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Dr. Ahmad Almulhem Email: ahmadsm AT kfupm Phone: 860-7554 Office: 22-324. COE 202: Digital Logic Design Sequential Circuits Part 1. Objectives. Sequential Circuits Memory Elements Latches Flip-Flops. Combinational vs Sequential. Combinational Circuits. A combinational circuit:

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COE 202: Digital Logic Design Sequential Circuits Part 1


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    1. Dr. Ahmad Almulhem Email: ahmadsm AT kfupm Phone: 860-7554 Office: 22-324 Ahmad Almulhem, KFUPM 2009 COE 202: Digital Logic DesignSequential CircuitsPart 1

    2. Ahmad Almulhem, KFUPM 2009 Objectives • Sequential Circuits • Memory Elements • Latches • Flip-Flops

    3. Ahmad Almulhem, KFUPM 2009 Combinational vs Sequential Combinational Circuits • A combinational circuit: • At any time, outputs depends only on inputs • Changing inputs changes outputs • No regard for previous inputs • No memory (history) • Time is ignored ! inputs X outputs Z

    4. Ahmad Almulhem, KFUPM 2009 Combinational vs Sequential Combinational Circuits • A sequential circuit: • outputs depends on inputs and previous inputs • Previous inputs are stored as binary information into memory • The stored information at any time defines a state • next state depends on inputs and present state inputs X outputs Z next state present state Memory

    5. Ahmad Almulhem, KFUPM 2009 Examples of sequential systems ATM Traffic light Vending machine What is common between these systems?

    6. Ahmad Almulhem, KFUPM 2009 Combinational Adder • 4-bit adder (ripple-carry) • Notice how carry-out propagates • One adder is active at a time • 4 full adders are needed

    7. Ahmad Almulhem, KFUPM 2009 Sequential Adder • 1-bit memory and 2 4-bit memory • Only one full-adder! • 4 clocks to get the output • The 1-bit memory defines the circuit state (0 or 1)

    8. Ahmad Almulhem, KFUPM 2009 Types of Sequential Circuits • Two types of sequential circuits: • Synchronous: The behavior of the circuit depends on the input signal values at discrete intervals of time (also called clocked) • Asynchronous: The behavior of the circuit depends on the order of change of the input signals at any instance of time (continuous)

    9. Ahmad Almulhem, KFUPM 2009 Clock Combinational Circuits • A clock generator produces a periodic train of pulses (alternating 0s and 1s) • A clock signal is required for generating discrete time intervals in synchronous circuits • The memory/state changes with the clock inputs X outputs Z next state present state Memory clock

    10. Ahmad Almulhem, KFUPM 2009 Memory - Latches • A latch is binary storage component • Can store a 0 or 1 • The most basic memory element • Built with gates (NORs, NANDs) • Can be used to build flip-flops

    11. Ahmad Almulhem, KFUPM 2009 SR Latch • What does this circuit do?

    12. Ahmad Almulhem, KFUPM 2009 SR Latch • Two states: Set (Q = 1) and Reset (Q = 0) • When S=R=0, Q remains the same, S=R=1 is not allowed! • Provide a simple form of memory • State of the circuit depends not only on the current inputs, but also on the recent history of the inputs

    13. Ahmad Almulhem, KFUPM 2009 S’ R’ Latch • How about this circuit?

    14. Ahmad Almulhem, KFUPM 2009 S’ R’ Latch • Similar to SR latch (complemented) • Two states: Set (Q = 0) and Reset (Q = 1) • When S=R=1, Q remains the same • S=R=0 is not allowed!

    15. Ahmad Almulhem, KFUPM 2009 SR Latch with Clock • An additional input signal (Clock) can be introduced to make sure that the operation of the latch is modified depending on the value of the Clock (C) signal • When C=0, the S and R inputs have no effect on the latch • When C=1, the inputs affect the state of the latch and possibly the output

    16. Ahmad Almulhem, KFUPM 2009 D Latch • How can we eliminate the undefined state?

    17. Ahmad Almulhem, KFUPM 2009 D Latch • Ensure S and R are never equal to 1 at the same time • Add inverter • D stands for data • Output follows the input when C = 1 • When C = 0, Q remains the same

    18. Ahmad Almulhem, KFUPM 2009 Characteristic Table • Similar to the truth table in combinational circuits • Next state is defined in terms of the current state and the inputs • K-Map can be used to obtain the characteristic equation • The indeterminate states can be expressed as don’t cares

    19. Ahmad Almulhem, KFUPM 2009 Characteristic Table – SR Latch

    20. Ahmad Almulhem, KFUPM 2009 Characteristic Table – SR Latch Characteristic Equation S=R=1 is not permitted, therefore this condition is all included here

    21. Ahmad Almulhem, KFUPM 2009 Characteristic Table – D Latch

    22. Ahmad Almulhem, KFUPM 2009 Characteristic Table – D Latch Characteristic Equation for the D Latch

    23. Ahmad Almulhem, KFUPM 2009 Excitation Table • Defines the input conditions for which a certain next state is reached from a certain current state for a circuit • Lists the inputs for moving from one state to another

    24. Ahmad Almulhem, KFUPM 2009 Excitation Table • Defines the input conditions for which a certain next state is reached from a certain current state for a circuit • Lists the inputs for moving from one state to another Example: SR Latch

    25. Ahmad Almulhem, KFUPM 2009 Clocked JK Latch extra feedbacks How does it work? Function table? Characteristic Table/Equations? Excitation Table?

    26. Ahmad Almulhem, KFUPM 2009 Clocked JK Latch • The JK latch improves on the SR latch by ensuring that the no input combination will lead to an indeterminate state • The outputs of this latch (Q, Q’) are coupled back to the latch input • J and K behave like S and R. • J implies the Set • K implies the Reset • For J=K=1, the latch switches to its complement state, i.e. Q becomes 0 if it was already 1 and vice versa

    27. Ahmad Almulhem, KFUPM 2009 JK Latch – Characteristic Table

    28. Ahmad Almulhem, KFUPM 2009 JK Latch – Excitation Table • For the next state and present state to be equal to 0, J must be equal to 0 • For both next and current states to be equal to 1, K must be equal to 0 • To change state from 0 to 1, J must be 1 • To change state from 1 to 1, K must be 1

    29. Ahmad Almulhem, KFUPM 2009 Clocked T Latch • The clocked T latch is a single input version of the JK latch • The letter ‘T’ stands for Toggle, as it toggles the current state when equal to 1 • When T=1, the next state is the complement of the current state

    30. Ahmad Almulhem, KFUPM 2009 T Latch (Tables) Characteristic table/equation Excitation table Functional table

    31. Ahmad Almulhem, KFUPM 2009 Q Clock D Q Q C Problem with Latches • What happens Clock=1? What will be the value of Q when Clock goes to 0? • Problem: A latch is transparent; state keep changing as long as the clock remains active • Due to this uncertainty, latches can not be reliably used as storage elements.

    32. Ahmad Almulhem, KFUPM 2009 Flip Flops • A flip-flop is a one bit memory similar to latches • Solves the issue of latch transparency • Latches are level sensitive • Flip-Flops are edge-triggered or edge-sensitive level positive (rising) edge negative (falling) edge

    33. Ahmad Almulhem, KFUPM 2009 Flip Flops Implementation • Two methods: • Master-Slave • Two latches (master and slave) and additional logic • Edge-Triggered • Usually implemented differently at the transistor level • Can also be built with latches similar to master-slave implementation

    34. Ahmad Almulhem, KFUPM 2009 Master-Slave SR Flip Flop • Built using two latches (Master and Slave) • C = 1, master is active • C = 0, slave is active • Q is sampled at the falling edge • Also called pulse-triggered: data is entered on the rising edge of the clock pulse, but the output does not reflect the change until the falling edge of the clock pulse.

    35. Ahmad Almulhem, KFUPM 2009 Master-Slave JK Flip Flop • Two SR Latches (master and slave) • Function is similar to JK Latch but • transparency issue is solved

    36. Ahmad Almulhem, KFUPM 2009 Edge-Triggered D Flip Flop • This flip-flop takes exactly the form of a master-slave flip-flop, with the master a D latch and the slave an SR latch. Also, an inverter is added to the clock input of the master latch. • Because the master latch is a D latch, the flip-flop exhibits edge-triggered rather than master-slave (pulse-triggered) behavior. -ve (falling) edge +ve (rising) edge

    37. Ahmad Almulhem, KFUPM 2009 Standard Symbols

    38. Ahmad Almulhem, KFUPM 2009 S D Q C Q R Direct Inputs • At power up or at reset, all or partof a sequential circuit usually isinitialized to a known state beforeit begins operation • This initialization is often doneoutside of the clocked behaviorof the circuit, i.e., asynchronously. • Direct R and/or S inputs that control the state of the latches within the flip-flops are used for this initialization. • For the example flip-flop shown • 0 applied to R resets the flip-flop to the 0 state • 0 applied to S sets the flip-flop to the 1 state

    39. Ahmad Almulhem, KFUPM 2009 D Flip Flop with Direct Inputs Question: What type of D-FF? Why?

    40. Ahmad Almulhem, KFUPM 2009 Flip Flops Sheet (Mano’s Textbook)

    41. Ahmad Almulhem, KFUPM 2009 Summary • In a sequential circuit, outputs depends on inputs and previous inputs • Previous inputs are stored as binary information into memory • The stored information at any time defines a state • Similarly, next state depends on inputs and present state • Two types of sequential circuits: Synchronous and Asynchronous • Two types of Memory elements: Latches and Flip-Flops. • A flip-flop is described using functional, characteristic table/equation and excitation tables