CHAPTER 6

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Sequential Logic Circuits

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SEQUENTIAL LOGIC CIRCUITS CHAPTER 6 Prepared by: RIC JR. V. BELOY

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LESSONS… EVALUATION??? CHOOSE FIRST…………..

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INTRODUCTION S o far, all of the logic circuits we have studied were basically based on the analysis and design of combinational digital circuits. The other major aspect of a digital system is the ANALYSIS and DESIGN of sequential digital circuits. MAIN MENU

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SEQUENTIAL LOGIC CIRCUITS The main characteristic of combinational logic circuits is that their output values depend on their present input values. Sequential logic circuits differ from combinational logic circuits because they contain memory elements so that their output values depend on both present and past input values.

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Combinational Circuits – the logic circuits whose outputs at any instant of time depend only on the input signals present at the time. Sequential Circuits – the logic circuits whose outputs at any instant of time depend the present inputs as well as on the past outputs.

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Sequential circuits can be Asynchronous or synchronous . Asynchronous sequential circuits change their states and output values whenever a change in input values occurs. Synchronous sequential circuits change their states and output values at fixed points of time, i.e. clock signals.

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Block diagram of a sequential circuit NEX T STATE LOGIC MEMORY ELEMENTS OUTPUT LOGIC CLOCK EXTERNAL OUTPUTS EXTERNAL INPUTS

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Memory Element – is a medium in which one bit of information ( 0 or 1 ) can be stored or retained until necessary, and thereafter its contents can be replaced by a new value .

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Memory Devices Latches A latch is a memory element whose excitation signals control the state of the device. A latch has two stages set and reset . Set stage sets the output to 1. Reset stage set the output to 0. Flip-flops A flip-flop is a memory device that has clock signals control the state of the device.

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FLIP-FLOPS Q Q’ INPUTS NORMAL OUTPUT INVERTED OUTPUT BLOCK DIAGRAM OF A FLIP-FLOP The basic 1-bit digital memory circuit is known as a flip-flop. It can have two states either the 1 state or the 0 state. It is also known as a bitable multivibrator. Flip-flops can be obtained by using NAND or NOR gates.

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TYPES OF FLIP-FLOPS • RS flip-flop •Master Slave JK flip-flop • T flip-flop • D flip-flop •Clock D flip-flop

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LATCH RS Latch The RS latch is the basic memory element consists of two cross-coupled NOR gates. It has two input signals, S set signal and R reset signal. It also has two outputs Q and Q' ; and two states, a set state when Q = 1 and a reset state when Q = 0 ( Q' = 1).

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RS Latch excitation table S R Q ( t ) Q ( t +1) 0 0 0 0 Hold 0 0 1 1 0 1 0 0 Reset Q ( t +1) = S ( t ) + R' ( t ) Q ( t ) 0 1 1 0 1 0 0 1 Set Q + = S + R'Q 1 0 1 1 1 1 0 X Forbidden 1 1 1 X

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S-R (SET-RESET) FLIP-FLOPS S R Q Q' 1 0 1 0 0 0 1 0 0 1 0 1 0 0 0 1 1 1 0 0 S R Q 0 0 hold 0 1 0 reset 1 0 1 set 1 1 unstable

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D FLIP-FLOP Make S and R complements of each other Eliminates 1s catching problem Can't just hold previous value (must have new value ready every clock period) Value of D just before clock goes low is what is stored in flip-flop Can make R-S flip-flop by adding logic to make D = S + R' Q. A D-FLIP-FLOP

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D Q Q' master stage slave stage P P' CLK R S Q Q' R S Q Q'

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MASTER SLAVE J-K FLIP FLOP

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T FLIP-FLOP T-FLIP-FLOP With a slight modification of a J-K flip-flop, we can construct a new flip flop called the T-FLIPFLOP . If the two inputs J and K of a J-K flip-flop are tied together it is referred to as a T-flip-flop . Hence a T flip-flop has only one input T and two outputs Q and Q’. The name T flip-flop actually indicates the fact that the flip-flop has the ability to TOGGLE . It has actually only two states TOGGLE STATE and MEMORY STATE .

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CLOCK D FLIP-FLOP The D flip-flop has only one input referred to as the D input, or data input, and two output as usual Q and Q’. It transfers the data at the input after the delay of one clock pulse at the output Q. So on some cases the input is referred to as a delay input and the flip-flop gates the name DELAY (D) flip-flop.

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MASTER SLAVE FLIP-FLOP A master slave flip-flop consists of two latches and an inverter. Master-slave RS flip-flop

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SEQUENTIAL CIRCUIT MODEL OUTPUT DECODER (combinational logic) MEMORY ELEMENTS NEXT STATE DECODER Outputs (in) Outputs (out) Next state variables(NS) NS=f( In,PS ) Present state Variable(PS)

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CLASSIFICATION OF SEQUENTIAL CIRCUIT From the General sequential circuit model discussed in the preceding section , sequential circuits are generally classified into five different classes; Class A Circuits Class B Circuits Class C Circuits Class D Circuits Class E Circuits

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The Class A circuit is defined as a Mealy machine named after G.H. Mealy . The basic property of a mealy machine is that the output is a function of the present input conditions and the present state of the circuit. The Class B and C circuits are generally defined as a MOORE machine named after E.F. Moore . In these types of circuits the output is strictly a function of the present state (PS) of the circuit inputs. CLASS B CIRCUIT

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OUTPUT DECODER MEMORY ELEMENTS NEXT STATE DECODER Outputs (in) Outputs (out)

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MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out) CLASS C CIRCUIT

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MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out) CLASS D CIRCUIT

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MEMORY ELEMENTS Outputs (out) CLASS E CIRCUIT Inputs (in)

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ANALYSIS OF SEQUENTIAL CIRCUITS The behavior of a sequential circuit is determined from the inputs, the outputs, and the states of the flip-flops. Both the outputs and the next states are a function of the inputs and the present state. The analysis of sequential circuits consists of obtaining a table or a diagram for the time sequence of inputs, outputs, and internal states. Boolean expressions can be written that describes the behavior of the sequential circuits.

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STATE DIAGRAM 00 11 10 01 0/0 1/0 1/0 0/0 0/0 1/0 1/1 0/0

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In the state diagram, a state is represented by a circle and the transitions between states is indicated by direct arrows connecting the circles. The binary number inside each circle identifies the state the circle represents. The circle arrows are labeled with two binary numbers separated by a /. The number before the / represents the value of the external input , which causes the transition , and the number after the / represents the value of the output during the present state .

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DESIGN PROCEDURE OF SEQUENTIAL CIRCUITS CLICK TO SHOW THE DESIGN

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REFERERENCES: www.meseec.ce.rit.edu/eecc341-winter2001/341-1-29-2002.pdF www.slideshare.net/ram_ari/sequential-logic-circuit

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END OF THE LESSON GET READY TO ANSWER THE FOLLOWING QUESTIONS…. …GO…

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EVALUATION; 1. ____________________ differ from combinational logic circuits because they contain memory elements so that their output values depend on both present and past input values. a. Sequential logic circuits b. Logic circuits c. Sequential circuits 2. The logic circuits whose outputs at any instant of time depend the present inputs as well as on the past outputs. a. Sequential circuits b. Logic circuits c. Flip-flops

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3. The logic circuits whose outputs at any instant of time depend only on the input signals present at the time. a. Combinational circuits b. Flip-flops c. Circuits 4. A sequential circuit that change their states and output values whenever a change in input values occurs. a. Asynchronous sequential circuits b. Circuits c. Flip-flops

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5. The basic 1-bit digital memory circuit is known as; a. Flip-flops b. Circuits c. Logic Circuits 6. The ____ is the basic memory element consists of two cross-coupled NOR gates. a. RS latch b. D flip-flop c. T flip-flop

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7.With a slight modification of a J-K flip-flop, we can construct a new flip flop called; a. T flip-flop b. D flip-flop c. RS latch 8. A ______ consists of two latches and an inverter. a. master slave flip-flop b. T flip-flop c. SR flip-flop

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9. The Class A circuit is defined as a Mealy machine named after; a. G.H. Mealy b. E.F. Moore c. William Moore 10. The behavior of a sequential circuit is determined from the following except; a. Sequence b. inputs c. outputs

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CONGRATULATIONS………. ….YOU DID IT…. EXIT

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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…VERY GOOD… …EXCELLENT….

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KINDLY REVIEW THE LESSONS

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KINDLY REVIEW THE LESSONS

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KINDLY REVIEW THE LESSONS

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KINDLY REVIEW THE LESSONS

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KINDLY REVIEW THE LESSONS