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.
Sorry my comment (which I removed) was hastily stated.
Consider the following logic table (I'm using ^
to represent XOR for brevity):
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.