Im trying to design a logic circuit for a 3 bit adder using 6 inputs, A2, A1, A0, B2, B1, B0 and 4 outputs, s0, s1, s2 and c (the carry out). I already have circuits for a half adder, full adder and a 2 bit adder. I thought I understood the concept behind it and iterated upon the 2 bit adder that I got working earlier but for some reason I'm not getting the correct answer.
a0 XOR b0 = s0 (a1 XOR b1) XOR (ao AND b0) = s1 (a2 XOR b2) XOR (a1 AND b1) = s2 ((a2 XOR b2) AND (a1 AND b1) OR (a2 AND b2)) = c (the carry out)
I'm testing values and when I try to following combination
a2 = 0 , a1 = 0 , a0 = 1, b2 = 1, b1 = 1, b0 = 1 . s2 shows 1 when it should be the carry I believe that should be 1 because, (001 + 111) = 1000, right? Could someone let me know where Im making a mistake? Thank you.