A 4-bit adder using a full adder and a half adder

Thread Starter


Joined May 1, 2007
Hi every1,

I need to design a 4-bit adder (which has 3 outputs: sum, carry low, and carry high) using a full adder and a half adder...I know how to do the truth table and I know how to draw the circuit diagram using a full adder and a half adder to get the sum (you take first 3 bits, say A,B,C and add them in full adder, then you take the output bit sum and put it in as input to the half adder whose other input is the fourth bit, say D, and then you get the final output sum)...But, I don't know how to get the low and high carry designs :confused: ....someone please help..I've been stuck on this since this week...Thanks, Farah.


Joined Mar 2, 2012
I have just built 4bits adder for my multiplier and googled for a simpler way of building it then I found this article:D
The warning says this is more than 1236 days old but seems there is no clear answer so I post a reply.

1 full adder, 2 half adders (ignore OR gate, used it because program did not allow wire-to-wire connection)
it adds 4 digits, A+B+C+D and makes output(Ss) up to 100(BNR)