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sequential?/combinational?

Discussion in 'Electronic Basics' started by [email protected], May 11, 2005.

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  1. Guest

    I have read a few different definitions of sequential and combinational
    and i still do not understand the difference between them. If anybody
    could put the major difference/s into simpler terms i would be grateful

    cheers
    sam
     
  2. Lord Garth

    Lord Garth Guest

    Sequential logic means one operation after another happens in sequence.
    This
    is controlled by using a timing pulse or a clock. Clocks can be synchronous
    meaning all devices are pulsed at the same time or the clock can ripple
    through
    the circuit meaning that the output from the first stage clocks the second
    and so on.
    It is generally better to design synchronous logic. Combinational logic
    does not
    use a clock. It uses a combination of logic gates to perform a task and is
    asynchronous
    to any system clock.
     
  3. Guest

    Thankyou very much thats a few extra marks i will gain on my exam
    tommorrow!

    Sam
     
  4. Lord Garth

    Lord Garth Guest

    Good luck on your test!
     
  5. Ratch

    Ratch Guest

    From Digital Design by M. Morris Mano

    "A combinational circuit consists of logic gates whose outputs at any
    time are determined directly from the present combination of inputs without
    regard to previous inputs. A combinational circuit performs a specific
    information processing operation fully specified logically by a set of
    Boolean functions. Sequential circuits employ memory elements in addition
    to logic gates. Their outputs are a function of the inputs and the state of
    the memory elements. The state of the memory elements, in turn, is a
    function of previous inputs. As a consequence, the outputs of a sequential
    circuit depend not only on present inputs, but also on past inputs, and the
    circuit behavior must be specified by a time sequence of inputs and internal
    states." Synchronous/asynchronous clocking pertain only to sequential
    logic. Ratch
     
  6. When I saw the OP's glee at getting an answer for a test from here
    (without trying to really understand it) I decided against saying
    anything more just then. It appears the test may be over now, so:

    To put it simply without distorting too much, combinatorial logic
    doesn't have state and sequential logic does.

    Jon
     
  7. Jeroen

    Jeroen Guest

    What about a 2 gate S/R flipflop? Or does logic with feedback not qualify as
    combinatorial?

    Jeroen
     
  8. If you read Ratch's quote, you will have your answer. If it amounts
    to retained state, the knowledge of which is required (as well as the
    state of the inputs) in order to understand the outputs, then it is
    not combinatorial.

    Jon
     
  9. Barry Jones

    Barry Jones Guest

    Combinational circuits are not clocked. You put some set of inputs into
    it, and the circuit produces results (outputs) just as fast as it can.
    Change the inputs, and the outputs follow, with a very small propagation
    delay.

    Sequential circuits are clocked. You control when the outputs will
    change with a clock. You can change the inputs over and over, and it
    won't make any difference to the outputs until the next clock pulse. At
    that time, whatever the inputs were at that moment get reflected in the
    outputs. Flip-flops are used to maintain the state of the outputs
    between clock pulses. Flip-flop outputs are generally considered to be
    the circuit outputs.

    Sequential circuits generally include a combinational component or
    section that provides the logic that determines what the outputs should
    be after the next clock pulse. The combinational component usually has
    external inputs and feedback inputs from the flip-flop outputs.

    I hope you work on understanding this rather than copying it. Put in in
    your own words.
     
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