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Flip-flop and Finite State Machine

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The basic memory unit is called an SR flip-flop. ... It is a variation to SR flip-flop, but it takes care of the S=R=1 combination. ... – PowerPoint PPT presentation

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Title: Flip-flop and Finite State Machine


1
Flip-flop and Finite State Machine
  • By
  • Radha Iyer
  • Prof Dr. Sin-Min Lee
  • CS147 Computer Organization and Architecture

2
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.

3
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.

4
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
5
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.

6
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.

7
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
8
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.

9
Truth table
Clock D Q Qprev
Rising edge 0 0 X
Rising edge 1 1 X
Non-Rising X Qprev
10
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.

11
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12
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.

13
Mealy Machine for JK flip-flops
14
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.

15
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16
Sources
  • http//en.wikipedia.org/wiki/Flip-flop_(electronic
    s)
  • http//en.wikipedia.org/wiki/Finite-state_machine
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