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Chapter 5

Sequential Circuits. Chapter 5. Sequential Circuits. Combinational Circuits + Storage element output depends both on previous state and input. Fig. 5-1. Storage element. (a): a buffer t G the delay the information enters the buffer at t and output at t + t G

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Chapter 5

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  1. Sequential Circuits Chapter 5

  2. Sequential Circuits • Combinational Circuits + Storage element • output depends both on previous state and input Fig. 5-1

  3. Storage element • (a): a buffer • tG the delay • the information enters the buffer at t and output at t+ tG • the stored information only retained in buffer by tG • longer storage time is necessary in most applications Fig. 5-2

  4. Synchronous clocked sequential circuit Use flip-flop Fig. 5-3

  5. 5-2 latch • A storage element can maintain a binary state indefinitely, until directed by an input signal to switch states. • The most basic storage elements are latches. Fig. 5-4 SR latch with NOR gates

  6. Simulation for SR latch Fig. 5-5

  7. - latch Fig. 5-6 latch with NAND gates

  8. - latch with control input Fig. 5-7

  9. D (Data) latch Fig. 5-8

  10. 5-3 Flip-Flops A change in value on the control input allows the state of a latch in a flip-flop to switch. This change is called a trigger The trigger enable (trigger) the flip-flops See Fig. 5-3 for sequential circuits A present (original) and next (new) state occur in flip-flop before and after the trigger, respectively The most important element in sequential circuits Can be derived from latch

  11. SR Master-Slave flip-flop Fig. 5-9

  12. Simulation for SR Master-Slave flip-flop • Pulse trigger • Pulse in the inputs SR will result wrong output Fig. 5-10 Initially unknown Unknown due to R=1 S=1 Pulse input results wrong output

  13. Edge-trigger Flip-Flop Negative-edge-trigger Flip-Flop Fig. 5-11

  14. Edge-trigger Flip-Flop Positive -edge-trigger Flip-Flop

  15. Symbols Fig. 5-13

  16. Symbols Fig. 5-14

  17. 5-4 Sequential Circuit Analysis • The output and the next state are a function of the inputs and the present state. • An example input equations output equation Fig. 5-15

  18. State table

  19. Two-dimensional state table

  20. Mealy model/Moore model • Mealy model circuits • Sequential circuits in which the outputs depend on the input, as well as on the states • The circuits in Fig. 5-15 • Moore model circuits • Sequential circuits in which the outputs depend only on the states • The circuits in Fig. 5-16

  21. A Moore model circuit (Fig. 5-16)

  22. State diagram (a): for Fig. 5-15 (b): for Fig. 5-16

  23. Example 5-1 States reduction equivalent

  24. Example 5-1 States reduction

  25. Example 5-1 States reduction equivalent

  26. Example 5-1 States reduction • Reduce from 4 states, 2 flip-flops to 2 states, 1 flip-flop • may or may not result in reduced cost

  27. Sequential Circuit Simulation • A simulator for the input/output of a designed circuit • Functional simulation • Timing simulation

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