ECE U322 Digital Logic Design

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ECE U322Digital Logic Design
Nov. 7, 2005

• Lecture 25
• Sequential Circuit Design
• Second Midterm Wednesday, Nov. 16.

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Announcements

• Midterm in class on Wed, Nov 16
• Homework 7 due this Thursday
• Practice MidtermII
• We will go over solutions Nov. 14 in class.

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Moore Machine

• Any Mealy machine can be turned into a Moore
  machine, and vice versa
• A Moore machine has the same number of states or
  more states than a Mealy machine
• Output timing is different between a Moore
  machine and a Mealy Machine
• Moore outputs change on clock edge
• Mealy machine output changes whenever inputs
  change

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01 Sequence Recognizer
Mealy machine
Moore machine
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State Table for 01 Recognizer
Output State A State B State C
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State Table for 01 Recognizer

• A 00, B 01, C 10

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Equations for 01 String Recognizer

• D1
• D0
• Z

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Circuit for 01 String Recognizer Moore version
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Timing Diagram for 01 string recognizer Moore
version
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State Diagram for 1101 string recognizer Mealy
version
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State Diagram for 1101 string recognizer Moore
version

• outputs marked on
  states for Moore model
• Arcs show only
  state transitions
• Add a new state E to
  produce the output 1
• The new state, E produces the same behavior
  in the future
  as state B. But it gives a different output at
  the present time. The timing of the Moore and the
  Mealy machines are different.

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Implement 01 sequence recognizer with J-K
Flipflops

• One extra design step determine J-K inputs
• We will do the Mealy version
• two states, one flip-flop A 0 B 1

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State Table for 01 recognizer
Output is the same as before
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Equations for 01 state recognizer

• J
• K
• Z

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Circuit Diagram for 01 recognizer with JK
flipflops
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Timing Diagram 01 recognizer with JK flipflops
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State Machine Implementations

• D or J-K flip-flops
• same number of states
• same number of flip-flops
• same timing
• Mealy or Moore machine
• Moore may have more states
• Timing is different
• output changes with state for Moore machine

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Candy machine example
Let us design a control unit for a simple
coin-operated candy machine. Candy costs 20
cents, and the machine accepts nickels and dimes.
Change should be returned if more than 20 cents
is deposited. No more than 25 cents can be
deposited on a single purchase therefore, the
maximum change is one nickel.
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State Diagram
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State Table