EE477 - s08 lab3

University of Southern California

Department of Electrical Engineering - Systems

EE 477 Laboratory #3

Module Design, Cadence and SPICE

Due 5/5/08 11:59 PM

There will be no extensions so plan your time accordingly

This lab addresses the design of a special-purpose circuit. The basic building block in the circuit is a very simplified digital neuron, along with a D flip flop. You will build a small network with this neuron and the D flip flop as basic building blocks.

The digital neuron

There are four data inputs to the neuron, A, B, C, and D, an inhibitory input I, load, reset, and a clock. You can assume /clock is an input too but you need to generate other inverted inputs. The neuron contains a flip flop, and the output of the flip-flop represents the output of the neuron. The neuron "fires" when 3 of the 4 data inputs are high, as long as I is held high, or when all 4 data inputs are high regardless of the value of I, and after the positive edge of the clock, the output of the flip-flop goes from low to high. After the next rising clock, the flip-flop output falls. The neuron cannot "fire" two clock cycles in a row. Load is normally held high, but is lowered if we want to emulate a neuron failing to fire due to lack of sleep or similar circumstance. Note that the function the neuron contains is not exactly the same as the compound gate you designed. A block diagram of the neuron is shown below.

The Neural Network

For the project, you are to assemble a "neural net" using 5 of these neurons. The network is designed to allow an automobile to start only under certain conditions. The first three neurons each take 4 bits of input derived from a code the driver enters into a keypad. The code is the last 3 digits of your student number, in binary coded decimal form, from R₁to R₁₂. So if you want to represent 7 in binary coded decimal form that is 0111, 8 is 1000 and 9 is 1001. The inhibitory input to each of these neurons is the output from a sensor that detects alcohol on the driver's breath. If alcohol is present, the inhibitory input I = 0 (active low) for these three neurons. So on clock cycle 1, the driver must enter his or her code in order to get to step 2 (clock cycle 2). The driver can make mistakes in the code if alcohol is not present but if alcohol is present, the driver has to be correct on all 4 inputs for each digit of the code. Each one of you is likely to have a slightly different circuit because your student number is different. To recognize 0 bits, of course, you have to invert the inputs. The circuit shown recognizes the digits 7, 8 and 9.

In clock cycle 2, all 3 of the code recognition neurons need to have fired, then if the key is in the ignition, the second stage neuron fires. For this second state (4th) neuron, tie the I input to Gnd because we want to be sure that all conditions are true to advance.

In clock cycle 3, the second stage neuron must have fired. The third stage neuron will then allow the car to start if the brake is depressed, the emergency brake is on and the car is in park. Again tie I ~~high~~ low because these all must be true to start the car.

You can use any combination of multiple clocks you wish, but only clock and /clock are clock inputs to the circuit and you will have to design circuits to generate the other clocks. /load is not an input to the circuit. You need to generate it if you need it. You can assume reset is asserted low if you like.

You should use the cells you designed in labs 1 and 2 to build your neuron and your circuit. Do not change the circuit structure of your cells or the transistor sizes unless the rise/fall times do not meet the requirements of lab 1. However, you can modify the layout in minor ways if you can see some ways to make the circuits smaller or faster. You can rotate and flip cells about the x and y axes.

Make sure your ohmic contacts meet the following requirement: Every cell should have a well and substrate contact, including transmission gates, and of course they should be connected to Vdd and Gnd. For larger cells, include at least two contacts per 50x50 lambda, one for the psubstrate and the other for the nwell. Every separate block of nwell should have at least one ohmic contact. For all your contacts, you will get the best performance if you use multiple minimum-size contacts for large contact areas and not large contacts.

You can use metal layers 1, 2 and 3 for the neuron. You can use metal layers 1-5 for the entire neural network.

The diagram below shows how the neurons are to be connected.

The Laboratory Goal

For this lab, you can use any layout strategy you choose as long as it fits the cell methodology you have already selected. The goal is to minimize the area·delay product. Designs that are more square, rather than long, thin rectangles, tend to be better designs. The designer with the lowest area-delay product will win a prize, a gift certificate from Amazon.com. There will be two prizes, one for the best undergraduate design and one for the best graduate design. Pay close attention to the specific delay you are to measure.

The Laboratory Steps

1. Design your circuit and create a Cadence circuit (schematic) diagram and a Cadence layout using the cells you have already designed. You cannot design new cells for Lab 3. The bulk of the work here should be the routing of your design. Include your logic/gate level diagram for your neuron. Use LVS to verify your layout prior to SPICE simulation. Important note: Any pins in your layout labeled the same should be physically connected.

2. Simulate your schematic and layout with SPICE to ensure that your design works correctly The inputs to your project circuit should be named clock, load, R₁, R₂, R₃, R₄, R₅, R₆,R₇, R₈, R₉, R₁₀,R₁₁, R₁₂, ignition, brake, emergency, park, and reset. The output should be named start_car. It is important you follow this naming convention so we can verify that your circuit works.

Use the following sequence of inputs (See timing diagram below for exact details):

a) Reset all flip flops (even though reset is shown asserted high you could assert it low) using whatever clock settings you need. This will vary depending on how you implemented reset.

b) Unassert Reset and then set load and the appropriate R's to 1. Set load high and keep it high. All other R inputs should be 0. I for the first three neurons should be 0. Clock the circuit. Then set ignition high, and all R inputs low. Clock the circuit again. Then set brake, emergency and park to high and ignition to low, and keep all R inputs low and clock again. Your output start_car should be high after the third upward transition of the clock.

c) Now reset the neurons and repeat the cycle with R₁ wrong, and with I low. Now your output should continue low after three positive clock edges.

d) Test the whole cycle again with I high and wrong R₁and show that the output start_car is high after the third upward transition of the clock. This case is not shown in the timing diagram.

e) Now hold your load input low. Return to the input sequence in part a and b. After three clock cycles, the output should remain low. This case is not shown in the timing diagram.

f) Now reset again and hold the R inputs with the proper code for two clock cycles, and show the output of neuron 1 (N1) to verify that it does not fire twice in a row. This case is not shown in the timing diagram.

g) Test your neural network with 2 bits wrong and I high in one of your code digits to verify the car will not start. This case is not shown in the timing diagram.

4. The delay you should measure with SPICE is the clock cycle. The faster your clock cycle, the faster your circuit will function. Measure the area of your design in square microns. Your area should be the bounding box area of your design. If your design is not rectangular, include the wasted area in your area calculation. Compute the area-delay product of your design.

5. Upload your report to the assignments function of Blackboard, incliding your layout and simulation files as a TAR file as in lab 2 so that we can test (simulate) your circuit to be sure it performs as specified Be sure to include your SPICE input files for the project.

Testing Strategy

Here is a testing strategy that might be useful to you: Build the neuron combinational schematic in Cadence and test it using SPICE. Add the flip flop and any logic needed to keep the flip flop from firing two clock cycles in a row. Test the entire "neuron." Now create your neural network schematic in Cadence and test it. Use the same incremental strategy to perform your layout and to test your layout.

Timing Diagram

Lab Report Contents:

A cover sheet (title page) giving your name, date you submitted the lab, title, and student number.
A description of the neuron and network you built, including a transistor-level or gate-level circuit diagram printed from Cadence.
The neuron and neural network layout screen capture.
A description of the simulation experiments you ran with SPICE
SPICE input netlist and simulation files generated by Cadence
SPICE outputs for the neural network schematic and layout as shown in MWAVES images.

Items 1-4 and 6 should all be in a single report. Tar this report along with the files specified in item 5, and upload to the assignments page of the blackboard.

IMPORTANT: Once you upload there will be no deletions or reuploads allowed. All files should be in a single Tar file, uploaded to blackboard. Files uploaded to the dropbox will not be graded. All layouts must be in color. Failure to follow these instructions could result in deduction of points.