ENGR338 Lab 2022 Fall
Lab 9 8-bit ALU
Name: Mason Brady
Email: mrbrady1@fortlewis.edu
8 Bit ALU

Introduction: This lab uses Electric VLSI and LTSpice to simulate a combination of gates created in the past to sucessfully create and 8-bit ALU.

Materials: LTSpice, Electric VLSI

Methods: First, I went to create the schematic for the ALU and as Dr. Li, I didn't have an 8-bit inverter. I added busses and created an array of 8 inverters in the layout before creating the ALU schematic which can be seen below in Figure 1.

Figure 1. 8-bit ALU layout.
Since it was 8-bit everything was done using buses except for the select pins on the MUX since the same signal goes to all 8 MUX gates. The first signal on the full adder's carry in is the first select signal because you have to add one to the inverted signal in order for it to be subtraction. The top MUX is also on the S0 signal so when S1 is toggeled, there is and or functionality. All of the logic was verified and is in the results section.

Finally, the ALU was laid out. This process seems daunting but when approached in sections is not too bad. The main idea is to keep everything in nice lines so that you don't cross anything and it is easy to keep track of where each connection goes. I tried my best at this but I chose to lay my ALU out without looking at the given one. I ended up putting a lot of my gates vertically because I thought it made the connections easier. The layout can be seen below in figure 2.


Figure 2. Overall view of the layout

In hindsight I should've put the full adder vertically but I thought it would make the connections easier with this layout. It ended up working out pretty well and went surprisingly smoothly.

Figure 3. NCC check and DRC.
Results:

The first simulation was to verify the and gate by getting all 0s from 11111111 and 00000000

Figure 4. 8-bit ALU and simulation.

Next, the OR gate was tested by using 11111111 and 00000000 and 1 0 for S0, S1
 
Figure 5. Simulation of OR gate in 8-bit ALU.

Next the ADD operation was tested using 11111111 and 10000000 to get 00000000 and 1
 
Figure 6. Addition using 8-bit ALU.

Finally, subtraction was tested using the same numbers as above in Figure 6. to get 11111110

Figure 7. Subtraction using 8-bit ALU.

Discussion: This lab was probably my favorite. I feel like I learned a lot more about being efficient in electric than I did in all the other labs. Learning small thing like if you click on a metal 2 wire with a metal 3 wire made a via automatically made a big difference in speed.