In a ripple counter the clock input of flip- flops is connected to output of other flip flops Ripple counters do not have a common clock (not synchronous circuit) Synchronous counters have common clock connected to clock input of flip-flops

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Ripple Counters

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Up-Down Counter

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BCD Counter

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BCD Counter

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Ring Counter

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Johnson Counter

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**Counters With Unused States**

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Counter Example

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bibliography Charles H. Roth- Jr., Fundamentals of logic design, Thomson Asia,5th edition (CH1,CH2,CH3,CH4,CH5,CH6,CH7,CH9,CH11, CH12) M. Morris Mano, Digital Logic and Computer Design, Prentice Hall of India Floyd, Digital Fundamentals, Universal Book Stall, 3rd Edition,1986 Morris Mano, Digital Design, Prentice Hall of India, 2nd Edition 1991 Bigell & Donovan, Digital Electronics, Thomson Asia Pvt. Ltd., 4th Edition

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Review questions State the difference between combinational circuits and sequential circuits? What do you mean by Latches? Give the difference between latches and flip flops? Define Flipflops? List the design procedures for designing a sequential circuit? State the operation of S-R flipflop? State the operation of T-fliflop? State the features of clocked T flipflop?

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Review questions Give the characteristics equation for S-R , T,J-K and D-flipflop? What is a counter? What is a buffer register? List the types of shift register? How do you convert JR flipflop into D flipflop? How do you convert T flipflop into D- flipflop? Give the counter applications?

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