# Design a full adder of two 1-bit numbers using multiplexers 4/1

How can i implement the full adder of two 1-bit numbers using only multiplexers 4/1? I created a truth table for a one-bit full adder, which looks like this:

A = first bit
B = second bit
Pu = bit from lower position (used to create an adder for multiple bit numbers)
S = sum
P = transfer to higher position (e.g. if A=1, B=1 and Pu=0, the sum is 0 and transfer 1)

I made K-maps and used AB as selection inputs of a multiplexer and Pu as information input.

And finally got to this solution:

Is it correct?

• Yes. It is correct. – nidhin Feb 19 '15 at 13:17

Yes, it's correct.

This is the sort of optimization that used to happen manually in the old days of FPGAs and before that with PALs. You had to rejigger your result based on selection and minimal logic gates.

I've not run across this solution, but then I've never been stuck with just multiplexors.

Oh incidentally,... one more multiplexor would do that inversion (generating the invert in the previous stage), so you could easily create a large adder with a three-pole switch for each binary digit. No logic needed. Nice.

Yes, your P MUX is very correct:

0+0 inhibits carry (switch carry for 0)

0+1 propagates carry (switch carry through)

1+0 propagates carry (switch carry through)

1+1 generates carry (switch carry for 1)

Especially advantageous now to use a switched MUX, never a combinatorial. Why? All multiplexers can switch simultaneously with regard only to A vs B. Carry then ripples with almost no propagation delay through closed switches. Using combinatorial logic slows this down to a ripple carry and no faster...

Now you might want a chain of borrow MUX to handle subtractions:

0-0 propagates borrow (switch borrow through)

0-1 generates borrow (switch borrow for 1)

1-0 inhibits borrow (switch borrow for 0)

1-1 propagates borrow (switch borrow through)

Note this MUX plan works with a non-inverting borrow.

SUM and differences are same XOR3(A,B,C) for A+(B+C), A-(B+C), B-(A+C)

I would rather an 8 way MUX for SUM than feed a 4way with the inverter. Feed it an 8bit carry conditional truth table that can do any function. Not just ADD. Even if stuck by some rule of 4way only, I would feed it a truth table.

And then you might want to propagate carry through all slices unchanged. Then you can set your truth table to give A vs B, AND vs OR, etc..

Beware if you go with 74CBT3253! There are two flavors of this dual MUX. Some offer independent /OE0 /OE1 shutdowns, others offer all or nothing. For example, you might want to shutdown the borrow MUX when using carry. That can be real difficult if you order the wrong chip thinking all same. Pay careful attention any suffix letters and doublecheck your spec sheet.

This MUX topology closely resembles a carry skip adder. Except it doesn't ever skip anything. Fast by virtue of switching, not skipping. Therefore I call it a carry switched adder, unless it already has some other name? Konrad Zuse invented this trick for the first relay computers, and might already have named it.

• Zuse also propagated a /carry, thus he didn't need the inverter... – KD5ZXG Jul 25 '20 at 3:28