Design 8 bit adder: Steps & Tips

[lisa92]

Hi I already created a 4 bit adder but for my assignment I need an 8 bit one. Is it correct if I design to separate 4bit adders and connect their outputs (Cout) with an AND gate?

4 bit adder:
https://www.dropbox.com/s/ao60s6dgtrnhal3/4bit adder.PNG

 

[phillip1882]

your picture is too small to read, so its hard to tell for sure; but i think you want to xor the values. for example, let's say your inputs are 11001010 and 00101111. breaking it up to two 4 bits additions, we get...
values 1110 = A 1001 = B
Bcarrybit = 1
(A << 4) xor B xor (Bcarrybit << 5)
hope that helps.

 

[rcgldr]

To make an 8 bit adder from two 4 bit adders, the carry output from the low order 4 bit adders goes into the carry input of the high order 4 bit adder. The outputs are not "connected", you just have an 8 bit output (4 from each adder) plus a carry outpur from the high order adder.