Cadence Tutorial A: Schematic Entry and Functional Simulation

Document Contents
Introduction
Environment Setup
Creating a Design Library
Creating a Schematic Cellview
Functional Simulation (transient analysis)

Introduction
This document is a tutorial for using CADENCE Custom IC Design Tools for a typical
bottom-up digital circuit design flow with the AMI06 process technology and NCSU design kit.
This document, Tutorial A, covers setup of the Cadence environment on a Linux platform, use
of the Virtuoso schematic entry tool, and use of the Affirma analog simulation tool.
Note: Your paths may be different depending on the project you are working on. Also note that
you can find additional tutorials and notes on the web from courses at other
universities. These may be helpful in learning Cadence, but because of differences in the
environment setup, you probably will not be able to follow a different tutorial step by step.

For more information about Cadence Virtuoso or the Affirma tool, see the manuals.

Environment Setup
Before beginning this tutorial you must setup Cadence to work with your account.

Some of the dialog boxes you see may be different from the ones given in this tutorial. This is
because different versions of Cadence are used in SEECS.

If you have not already done so, launch Cadence now by going to your working directory and
typing icfb& at the command prompt. A Command Interpreter Window (CIW) similar to the
example below will appear. When all the configuration files have been read, the END OF SITE
CUSTOMIZATION message will be displayed indicating the startup was successful. With each
new session, Cadence starts a new CDS.log file in your home directory where all the messages
that appear in the CIW will be stored.

Along with the CIW window, you should also see the Library Manager window that lists the
libraries in your working directory. For now the NCSU-Analog-Parts library is the important
one since it has basic circuit elements like transistors, current sources, voltage sources, ground,
resistors, capacitors etc.

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 Command Interpreter Window

Library Manager Window

In this tutorial, a simplified convention will be used to show the sequence of steps for the pull
down menu. For example, File => Exit will indicate that you open the pull down menu for File
and then click on Exit. Another example could be Tools => Analog Artist => Simulation, which
will indicate that you pull down the Tools menu, then click on the Analog Artist button and
finally click on the Simulation button.

Note: If at anytime during this tutorial you want to quit Cadence, make sure you save your work
by selecting Design => Save and close the design windows by selecting Close from the menu.
After you have closed all your working windows, select File => Exit and click Yes in the pop-up
confirmation window to end the Cadence session.

Cadence File Organization

To start a design in Cadence, you must first create a library where you can store your design
cells. Every Library is associated with a technology file and it is the technology file that supplies
the color maps, layer maps, design rules, and extraction parameters required to view, design,
simulate and fabricate your circuit. Cadence stores its files in libraries, cells, and cellviews.

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A library (which actually appears as a directory in Linux) contains cells (subdirectories), which
in turn contain views. Each library contains a catalog of all cells, viewed along with the actual
LINUX paths to the data files. Each cell in a library uses the same mask layers, colors,
design rules, symbolic devices, and parameter values (i.e. the information contained in the
technology file). A cell is the basic design object. It forms an individual building block of a chip
or system. It is a logic, rather than a physical, design object. Each cell has one or more views,
which are files that store specific data for each cell. A cellview is the virtual data file created to
store information in Cadence. A cell may have many cellviews, signifying different ways to
represent the same data represented by the cell (for example, a layout, schematic, etc).

Example Organization:

Library: logic_gates
Cell: inv
View: schematic
View: symbol
Cell: nand2
View: schematic
View: symbol
View: layout
View: extracted
Library: ripple_carry_adder
Cell: 1bit_adder
View: schematic
Cell: 2bit_adder
View: schematic

The Custom Design Process

For a full custom design (as opposed to a coded/synthesized design using, e.g., Verilog HDL),
the design process begins by creating a schematic. The schematic is then simulated to verify
operation and optimize performance. A layout of the circuit is generated and checked for design
rule violations (DRC). The layout is then extracted and a layout vs. schematic (LVS)
comparison is run to ensure the cell layout exactly matches the schematic. Finally the extracted
layout is simulated to observe the effect of parasitics that will be present on the fabricated chip.
These post layout simulation are closer to reality and will show if your design would work if
fabricated. In this tutorial you will create a schematic for a basic digital logic gate, and inverter,
and perform some basic simulations on the schematic to verify it is functioning properly.

Creating a Design Library

STEP 1.
In the Library Manager window, select File => New => Library to open the Create Library
window shown below.
Enter a Name for your library. The example shows the library name tutorial, but since you
will probably be using the cells in this library (and adding more to it) choose a name like lab,
cmos, or digital. Leave the Path blank. Click enter.
A new dialog box opens. Click on the Attach to Existing Tech Library button and choose
NCSU_TechLib_ami06 as the technology file to be associated with your new library.
Finally, click on the OK button and you now have an empty library to start adding cells to.

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Creating a Schematic Cellview
STEP 2: Create a new schematic
Go to the Library Manager window and click/select your library (for example tutorial).
Now select File => New => Cellview. Use the Create New File window that pops up to
create the schematic view for an inverter cell.
Enter the Cell Name inv.
Click on Tool drop-down list and select Composer-Schematic. This is where you choose
which Cadence tool you want to use and the appropriate View Name for each tool will be
filled in automatically. Here we will be creating the schematic view.
Click the OK button. The Virtuoso schematic editing tool will open with an empty
Schematic Editing window as shown below.

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STEP 3: Add an nFET to the schematic
In the Schematic Editing window Select Add => Instance to activate the Add Instance tool
for adding components (transistors, sources, etc.) to your schematic. You can also invoke
this tool by clicking on the Instance icon on the left-hand toolbar, or by typing the hot key i
with your mouse over the Schematic Editing window. Two windows (Component Browser
window and Add Instance) will pop open.

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 In the Component Browser window, under Library select NCSU_Analog_Parts. A list of
parts will be displayed near the bottom of this window. From the parts list, click on
N_transistors and a list of available nFET transistor elements will be displayed. Pick up the
nmos4 element by clicking on it. This will attach the component to your mouse pointer.
If necessary, click on the Rotate, Sideways, or Upside-down buttons in the Add Instance
window to manipulate the component you are adding.
Click on schematic area of the Schematic Editing window (main black area of the window)
and the nmos4 component will appear in your schematic. Clicking on the schematic window
again will add another copy of the component (dont do this). Pressing ESC on the keyboard
will end the Add Instance function (but dont do this yet).

STEP 4: Add other components to the schematic

In Component Browser window, go to the P-Transistors category in NCSU_Analog_Parts,
and pick up pmos4 and add it to your schematic. Place it above the nFET as appropriate for a
CMOS inverter. If you need to move a component, press ESC and left-click on the
component and drag it to a new location. Pressing ESC will cancel the Add Instance
command so you will need to restart it to add more components.
In Component Browser window, go to the Supply-nets category and pick up vdd (supply
voltage) and gnd (ground reference) and add them above and below, respectively, the
MOSFET components. Now the Schematic Editing window look like the figure below.

STEP 5: Add I/O pins

In the Schematic Editing window select Add => Pin (or use hot key p) to add an input pin.
In the Add Pin window enter A for the Pin Name and select input for the Direction.
Click on the Schematic Editing window and drop the pin to the left of the transistors
(between the gate inputs).

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 Follow the same procedure to add output pin Y. to the right of the transistors between the
drain nodes. Be sure to use the correct pin direction. Press ESC when you are done.

STEP 6: Add wire connections

In the Schematic Editing window select Add => Wire (or use hot key w) to begin placing
wires which will connect the terminals of the components and pins in the schematic.
Wire the schematic as appropriate for a CMOS inverter. Be sure to connect the body
terminal of each transistor (nMOS to ground, pMOS to VDD). Press ESC to end wiring.

STEP 7: Setting global labels

In the Schematic Editing window type the letter l to invoke the labeling tool. Since we will
be using the same vdd and gnd supplies for all our cells, we want to make a global
declaration for these labels. Cadence uses the ! symbol after a label-name to indicate a
global label, i.e., one that is common to all cells in a design.
In the label pop-up window, enter the Name vdd!
Move your mouse to the Schematic Editing window and drop the vdd! label on the wire that
connects to the vdd pin.
Repeat this procedure to label the global gnd! net. Press ESC when you are done.

Final schematic of an inverter

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STEP 8: Check and Save the cellview
Now that you are familiar with the schematic editing tool, explore the menu commands in the
Schematic Editing window to do the following:
Check the cell for errors. If errors exist the error section will blink in the Schematic Editing
window.
Save the cellview.

General Editing Tips

Mouse Buttons: In most Cadence tools, the left mouse button is used to select components,
wires, etc., and the middle mouse button can be used to change object properties, e.g., the width
and length of a transistor.
Moving Objects: If you want to move any object, just move the cursor on top of the object and
type m. The object will then move with the cursor. Or you can select the objects to be moved
by left-click and drag to draw a box around the objects. After highlighting the objects to be
moved, type m and the highlighted objects will move with your cursor.
Deleting Objects: If you want to delete an object, move the cursor on top of the object and hit
the DEL key. You can also highlight an object or a group of objects by drawing a box around
it, as described above, and pressing the DEL key.
Undo Operations: When you make a mistake (accidentally delete a component, etc.), you can
undo the action by click on the Undo icon in the toolbar.
BindingKey: The following hot keys are available for the schematic editing tool.
1). Press p to add pins
2). Press q on the device/instance to edit properties for the device
3). Press w to add wires
4). Press f fit the schematic in your schematic window
5). Press z to zoom in the window
6). Press shift+z to zoom out the window
7). Press l to label a wire
8). Press Up and Down arrows to move up and down within a schematic window
9). Press ESC to terminate an operation in the schematic window
10). Press u to undo an operation in the schematic window

STEP 9: Create a symbol

In the Schematic Editing window, select Design => Create Cellview => From Cellview.
In the Cellview From Cellview window that pops up, you should notice source (From View
Name) set to schematic and the destination (To View Name) set to symbol, which is linked to
the Composer-Symbol tool. You should not have to modify this window.
Click on OK in the Cellview From Cellview window.

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 The Symbol Editing window will pop up showing the default symbol, a rectangle with red
square dots for input and out pins. You can keep this symbol, but it would be helpful if you
modified it to a more meaningful symbol (such as a triangle for an inverter). Explore the
options on this window and the tips below to define your symbol graphic.
When the symbol is complete, save it. In the Symbol Editing window select Design => Save
or click on the Save icon at the top of the toolbar.
In the Symbol Editing window, select Window => Close to close the symbol.

Final inverter symbol

Symbol Editing Tips

General Notes:
o The red box around your symbol is called a selection box. When you place your symbol
in a schematic, this box represents the area you can click in to select the symbol. You can
change the size of selection box. It is a good practice to fit the symbol entirely within the
red box; otherwise, you may not easily be able to select it when you instantiate it later on.
o The red square dots indicate the pin connections.
o [@InstanceName] and [@PartName] are display variables, which you may keep or
delete. These are generally annoying for small cells/symbols but can be useful for
higher-level circuits.
Drawing Lines: To draw a line, move your cursor to the Line icon (the icon with a descending
line) and click on the left mouse button. Icons are to the left of the symbol window. Move your
cursor to the location where you want to draw a line and click on the left mouse button. If you
draw a closed object, all you have to do is to click on the points where you want to change the
direction of the line. However, if you are drawing just a straight line, you need to double click
the left mouse button on the end point of the line to signify the completion of drawing the line.
Drawing Shapes: To draw a circle (or any other shape), first click on the Line icon and then click
on the options icon (the second to last icon). A pop-up menu will appear in which you can select
different shapes and fill styles. Then move your cursor to the location where you want to draw
the circle/shape and left-click. That is the location of the center of your circle. Move your mouse
so that the circle is as big as you desire it to be and then click on the left mouse button again.

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Simulation with Affirma Analog Circuit Design Environment
To verify a circuit is working and test the functionality of the schematic we must simulate the
circuit. For this we will use the Cadence Affirma analog simulation tool. For now we will just
run a simple transient analysis to confirm the circuit designed above is operating as an inverter
should be. Additional information for running simulations with Cadence tools will be provided
later in the class.

STEP 10: Launch the Affirma analog simulation tool

With your inv schematic open, in the Schematic Editing window select Tools => Analog
Environment, and the Affirma Analog Circuit Design Environment window will open.
o You can also launch this tool from the CIW by selecting Tools => Analog Environment
=> Simulation in the CIW. When the Affirma Analog Circuit Design Environment opens
you have click on the Setup => Design to specify the library and cell, for example
tutorial and "inv".

In Affirma Analog Circuit Design Environment, click on Setup => Simulator/Directory/Host.

Choose spectre as the Simulator (if not default).
The default Project Directory should be ./simulation. This points to a directory named
simulation inside of the directory you launched Cadence from. This is where your
simulations files and results will be saved. You may set the Project Directory to any valid
path, but you might find it useful to keep all simulations in one directory. To run multiple
simulations on the same cell, you can use different paths for each simulation. Note that upon
closing and reopening this screen, the project directory will automatically be transformed to
the absolute path name: /home/<groupname>/<userid>/simulation/

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 In Affirma Analog Circuit Design Environment, click on Setup => Environment.
Next to Use SPICE Netlist Reader(spp):, click the box Y. This is necessary because the
AMI06 transistor model libraries use SPICE netlist syntax. Click OK to close this window.

STEP 11: Set up stimulus

The schematic defines the components within the cell but does not define the control signals
(typically voltage sources) necessary to test the operation of the circuit, such as the power supply
voltage and an input voltage signal. These signals are referred to as the stimulus, and here we
will define use a spectre stimulus text file to define these signals.

You will need to use a text editor in order to create a stimulus text file. You can use any editor
you are familiar with. You can name the stimulus file any name, and put it wherever you wish.
In this tutorial, we will call the file stimulus.txt and place it in your directory. If you are new
to LINUX, you may want to nedit by doing the following:

Open a terminal screen (or use any one that is already open).
Make sure you are in your Cadence directory. If unsure, type
cd /home/<groupname>/<userid>
Type nedit & to create a new file.

Once you have opened a text editor, type in the following three lines to define the simulator
language (spectre), the DC supply voltage and a square wave pulse input voltage that will be
useful for a transient analysis simulation to verify functionality of the inverter circuit. Note
that the input waveform used may vary with the type of analysis needed, although the supply
voltage source will generally remain constant.
o simulator lang=spectre
o vdd (vdd! 0) vsource dc=3
o V1 (A 0) vsource type=pulse val0=0 val1=3 delay=0 rise=0.05n
fall=0.05n width=10n period=20n
Note: Each bulleted item represents one line in your stimulus file. The third bullet item
(voltage pulse) is broken into two lines due to the document margins, but should only take up
one line in your stimulus file.
Make sure that you always end the last line by pressing Enter. This adds a new line character
to the line, and informs the simulator that the voltage definition is complete. If you do not do
this, you will likely get a netlist read error when you try to Netlist and Run the simulation.

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 Save the text file. If you are using Nedit, this can be done by
o Selecting Files => Save As
o In the New File Name: box, the current path will be displayed. Click on the
blinking cursor and add stimulus.txt to the path.
o Click OK. This will save the file
You can exit the text editor at this point (nedit: select File=>Exit). However, you may want
to keep it open if you plan on making changes to the file during simulation.

Stimulus File Tips

The voltage sources above are defined using spectre syntax. The DC supply voltage named
vdd is between nodes vdd! and 0 (ground) with value of 3V. The input square wave is defined
using the spectre pulse syntax

<vname> (node+ node-) vsource type=pulse val0=min_voltage val1=max_voltage

delay=delay_length rise=rise_time fall=fall_time width=pulse_width
period=period_length

where the default units are volts and seconds. The n on the time values sets them to
nanoseconds (i.e. n is a 10-9 multiplier). Although you can vary the timing values as necessary
to meet simulation goals, the rise and fall times listed are good for the AMI-C5N technology and
should not be modified unless you are sure you know what you are doing.

Once you have written a stimulus file, open (refocus) the Affirma Analog Circuit Design
Environment window. Click on Setup => Simulation Files.
Click on the box that is labeled Stimulus File so that a blinking cursor appears in it.
Click the Browse button to open a file browser window.
Navigate to where you have saved the file stimulus.txt if you have followed the
tutorial exactly, it should appear in the default directory opened by the file browser.
Click on the name stimulus.txt and then click OK. The file browser should close, and
the absolute pathname for your stimulus file should appear in the Stimulus File box.
Click OK to apply changes and close this window.

STEP 12: Setup Model Files

In Affirma Analog Circuit Design Environment, click on Setup => Model Libraries
You can add the necessary Model Library Files by
o Click the bottom Model Library File box
o Type /usr/ee424/ami06N.m
o Click the Add button. This should add the model to the Model Library File list and
clear the Model Library File box
o Repeat the process or /usr/ee424/ami06P.m
o Your Model Library Setup dialog box should look like the one below. Once it does, you
can click OK to close the dialog.

STEP 13: Setup analysis

With the stimulus defined, we need to choose what type of analysis we wish to perform.
Different analyses are useful for getting a variety of different data from the simulations. Here we

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wish to simulate the response of our inverter to a simple square wave input and verify that the
output is indeed being inverted. For this we will setup a transient (over time) analysis.
Alternative analysis types, including DC analysis, that are used to simulate different performance
characteristics of a circuit are covered later in the course.

In Affirma Analog Circuit Design Environment window, select Analyses => Choose.
In the window that pops up, select tran to choose a transient analysis.
Enter the time limits for simulation by entering a Stop Time of 50n.
Choose Enabled at the bottom of the screen and press OK.

STEP 13: Setup output traces

In Affirma Analog Circuit Design Environment window, select Outputs => to be plotted =>
Select on Schematic. This will activate the Schematic Window allowing you to pick which
signals (nets/wires) you would like to have plotted during the simulation.
In the Schematic Window click on the wire that is the input to your inverter and also click on
the output wire. This will complete the simulation setup.

STEP 14: Run simulation

In the Affirma Analog Circuit Design Environment window select Simulation => Netlist and

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 Run.
When the simulation is complete, the CIW should show "Reading Simulation Data ......
Successful". If the simulation was not successful, go to Simulation => Output Log in your
Affirma Analog Circuit Design Environment to find out what the problem was.
If the simulation results shows a typical inverter response (i.e. output high when input is low
and visa versa) then the circuit is working properly. If not, you will need to go back to the
schematic to track down the problem and rerun simulations until you get the proper response.
To separate input and output signals, in the Waveform Window click on Axes => To Strip.
When this simulations results are correct, you have completed this tutorial.

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