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Designing a Carry Lookahead adder in Excel sheet

  1. Oct 16, 2014 #1
    I'm designing a carry lookahead 8bit adder. My problem is that i don't know how to calculate the group generate signal.

    I have 4 G signals:

    First level:
    G1:0
    G3:2
    G5:4
    G7:6

    And these have to be added together:

    Second level:
    G3:0
    G7:4

    Third level:
    G7:0

    Two different formulas for computing G:

    Gi:0 = Gi +Pi * Gi-1:j eq. (1)

    Gi:j = Gi:n+Pi:n * Gn-1:j eq. (2)


    I know how to use (1) for G1:0 but for G3:2 and further on i don't know how to do.

    My book doesn't explain this well at all. (Integrated Cirucit Design, Pearson)
     
  2. jcsd
  3. Oct 16, 2014 #2

    berkeman

    User Avatar

    Staff: Mentor

    Could you post a diagram of what you are designing? What is the difference between a standard adder and a carry-lookahead adder?

    And an adder has two sets of inputs. What is your second set of inputs?
     
    Last edited by a moderator: Oct 16, 2014
  4. Oct 17, 2014 #3

    I have a propagate signal as input and Carry in as input.

    http://sv.tinypic.com/view.php?pic=9h1euo&s=8#.VED-NPl_vkg
     
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