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CS/EE 5720/6720 – Analog IC Design Tutorial for Schematic Design and Analysis using Spectre Introduction to Cadence EDA: The Cadence toolset is a complete microchip EDA (Electronic Design Automation) system, which is intended to develop professional, full-scale, mixed-signal microchips. The modules included in the toolset are for schematic entry, design simulation, data analysis, physical layout, and final verification. The Cadence tools at our university are the same as those at most every professional mixed-signal microelectronics company in the United States. The strength of its analog design/simulation/layout and mixed-signal verification and is often used in tandem with other tools for digital design/simulation/layout, where complete top-level verification is done in the Cadence tools. the Cadence tools is in An important concept is that the Cadence tools only provide a framework for doing design. Without a foundry-provided design kit, no design can be done. The design rules used by Cadence set up in this class is based for AMI’s C5N process (0.5 micron 3 metal 2 poly process). So, how is Cadence set up? Broadly, there are three sets of files that need to be in place in order to use Cadence. 1) The Cadence tools These are the design tools provided by the Cadence company. These tools are located in the /home/cadence directory. They are capable of VLSI integration, project management, circuit simulation, design rule verification, and many other things (most of which we won't use). 2) The foundry-based design kit As mentioned before, the Cadence tools have to be supported by a foundry-based design kit. In this class, we use Cadence design kit developed by the North Carolina State University (NCSU CDK). NCSU CDK provides an environment that has been customized with several technology files and a fair amount of custom SKILL code. These files contain information useful for analog/full- custom digital CMOS IC design via the MOSIS IC fabrication service (http://www.mosis.org). This information includes layer definitions (e.g. colors, patterns, etc.), parasitic capacitances, layout cells, SPICE simulation parameters, Diva rules for Design Rule Check (DRC), extraction, and Layout Versus Schematic (LVS) verification, with various GUI enhancements. For more information on the capability of the NCSU CDK, go to http://www.cadence.ncsu.edu/CDKoverview.html 1

This design kit is located in the /home/cadence/ncsu/local directory. All the design parameters that are needed by the Cadence tools are located in various files in the sub-directories you will find here. The nominal spice parameters for n type transistors for AMI’s 0.5 micron process used in this class can be found in /home/cadence/ncsu/local/models/spectre/nom/ami06N.m. 3) The set up files in your local cadence directory There are set up files that should be in your local Cadence directory (i.e. the directory from which you invoke Cadence) that sets up the required local environmental variables for Cadence to work on your computer terminal. They are as follows: .cdsinit, .cdsplotinit, .simrc (sets up the variables to be used by NCSU CDK) .cdsenv (not essential, but sets your preferences which can be different from user to user) Also we need a .cshrc file to source the current version of cadence we are using in this class. Now, of the three sets of files, the first two sets containing the cadence tools and the NCSU CDK have been already set up by the Cadence Administrators for the class. In this tutorial, the process of setting up the required files in your local cadence directory is explained. Setting up Cadence2000 Note: People who have already set up Cadence before still need to follow the steps below. Before you start using cadence you need to complete the following steps: 1. First, before anything else, make a directory from which to run Cadence. This is important so that all of Cadence’s files end up in a consistent location. I recommend making an IC_CAD directory and then under that making a cadence directory: cd mkdir IC_CAD mkdir IC_CAD/cadence 2. You need to add the following lines in your .tcshrc file (or whatever shell setup file you use…) Just open it up with emacs and add: set path = (/uusoc/facility/cad_common/local/bin $path) setenv ICDIR IC_CAD setenv CADENCEDIR $ICDIR/cadence 2

setenv LOCAL_CADSETUP /uusoc/facility/cad_common/local/class/5830/cadence Adding the first line will update your search path to include the path of the customized CAD tool startup scripts. The next lines set your working directory for cadence as ‘IC_CAD/cadence’. The fourth line sets up the path to a directory that contains class-specific settings. After you save this file you can log out and log in again, or you can source it from the command prompt in the following way. The sourcing only needs to be done the first time. After that the .tcshrc file will be sourced automatically when you log in and start up a shell. :~> source .tcshrc 3

Starting Cadence 2000 and Making a new Working Library Now that you have your own Cadence directory (called IC_CAD/cadence if you’ve followed the directions up to this point), you need to remember to connect to this directory before you start the Cadence tools. That way Cadence will see the init files that you’ve put in that directory, and find the circuits you’ve designed since all the design files will be stored in this directory. In order to organize your new circuits, you now need to create a new library using the Cadence library manager to hold your design files. 1. Connect to your class cadence directory (cd ~/IC_CAD/cadence) and run the command ncsu-icfb (this stands for North Carolina State University and “ic front to back,” in case you’re curious). You should get a window (called the Command Information Window – CIW) as shown below: 2. Library Manager will automatically be opened. If not, in the CIW, select Tools → Library Manager…. You should get the following window, with the following list of libraries: 4

3. In order to build your own schematics, you’ll need to define your own library to keep your own circuits in. To create a new working library in the library manager, select File → New → Library. In the Create Library window that appears fill in the Name field as CSEE5720 (or whatever you’d like to call your library). Select ‘Attach to existing tech library’ for Technology Library. Select “UofU AMI 0.6u C5N” process and press OK. Path field is left blank. 5

NOTE: This may take a few minutes to execute Now the working library has been created. All the project cells (components) that you generate should end up in this library. When you start up the Library Manager to begin working on your circuits, make sure you select your own library to work in. Creating a New Cell When you create a new cell (component in the library), you actually create a view of the cell. For now we’ll be creating “schematic” views, but eventually you’ll have other different views of the same cell. For example, a “layout” view of the same cell will have the composite layout information in it. It’s a different file, but it should represent the same circuit. This will be discussed later in more details. For now, we’re creating a schematic view. To create a cell view, carry out the following steps: Creating the Schematic View of an RC filter 1. Select File → New → Cell View… from the Library Manager menu or to the CIW menu. The Create New File window appears. The Library Name field is CSEE5720. Fill in the Cell Name field as RC_filter. Choose Composer - Schematic from the Tool list and the view name is automatically filled as Schematic. The library path file is automatically set. Click OK. 6

2. A blank window called Virtuoso Schematic Editing: CSEE5720 RC_filter Schematic appears. 3. Adding Instances An instance (either a gate from the standard cell library, or a cell that you’ve designed earlier) can be placed in the schematic by selecting Add → Instance… or by pressing ‘i’, and the following Component Browser window appears: 7

4. For this example, we need to add the following components: Capacitor of 1 µF and two resistors of 1kΩ ohm and 10kΩ respectively. To add a capacitor of 1 µF, select the NCSU_Analog_Parts Library and the R_L_C menu. Choose cap in the sub-menu that appears. This opens the Add Instance window: Now, enter the capacitance value of 1u F and hit Hide. Place the capacitor in the schematic window. Other instances can be added in the similar fashion as above. Resistors can be found in R_L_C → res. Enter the required resistor value. To come out of the instance command mode, press Esc. (This is a good command to know about in general. Whenever you want to exit an editing mode that you’re in, use Esc. I sometimes just hit a bunch of Esc’s whenever I’m not doing something else just to make sure I’m not still in a strange mode from the last command. ) 5. Connecting Instances with Wires To connect the different instances with wires we select Add → Wire (narrow) or press “w” to activate the wire command. Now go to the node of the instance and left-click on it to draw the wire and left-click on another node to make the connection. If you need to end the wire at any point other than a node (i.e. to add a pin later on), double left-click at that point. To come out of the wire command 8

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