# Circuit that acts as an octal Full Adder (FA)

Discussion in 'Homework Help' started by twis19, Nov 30, 2010.

1. ### twis19 Thread Starter New Member

Nov 30, 2010
2
0
I am need some direction how to begin this project; design a circuit that act as an octal full adder using only binary full adders.

Input: A, B = octal digits; Cin = binary digit
Output S = octal digit; Cout - binary digit

can you suggest a starting point or additional resource to get started?

Thanks

2. ### tyblu Member

Nov 29, 2010
199
16
From which level are you starting? Do you design individual transistors (gate widths) and work up from there, or do you have access to higher level blocks? I would start by understanding a 'Full Adder', as you just need 8 of them. (For a ripple adder, at least; for a look-ahead adder, you'll have to do implement some extra circuitry.)
Homework should go in the homework forum.

3. ### twis19 Thread Starter New Member

Nov 30, 2010
2
0
I will have to start the individual transistors and work way up.
Thank you for your prompt feedback.

4. ### tyblu Member

Nov 29, 2010
199
16
Can you implement a simple ripple adder instead of a look-ahead or other type? They are the simplest. A single Full Adder implements the following:
Co = AB+ACi + BCi
S = A(+)B(+)Ci, where (+) is NOR

They can be implemented together with this circuit, using 26T (+4T for inverters):

You will have to design the pull-up and pull-down networks to have equal strengths using gate widths, then daisy-chain 8 of them together.

Last edited: Dec 2, 2010