Propagation delay for carry lookahead adder

Discussion in 'Homework Help' started by laguna92651, Apr 17, 2008.

  1. laguna92651

    Thread Starter Active Member

    Mar 29, 2008
    101
    0
    1) I'm using a 4 bit carry lookahead adder to build a 8 bit parallel adder. I'm to calculate the maximum propagation delay time, assuming each gate introduces a unit time of propagation delay. I'm assuming not carry lookahead generators.

    Would it just be 8 units of propagation delay?

    2) If I had a 16 bit adder using carry lookahead generators, what would the maximum propagation delay time, assuming each gate introduces a unit time of propagation delay.

    Thanks
     
  2. techroomt

    Senior Member

    May 19, 2004
    198
    1
    are you building the adder from gates? if so, follow the inputs through the gates and move them one gate at a time. after each gate count one (e.g. t1, t2, t3, etc.). when all signals have reached the output you have your total.
     
  3. Caveman

    Active Member

    Apr 15, 2008
    471
    0
    Carry lookahead generators still have propagation delay, just less than going all the way through the carry outputs.
     
  4. laguna92651

    Thread Starter Active Member

    Mar 29, 2008
    101
    0
    Is there a rule of thumb to use for propagation delay for 4 bit carry lookahead adders, that is if I were building a 8 bit adder with 4 bit parts the delay is, for example 2 x 4 units of propagation delay?

    The same question if I were to use parts with carry lookahead generators, is there a rule of thumb to use to calculate delay for larger adders?

    Is there any reference material where I could see how the propagation path is calculated?

    Thanks
     
  5. laguna92651

    Thread Starter Active Member

    Mar 29, 2008
    101
    0
    I cascaded tow 4 bit lookahead adders and traced out the longest path. 4 delay units for the 1st 4-bit and 4 delay units for the 2nd 4-bit, is the the correct reasoning?
     
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