# 2 bit half adder truth table?

1. Oct 27, 2012

### delta59

I had a cpre exam last night and this was the last question. Can some one tell me the answer, I'm curious if I got it right or not. The part that tripped me up was a 2 bit "half adder". I assumed I wasn't supposed to use a full adder. So I'm not sure what to do with the carry from the first adder.

In this problem we will design a 2-bit half adder(add without carry) that adds two 2-bit number, denoted by ab/cd respectively and produces a 3 bit output (c,s1,s0)

2. Oct 27, 2012

### lewando

What did you come up with?

3. Oct 27, 2012

### mynick

i am sure you know that 2 H.A.= 1 F.A.
and that carry 'travels' from one adder to the next one. All digital design books show you how it's done.
right?

Last edited: Oct 27, 2012
4. Oct 27, 2012

### aralbrec

You want to make sure it will produce the right answer, I think that should indicate what to do with the result of adding the first two bits together.

A half-adder means there is no carry in. A 2-bit half adder is a box that adds two 2-bit numbers but there is no carry coming in.

5. Oct 27, 2012

### delta59

Here is what I got and my assumption of the circuit. Sorry in the drawing s1 is s1 and s2 is s0

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Last edited: Oct 27, 2012
6. Oct 27, 2012

### delta59

this look right

7. Oct 27, 2012

### lewando

I think it is safe to say you got this one wrong. Your tables do not look right. For example, c should be a 1 only when ab + cd > 3.

[edit-- fyi, a half adder has a carry output. A full adder has a carry input and a carry output.]

Last edited: Oct 27, 2012