Flip-flop and Finite State Machine

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Flip-flop and Finite State Machine

• By
• Radha Iyer
• Prof Dr. Sin-Min Lee
• CS147 Computer Organization and Architecture

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What is Sequential Circuits?

• A Sequential Circuit can be defined as circuit
• having sequential logic. Sequential logic is a
  type
• of a logic circuit whose output depends not only
  on
• current inputs but also on previous inputs.

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What are flip-flops?

• In order to remember previous inputs, sequential
  circuits must have some sort of storage element.
  This storage element is called flip-flop.
• Flip-flop depends on previous inputs to the
  circuit.
• The basic memory unit is called an SR flip-flop.
• We can describe flip-flops using characteristic
  table.

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SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6 SR Flip-Flop operation (BUILT WITH NOR GATES) 6
Characteristic table Characteristic table Characteristic table Excitation table Excitation table Excitation table Excitation table Excitation table
S R Action Q(t) Q(t1) S R Action
0 0 Keep state 0 0 0 X No change
0 1 Q 0 0 1 1 0 reset
1 0 Q 1 1 0 0 1 set
1 1 Unstable combination 1 1 X 0 race condition
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Unstable Circuit

• S-stands for set
• R- stands for re-set
• If both S and R are set to1 in the SR flip-flop
  at the same time then it resulted in an unstable
  circuit, so this combination is not allowed.
• To take care of this problem JK flip-flops were
  developed.

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What is JK flip-flop?

• It is a variation to SR flip-flop, but it takes
  care of the SR1 combination.
• (JSet, KReset)
• When it comes to the SR1 it toggles. It is a
  great improvement over the SR flip-flops.

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JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6 JK Flip Flop operation 6
Characteristic table Characteristic table Characteristic table Characteristic table Excitation table Excitation table Excitation table Excitation table Excitation table
J K Qnext Comment Q Qnext J K Comment
0 0 hold state 0 0 0 X No change
0 1 reset 0 1 1 X Set
1 0 set 1 0 X 1 Reset
1 1 toggle 1 1 X 0 No change
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What are D flip-flops?

• Another modified version of SR flip-flops are D
  flip-flops.
• D stands for data flip-flops.
• It represents physical computer memory.
• The output always takes on the state of the D
  input at the rising clock edge.
• D flip-flops are very useful as they capture the
  signal when the clock rises.
• Flip-flops are very useful and are found in a lot
  of electronic devices.

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Truth table
Clock D Q Qprev
Rising edge 0 0 X
Rising edge 1 1 X
Non-Rising X Qprev
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Finite State Machine

• It is a model of behavior composed of a finite
  number of states, transitions between those
  states, and actions, (wikipedia,2000).
• A finite state machine is an abstract model of a
  machine with a primitive internal memory.
• It depicts the graphical part of the flip-flops.

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Moore/Mealy Machines

• Moore machine represents the JK flip-flops
• Output depends only on the state.
• Example for the Moore machine model will be the
  elevator door, which just opens and closes
• Mealy Machine depends on output and state.
• Example of Mealy Machine will be Microwave oven.

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Mealy Machine for JK flip-flops
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Conclusion

• We learned about flip-flops which are basic
  storage elements.
• There were different kinds of flip-flops versions
  developed SR, JK, D flip-flops.
• Then we learned about Finite State Machines which
  are primitive internal memory.
• We saw Moore Machine, Mealy Machine which are
  types of Finite State machines.

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Sources

• http//en.wikipedia.org/wiki/Flip-flop_(electronic
  s)
• http//en.wikipedia.org/wiki/Finite-state_machine