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Fast 80-Bit CMOS Adder

IP.com Disclosure Number: IPCOM000100656D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Steimle, A: AUTHOR

Abstract

This article shows how to speed the carry generations of a large adder using a standard CMOS library and how to avoid the pseudo end around carry adder loop.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Fast 80-Bit CMOS Adder

       This article shows how to speed the carry generations of
a large adder using a standard CMOS library and how to avoid the
pseudo end around carry adder loop.

      For an 80-bit carry look ahead adder, it is known that the
adder is composed of:
      1. 80 generate  blocks Gi= Ai AND Bi
      2. 80 propagate blocks Pi= Ai XOR Bi
      Using these equations, the carry expressions is given by:

                            (Image Omitted)

      Ci+1= Gi + PiGi-1+ PiPi-1Gi-2+ PiPi-1Pi-2Gi-3 +
      PiPi-1Pi-2Pi-3Ci-3
      The above expression can be written:
      Ci+1= Gk + PkCi-3
      3. 20 generate/propagate group 4
           The propagate group 4 is produced by 1 level AND
      gate but the generate group 4 is produced by 2 levels
      of NAND gates in the known carry lookahead adder.
      C4i= G4k + P4kG4k-1 + P4kP4k-1G4k-2 +
      P4kP4k-1P4k-2G4k-3
          + P4kP4k-1P4k-2P4k-3C4(i-4) for O < k < 19,
      i = 4, 8, 12, 16
      4. 5 generate/propagate GROUP 16
      From the above, it can be deduced that:
      a) For the generate group 16, it is worthwhile to use the 2 x 4
AND-OR gate for better area and delay ratio versus two levels of NAND
as explained below.

      With the group propagate being produced 1 logic level before
the group generate, they can be combined by AND gates:
      P4k,k-1 = P4kP4k-1
      P4k,k-1,k-2 = P4kP4k-1P4k-2
      P4k,k-1,k-2,k-3 = P4kP4k-1P4k-2P4k-3
      The carry expression can now be written:
      C4i= G4k + P4kG4k-1 + P4k,k-1G4k-2 + P4k,k-1,k-2G4k-3
         + P4k,k-1,k-2,k-3C4(i-4)

      The four left terms represent the group generate 16 (2 x 4 way
AND-OR); the...