EE 421L Project 

Design, layout, and simulate an 8-bit ALU that can perform: A AND B, A OR B, A+B(addition), and A-B(subtraction).

Authored by: Adam James Wolverton

Email: Wolvert9@unlv.nevada.edu

Today's date: November 20th, 2013

Project description:

First half of the project(no layout), a jelib of your design and an html report detailing operation (including simulations), is due at the beinning of lab on Nov. 8. The top level ALU icon used for simulations should have the following inputs (all bus connections): A (8-bits), B(8-bits), F(2-bits), and Z(8-bits). Ensure that you have input vectors saved for simulations . Put your report (proj.htm) in a folder called /proj in your directory at CMOSedu.

Second half of the project will be a verified layout and documentation (in html), is due at the beginning of lab on Nov. 22. Finishing the projects by Nov. 22 will give us time to assemble chips for fabrication through MOSIS. 

First Half:

 In this portion we combine all of the elements necessary to make an ALU, such as a Full adder, an arrays of OR and AND gates, and Multiplexers. As seen below, this is my implementation of an 8-bit ALU. The way I set up the 8 bit buses, please NOTE that the Most Significant Bit (MSB) is the 7th bit and the Least Significant Bit (LSB) is the 0th bit. Note that I made an icon for implementation of a Full Adder/Subtractor.

Here is what is implemented in my 8 bit full adder/subtractor. I had noticed that a ton of people, if not everyone was using an inverter and an extra Demux to implement subtraction. I wanted to be different and try my chances using an XOR gate to use the control input F0 to accomplish subtraction.  In order to accomplish subtration we must have A-B = A+(-B)=AtB*+1. In order to perform the 2's complement, we will set the initial carry-in to 1 instead of 0, which will add an extra 1 to the sum.  In conclusion, when F0 = 0 this will have the Adder/subtractor perform addition, and when F1 =1 this will have it perform the subtraction operation. 

The overall control operation for my 8-bit ALU is as seen below:

Here are my simulations using the icon I made for the ALU and  IRSIM and LTSpice of my 8-bit ALU to ensure it is operating correctly:

Observing the above IRSIM simulation of my 8-bit ALU you can see that it is working correctly for the chosen values of A = 0x0D (=0000 1101 in binary) and B = 0x1B (= 0001 1011 in binary).

When (F1,F0)=(0,0), we get the correct addition value  of A+B = 0x0D + 0x1B = 0x28

When (F1,F0)=(0,1), we get the correct subtraction value of A-B = 0x0D - 0x1B = 0xF2

When (F1,F0)=(1,1), we get the correct AND operation value of A AND B = 0x0D AND 0x1B = 0x09

When (F1,F0)=(1,0), we get the correct OR operation value of A OR B = 0x0D OR 0x1B = 0x1F

Below is the simulation of the operation of my 8-bit ALU using LTSpice:

Here are the Bits for A:

Here are the bits for B:

Here are the result bits for Z:

Here are the Control inputs F1 and F0, and the Carry out Cout:

Second Half: 

In this portion of the project we will proceed to designing the layout of the 8-bit ALU after proper operation has been verified. In order to make an 8-bit ALU, I decided to first design a simple 1-bit ALU then cascade 8 of them together so that it can form an 8-bit ALU. Below, you will find some "new" layouts that haven't been created in my propior labs, which I implemented in the design of the layout for the 8-bit ALU. 

Here is the schematic and layout of a 1-bit Full Adder used in the 1-bit ALU.

In the previous lab, we were given the schematic of a 2-to-1 Demux, but we weren't not given a layout, so I made the layout to be used in my 8-bit ALU. In the ALU itself, I used an inverter between S and Si to accomplish only 1 control input.

Once I made these layouts, I used them as well as previously created layouts to put together the 1-bit ALU layout design shown below. 

Cascading 8 of these 1-bit ALU layouts will give me an 8-bit ALU as shown below:

After the layout is completed, I backed up all of my work again as shown below:

To gain access to my Project Library for this 8-bit ALU right click and save HERE

To return to Adam's Lab directory click HERE