# 4-bit Incrementor Circuit

Discussion in 'Homework Help' started by Kuros, Sep 23, 2010.

1. ### Kuros Thread Starter New Member

Sep 23, 2010
3
0
Hello all,

I'm hoping I can get some help with this homework problem. I'm taking a class about compilers and low level computer architecture, and there is a section on building circuits. I've managed to do a couple, but this one has stumped me:

I have 4 inputs and 4 outputs, each one corresponding to a single bit. I need to take the input and add 1 to it and output the result. So:

I need to change 0000 to 0001 and display it.

What I've tried so far:
I have two switches. Each switch goes to both an AND gate and an XOR gate. This lets me add two bits, but I am unsure how to extend this to 4 bits, or for that matter, how to specify that it is a 1 I am always adding.

I've written out a truth table for it, but that doesn't seem to help my understanding.

I would greatly appreciate any help.

2. ### Kuros Thread Starter New Member

Sep 23, 2010
3
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Update: I have an incrementer working that only has 2 inputs. I generated a truth table and used Sum of Products to design the circuit with 4 AND gates and 2 ORs. I could do this with a 4 bit incrementer, but I would have a horrible time figuring out the boolean algebra from the truth table. I'm sure there must be a simpler way to think about this and I'm just missing it.

3. ### nick.dellimuti New Member

Aug 30, 2010
3
0
an easier way that you could of done this is with a clock d flip flop set up for toggle and a 2- 4bit decoder the you could have one time signal and what the flip flop dose is dived the signal and you make the faster clock frequency bit be lsb (or input a of the decoder) and the slower frequency msb(or B of the decoder)and then the decoder would count 0001 all the way to 1111

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Google the circuit called a Full Adder. It is a combinatorial circuit with 3 inputs and 2 outputs. You can use a single full adder to make an incrementer that works serially on one bit at a time, or you can use 4 full adders to do the increment function in parallel.

5. ### Kuros Thread Starter New Member

Sep 23, 2010
3
0
@papabravo
Thanks for the reply. I looked up a full adder, and I think I understand how it works. However, I am not sure how to go from that to an incrementer. I need to be able to have 4 inputs, and 4 "light" outputs that turn off/on to represent the added number in binary...

I am thinking I will actually need to have an input (I can add inputs/outputs if needed) that is always on to represent the one being added?

Thanks again.

6. ### AMIT_GOHEL Member

Jul 13, 2010
67
7
Hey i've a idea..

Full adder having 2 inputs and 2 outputs(sum & carry).
Take one input as your default once and anather as carry of previous one..
output will be your required once and same in number of input..

and to increment give 1st adder(lsb) 1 logic as carry..

Sorry i cant drow schemetic...
But you may ask if you having dought in disc..

Regards...

7. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
An Incrementer is an adder where one of the operands is a constant 0001 and the other operand comes from the input device