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2.4.4   Exclusive-OR Cell

The two-input exclusive-OR ( XOR , EXOR, not-equivalence, ring-OR) function is

  A1   A2 = XOR(A1, A2) = A1 · A2' + A1' · A2.(2.32)

We are now using multiletter symbols, but there should be no doubt that A1' means anything other than NOT(A1). We can implement a two-input XOR using a MUX and an inverter as follows (2 gates):

  XOR(A1, A2) = MUX[NOT(A1), A1, A2],(2.33)

where

  MUX(A, B, S) = A · S + B · S '.(2.34)

This implementation only buffers one input and does not buffer the MUX output. We can use inverter buffers (3.5 gates total) or an inverting MUX so that the XOR cell does not have any external connections to source/drain diffusions as follows (3 gates total):

  XOR(A1, A2) = NOT[MUX(NOT[NOT(A1)], NOT(A1), A2)].(2.35)

We can also implement a two-input XOR using an AOI21 (and a NOR cell), since

  XOR(A1, A2) = A1 · A2' + A1' · A2 = [ (A1 · A2) + (A1 + A2)' ]'

  = AOI21[A1, A2, NOR(A1, A2)],(2.36)

(2.5 gates). Similarly we can implement an exclusive-NOR (XNOR, equivalence) logic cell using an inverting MUX (and two inverters, total 3.5 gates) or an OAI21 logic cell (and a NAND cell, total 2.5 gates) as follows (using the MUX function of Eq. 2.34):

  XNOR(A1, A2) = A1 · A2 + NOT(A1) · NOT(A2)

  = NOT[NOT[MUX(A1, NOT (A1), A2]]

  = OAI21[A1, A2, NAND(A1, A2)](2.37)


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