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CSCE 613 Working in the Mentor Environment Page 1

CSCE 613 Fall 2005

Working in the Mentor Environment Part 1

The following description shows what steps are involved in the complete design of a transistor-levelcustom circuit and the Mentor Graphics applications used at each step in the design process.

Capturing the circuit schematic is done in Design Architect. If the schematic is needed in higherschematic, it needs to be represented by a symbol. This is done using Design Architect.

Layout is done IC Station. Verification to confirm that the layout conforms to the process rules andschematic is done using DRC check and LVS. Extraction of the resulting capacitance is done usingICextract. All these features (DRC check, LVS, ICextract) are in the IC Station.

Hierarchical design using previous design circuits can also be done in Mentor Graphics.

1. Environment Set-up1.1 Cell Directories

Each circuit created using Mentor Graphics is called a cell. All of your cells should be kept within yourhome directory. You may create any subdirectories that will help you organize your files, but youshould keep all your files within your directory. You wont have write permissions into the mgc_treedirectory hierarchy.

1.2 Setting your Mentor 'Working Directory'Before you start any Mentor Graphics application you need to tell Mentor what directory is yourworking directory. Typically yourworking directorywill be your home directory, $HOME, but it maypossibly be a subdirectory or a directory within your teammates home directory. In a commandwindow, navigate to the directory which contains the cells on which you will be working using the cdcommand. You can use the pwd to determine which directory you are currently in.

You can set the directory by using the $MGC_WD environment variable, which you can set using the

following Unix syntax:

% setenv MGC_WD $HOME/csce613

This assumes you have created a csce613 directory in your home directory located in /s1. It alsoassumes you are running C shell. Im not sure if this command works if you are running bash orother command shell.

1.3 Required Files for Mentor GraphicsYou will need to access several files used by Mentor Graphics which contain information about thespecific process we are using for our design projects (process file, library file, design rules file, etc.).These files will be located on \\Polaris in the directory:

/usr/local/mgc_tree/ADK/technology/ic/ami05

Well reference these files when we are using the IC Design tools, which we use later in this tutorial.

2. Design Architect

This document contains a step-by-step tutorial for creating an inverter in the Mentor Graphicsapplication Design Architect. It covers entering the schematic, creating a symbol, and creating aviewpoint. Separate documents continue the tutorial for other Mentor applications.


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2.1 General Information

The first thing that we need to do to begin the circuit design process is to capture our schematic inDesign Architect, referred to as DA. You will use N-FETS and P-FETS from a special library as thesecomponents have the necessary properties assigned to drive the entire process. After placing theseFETS, you can wire them together and add ports and net names as you would any other design. You

can even create hierarchical designs and then design your layout from that.

2.2 ADK Libraries

ADK (ASIC Design Kit) is an add-on to Mentor that does a lot of the work for us. For current projectwe will use many of the ADK features, including schematic libraries with basic elements (transistors,VDD, etc.) and several standard gates. The basic elements for transistor level design are in the SDLLibrary. You should use these elements for all transistor level design.

Cells in the ADK library are sized (W & L) in units of 'lambda', not microns. We will be using the 0.5umprocess with a lambda of 0.3, so if you set L=2 you will get a length of 0.6um.

2.3 Opening DA

Using a command terminal, go to your working directory (generally yourhome directory) and set thisas your Mentorworking directoryusing the Unix setenvcommand before opening DA, as discussedearlier.

To start DA, simply type% da &

This command will load DA so that we can create a layout using our Inverter model we created earlier.

The '&' at the end of the command will allow you to enter commands in the terminal window afterDAstarts running.

Once DA is running, you should see the window shown in Figure 1, and you can then follow the stepsbelow to capture an inverter schematic.

2.4 Schematic Capture

1. To begin a new design, you must open a new sheet from the DA menu bar. SelectFile > Open > Sheet

(Alternatively, you can click on OPEN SHEET on the side menu). Enter the name of your cellin the Component Name slot. Do not change the value of the Sheetslot. Since we want tocreate an inverter, call the cell inv, as shown in Figure 2. Click on OKand a new blank sheetwill open.


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Figure 1

Figure 2


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2. Now we can use transistors to draw an inverter gate. To findpmos and nmos transistor, fromthe top menu select the Symbol button on the palette. Navigate to the directory as shownbelow in order to open the ADK library:

/usr/local/mgc_tree/ADK/parts/lib/sdl

3. Open the specified device so that you can create and edit it in the Canvas.

4. Locate the transistor named n-fet-4 fornmos and p-fet-4 forpmos. Note that thesetransistors are four-terminal transistors, which include a body (substrate) terminal. When youdraw a circuit, you must connect nmos body to GND andpmos body to VDD.

6. Add transistors to your schematic. Select an nmos device from directory and place it into theinvschematic window as shown in Figure 3. Next, place apmos device into the window. Youcan see the instance (part) that you place in the window is highlighted, meaning that thisinstance is selected and will be affected by the next operation. You should de-select theinstances you've added by pressing the F2 key. You can also de-select the object by clickingit again with the left mouse.

7. The Mentor applications have many basic commands programmed into the function-

keys. Study the menu at the bottom of the screen and notice that commands are also listedfor shift, alt, and ctrl combinations with the function-keys. You will want to use many of theseshortcut keys to simplify your work and should practice them on your own. For now, we willonly point out F1 and F2, which are used to select and deselect.

Figure 3


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7. Add power ports to your schematic by selecting and placing the vdd and gnd instances fromthe library menu pointing into the SDK directory.

8. Next you need to add an input and output port. Select and place the portin and portout partsfrom the directory menu. After putting all components that you need into schematic window,press Esc to cancel theplace parts operation. Note that you should always remember topress Esc to exit each operation when you are finished with that operation.

9. Right-click on the palette menu and selectDisplay schematic palette

This will take you back to main palette. You can edit your circuit by using this paletteto add the appropriate wire connections to your schematic.

10. Now, move the instances around to get them in the proper position for an inverter, VDD on top,then pmos, then nmos, etc. You can move the instances by selecting each instanceindividually and selecting

MOVE

from the main schematic palette.When the instances are in the proper location, you can add wires to connect yourcircuit. From the main palette, select

ADD WIREand wire your circuit by clicking at each wire node and double-clicking to end awire. Remember, you will be in the ADD WIRE operation until you hit Esc.

11. Next, you should name the input and output ports. From the main palette, selectTEXT > CHANGE VALUE

and then clicking on the appropriate port. Enter the name of the input port as in and repeatthe procedure to name the output port out.

12. The default W/L value of the MOSFETs from ADK library is 5/2. To change the size of thetransistor, hit F2 to deselect everything, select a transistor, and then right-click over thetransistor. A menu will pop up showing yet another way to access commands in DA. Select

Properties > Modifyand the the property you want to modify. For this tutorial, change the width of the pMOStransistor to 10 (for 10 = 3 m).

13. Now the schematic of the inverter gate is finished and should look similar to the one in Figure4. Before saving your cell, you need to check the sheet. You can access the command fromboth the main DA menu or by right-clicking on the background of the sheet. From the mainmenu select

Check > Sheet > With DefaultsIf you have any errors, be sure to fix them before continuing. You can expect to get a fewwarnings which you can ignore, but you can't ignore errors.


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14. Once you have successfully checked your sheet you can finally save the cell. SelectFile > Save Sheet > Default Registration

Figure 4

2.5 Creating a Symbol

Like most circuit design CAD software, Mentor Graphics provides for hierarchical design, which meansthat you can instantiate your lower level cells (circuits) into upper level cells to create a treestructure. This allows you to create each basic cell only once and then use it over and over again byadding it to other cells (called, instantiating -adding an instance, or cell). Since, at higher levels, wereally don't need to see the detailed transistor-level description of the base cells, we create symbols,which will appear when we add the cell to higher-level cells. Generally, it is preferred to have thesymbol appear in a way that represents the function of the cell, like using standard gate symbols forinverters, nands, nors, etc. The following will explain how to create a symbol for your inverter and howto make it look like an inverter.

1. While still in DA, from the main menu selectMiscellaneous > Generate Symbol

to automatically generate a symbol. Do not change any of the options in the dialog box and clickon the 'OK' button to generate a symbol for your cell. The generated symbol will be a simplerectangle with two pins, IN and OUT, which does not look much like an inverter, so we will modifythe appearance of this symbol.


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2. To make your inverter symbol look like an inverter:- Select the body (rectangle) of the symbol and delete it. Dont delete the two pins of the symbol.- Click DRAW ADD POLYLINE and draw a triangle as shown in Figure 5. This can also bedone using the right mouse to select the operation from the menu.- Click DRAW ADD CIRCLE to draw a circle at the front of the triangle.- Press F2 to unselect these shapes.- Individually select the two Pins, and click MOVE to move them to the proper position relative tothe symbol you have drawn.- Because this symbol is small, it will look much better if we place the pin text outside of thesymbol (this is an optional step -just makes your symbol look better). Click TEXT CHANGEVALUE, select pin names, and then click MOVE to move pin names to proper position as shown inFigure 6.- Choose File Save Symbol to save the symbol.

3. To further identify you cell, you can add the name of the cell on the symbol so that it will show upwhen you instantiate it. To do this, from the main menu select

Edit > Add Graphics > Textand type INV as the text. Place this text in the symbol. You can use the move command to moveit around as suits you.

4. To finish, we need to check the symbol for errors. From the main menu selectCheck > With Defaults

You can ignore the warnings about the properties not being on the interface. If you have any othererrors, however, you need to fix them before moving on.

5. Save your symbol by selectingFile > Save Symbol > Default Registration

6. Close the symbol window by clicking on the X in the upper right corner of the window.

Figure 5


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Figure 6

2.6 Creating a Viewpoint

To complete the layout of the circuit, you should create a viewpointof your circuit. A viewpoint issimilar to a netlist and is used by the other Mentor Graphics applications as a representation of your

circuit.

1. Select the Session Palette -> Set Viewpoint from the menu. His can also be accessed usingthe F3 function key.

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