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CHOOSE FIRST…………..PowerPoint Presentation: 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 MENUPowerPoint Presentation: 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.PowerPoint Presentation: 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.PowerPoint Presentation: 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.PowerPoint Presentation: Block diagram of a sequential circuit NEX T STATE LOGIC MEMORY ELEMENTS OUTPUT LOGIC CLOCK EXTERNAL OUTPUTS EXTERNAL INPUTSPowerPoint Presentation: 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 .PowerPoint Presentation: 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.PowerPoint Presentation: 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.PowerPoint Presentation: TYPES OF FLIP-FLOPS • RS flip-flop •Master Slave JK flip-flop • T flip-flop • D flip-flop •Clock D flip-flopPowerPoint Presentation: 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).PowerPoint Presentation: 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 XPowerPoint Presentation: 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 unstablePowerPoint Presentation: 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-FLOPPowerPoint Presentation: D Q Q' master stage slave stage P P' CLK R S Q Q' R S Q Q'PowerPoint Presentation: MASTER SLAVE J-K FLIP FLOPPowerPoint Presentation: 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 .PowerPoint Presentation: 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.PowerPoint Presentation: MASTER SLAVE FLIP-FLOP A master slave flip-flop consists of two latches and an inverter. Master-slave RS flip-flopPowerPoint Presentation: 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)PowerPoint Presentation: 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 CircuitsPowerPoint Presentation: 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 CIRCUITPowerPoint Presentation: OUTPUT DECODER MEMORY ELEMENTS NEXT STATE DECODER Outputs (in) Outputs (out)PowerPoint Presentation: MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out) CLASS C CIRCUITPowerPoint Presentation: MEMORY ELEMENTS NEXT STATE DECODER Inputs (in) Outputs (out) CLASS D CIRCUITPowerPoint Presentation: MEMORY ELEMENTS Outputs (out) CLASS E CIRCUIT Inputs (in)PowerPoint Presentation: 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.PowerPoint Presentation: STATE DIAGRAM 00 11 10 01 0/0 1/0 1/0 0/0 0/0 1/0 1/1 0/0PowerPoint Presentation: 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 .PowerPoint Presentation: DESIGN PROCEDURE OF SEQUENTIAL CIRCUITS CLICK TO SHOW THE DESIGNPowerPoint Presentation: REFERERENCES: www.meseec.ce.rit.edu/eecc341-winter2001/341-1-29-2002.pdF www.slideshare.net/ram_ari/sequential-logic-circuitPowerPoint Presentation: END OF THE LESSON GET READY TO ANSWER THE FOLLOWING QUESTIONS…. …GO…PowerPoint Presentation: 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-flopsPowerPoint Presentation: 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-flopsPowerPoint Presentation: 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-flopPowerPoint Presentation: 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-flopPowerPoint Presentation: 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. outputsPowerPoint Presentation: CONGRATULATIONS………. ….YOU DID IT…. EXITPowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: …VERY GOOD… …EXCELLENT….PowerPoint Presentation: KINDLY REVIEW THE LESSONSPowerPoint Presentation: KINDLY REVIEW THE LESSONSPowerPoint Presentation: KINDLY REVIEW THE LESSONSPowerPoint Presentation: KINDLY REVIEW THE LESSONSPowerPoint Presentation: KINDLY REVIEW THE LESSONS You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.