Chapter 4 Sequential Circuits Sequential Circuits Combinational Circuits

Chapter 4 Sequential Circuits

Sequential Circuits • Combinational Circuits + Storage element • output depends both on previous state and input Fig. 4 -1

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

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

4 -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. 4 -4 SR latch with NOR gates

Simulation for SR latch Fig. 4 -5

- latch Fig. 4 -6 latch with NAND gates

- latch with control input Fig. 4 -7

D (Data) latch Fig. 4 -8

4 -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. 4 -3 for sequential circuits ¡ A present (original) and next (new) state occur in flipflop before and after the trigger, respectively ¡ The most important element in sequential circuits ¡ Can be derived from latch

Negative-Edge-Triggered D flip-flop

Positive-Edge-Triggered D flip-flop

Symbols

Direct Inputs

4 -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. 4 -13 15

State table

Two-dimensional state table

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. 4 -13 ¡ Moore model circuits Sequential circuits in which the outputs depend only on the states The circuits in Fig. 4 -14

A Moore model circuit (Fig. 4 -14)

State diagram (a): for Fig. 4 -13 (b): for Fig. 4 -14

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