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design digital comparator using Manchester carry chain

Discussion in 'General Electronics Discussion' started by sala, Jan 29, 2011.

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  1. sala

    sala

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    Jan 29, 2011
    does anyone knows the steps for me to integrate Manchester carry chain adder to a CMOS digital comparator?
    i am using microwind software..if can i need the schematic ..really need :(
     
    Last edited: Jan 29, 2011
  2. KD5ZXG

    KD5ZXG

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    Saturday
    You don't need a carry chain. You need a borrow chain, or better yet a magnitude comparator chain.

    Lets imagine a chain of 4way pass-through multiplexers, each selecting a path to connect based upon An-Bn.

    We might initialize the chain with borrow from some prior operation.

    The case of 0-0 passes Borrow through to the next higher n+1.
    The case of 0-1 passes VDD through to the next higher n+1.
    The case of 1-0 passes GND through to the next higher n+1.
    The case of 1-1 passes Borrow through to the next higher n+1.

    But the above chain gives only borrows for An-Bn, we can do better.
    Instead of VDD, give the 0-1 case a control line called LT, because An is proven "Less Than" Bn.
    Instead of GND, give the 1-0 case a control line called GT, because An is proven "Greater Than" Bn.
    Now we gain the option to reverse subtract Bn-An by setting LT high and GT low.

    Lets call the borrow input EQ, for Equal. No circuit change, only our thinking...
    We can test for any comparison An vs Bn using LT EQ GT as < = >.
    The high borrow output shows the comparison result.

    Great thing about this, multiplexers can set switches in parallel time.
    The comparison (or borrow) slides through with hardly any propagation delay.

    XOR3 gives the difference. Or NXOR3 if you prefer to use an inverse borrow.

    You can turn a borrow chain to carry by inverting B, or vice versa...
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    @KD5ZXG Don't try to revive an 11 year old thread -> closed.
     
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