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Reverse polarity

  1. Sep 24, 2005 #1
    So, we are just starting ABEL... and I don't quite understand reverse polarity usefullness or application. Here's the extract from my textbook (Wakerly, 3rd ed.):
    "In this example, the reverse-polarity equation has one less product term than the normal polarity equation (whatever...the example), so the compiler will select this equation if the targeted device has selectable output inversion." :confused:

    could someone explain or give an example how this concept is used... :uhh:
    ok, so i understand that compiler will go for a more optimal option, but what does not make sense is that it would select let's say F' over F if F' is more optimal, but the circuit should operate on F not F'!! how is this making sense?
    Maybe I am asking the wrong question...anyway, I'm confused.

    thanks much.
     
    Last edited: Sep 24, 2005
  2. jcsd
  3. Sep 26, 2005 #2

    berkeman

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    Staff: Mentor

    I believe it refers to the fact that you can write an equation using OR or AND functions to gather up terms. Like:

    A & B = !(!A + !B)

    In some equations, it may take fewer terms when you use one form or the other, which may help you fit in a smaller CPLD, for example.
     
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