Microsoft Word - Digital Logic Design v_4_6a

4.6. Sequential Circuit Analysis

Flip-flops may be used to design circuits with feedback, such as counters, shift registers, sequence
detectors and controllers.

Feedback systems are classified as synchronous when all changes are synchronized with the system
clock. Feedback systems that do not use the system clock and change as the input change are called
asynchronous.

Synchronous systems are preferred over asynchronous systems since they will not have hazards and
other synchronization issues.

Synchronous systems are also referred to as synchronous finite state machines (FSM).

A FSM utilizes a system clock that adheres to the following definitions:

• Each period of system clock represents a State-Time


• A state represents the state of the flip-flop outputs (value of outputs)


• Synchronous external inputs  Xs


• Current machine state, flip-flop outputs  Ys


• Synchronous state machine external outputs  Zs

Synchronous state machines may be implemented in one of three models based on the characteristic of
its output (Moore, Mealy or mixed-type synchronous state machine):

 Moore-type Synchronous Finite State Machine

outputs are a function of the state of the machine Z1(Y1, Y2, Y3)

A Binary counter is a good example of Moore machine since the output of the flip-flops can be

used to represent the count.

 Mealy-type Synchronous Finite State Machine

Outputs are a function of the state of the machine and external inputs. Z1 (Y1, Y2, ...X1, X2, ...)

Excitation

Forming Logic

(Combinational)

Flip Flops

Input

Output

System Clock

Clock

External

Inputs (Xs)

Moore output

Forming Logic

(Combinational)

Moore

External Output

Zs(Ys)

Ys

Excitation

input

Feedback