additionlogical-operatorsdigital-design

How is the full adder's carry out term derived?


I'm reading the section of the full adder in Digital Design by Morris Mano and I can't seem to figure out how it got from equation A to equation B.

From a full adder's truth table and k-map using inputs x, y, and z, the carry out term, C, is defined as:

C = xy + xz + yz (equation A)

I could understand the above, but in order to leverage the xor already used by the summation term of x, y, and z, the book redefines C as:

C = z(xy' + x'y) + xy = xy'z + x'yz + xy (equation B)

How are these two equivalent? I've tried to derive one from the other on paper but I'm not able to come up with the steps in between.


Solution

  • Sorry my comment (which I removed) was hastily stated.

    Consider the following logic table (I'm using ^ to represent XOR for brevity):

    enter image description here

    The results of xy + xz + yz are the same as xy + (x ^ y)z because, for the first 6 cases, the value of x + y and x ^ y are the same. For the last two cases where they are different, the xy term being OR'ed in is 1 which makes their difference irrelevant to the final value.