EECS 211 CAD Tutorial 1. Introduction This tutorial has been devised to run through all the steps involved in the design and simulation of an audio tone control amplifier using the Mentor Graphics CAD tools. Before invoking the Mentor Graphics tools it is essential to first set up the environment under which these tools will be used throughout the semester. 1.1 Setting up the links For the remainder of the course you will be working in your EECS 211 class directory which will store all the files that you create. Since Mentor files occupy a large amount of space it is essential that you work in your class directory, else your home directory quota will be quickly used up. Note: To avoid typing long pathnames, a link called eecs211 is automatically created in your home directory when your class directory is set up. This eecs211 link points to your class directory and hence, you can change to your class directory from your home directory simply by typing: % cd eecs211 a) The % symbol indicates the UNIX prompt of the c shell and is usually preceded by the name of the machine you are working on. b) LMB, RMB refer to the left and right mouse buttons. The LMB is always used for clicking on, selecting, and positioning components, shapes within the various Mentor programs, and for OK/NO/Cancel operations in user interface items like dialog boxes, prompt bars, etc. The RMB is pressed to bring up the various pop up menus.
2. Getting Started The following is an overview of the steps involved in creating and simulating an audio tone control amplifier in Mentor Graphics Design Architect and Accusim SPICE model simulators. If you recollect, this was the last experiment you did in EECS 210. 1.2 Schematic Capture with DA Invoke DA by typing the following at the unix shell prompt: (make sure you are in your class directory). % da & A window with the title Design Architect will open in a short while. To expand this window to its maximum size click the LMB on the square button at the extreme top right corner of the DA window. In the Menu Palette on the right part of the window, click the LMB on the OPEN SHEET icon. A pop up dialog box will appear, prompting you for the name of the component. Use the mouse to highlight the component box (a red border should appear around the box) and enter the name of the component, say tonecontrol. Then click the LMB on the OK button. A schematic window will appear and you are now ready to edit a schematic. The circuit we need to model is shown in figure 1. This is the tonecontrol amplifier of EECS210. You need to start assembling the components and connecting them together in the circuit. First let us find the LM741 op amp. Go to the Libraries menu at the top of the menu bar and select MGC Analog Libraries>Display Libraries Palette. From the palette menu of the right side of the window, choose AccuParts. Select OPAMPs then lm6365 and then scroll down to find the lm741. Click on lm 741. A dialog box will appear at the bottom of the schematic window and if you move the mouse in the schematic window, a ghost image of the lm741 symbol will be seen. Position the op amp appropriately in the schematic window with the mouse and click the LMB. This causes the instance (in this case, the lm741) to be fixed at one place. It will appear highlighted as it is still selected. Press the F2 function key on the keyboard to unselect the op amp instance. Now, it should appear blue in color. This completes the placement of one component. Next we must find the passive components such as the resistors and capacitors and connect them to the op amp. To place a resistor, do the following select Libraries>MGC Analog Libraries>Display Libraries Palette. From the palette menu, select Generic Parts by clicking on it. In the palette, a menu will appear showing various generic circuit elements such as resistors, capacitors, etc. To place a vertical resistor, click on the symbol of the resistor(resist). To place a resistor that is placed horizontally in the schematic window, click on RESIST H. Again, a dialog box will appear at the bottom of the schematic window and if you move the mouse in the schematic window, a ghost image of the resistor will be seen. Position the resistor
appropriately in the schematic window with the mouse and click the LMB. Capacitors also can be placed in a similar way by clicking on CAP or CAP H. All components are placed with default values (10K for resistors and 10pF for capacitors, etc). You will learn below how to set the correct component values. To find the potentiometer, from the menu select Libraries>MGC Analog Libraries>Display Libraries Palette. From the palette menu, select System Model, System Model Blks, then Generic Elements. Click on POT to select the potentiometer. A pop up menu appears prompting you to enter the potentiometer parameter. Use R=100 and K=50. To rotate any component, for instance the potentiometer, select it first by clicking the LMB on the component (you can also select it by placing the cursor on the component and clicking F1 on the keyboard). Then, click the RMB anywhere in the schematic window and a pop up menu appears. Follow the link Rotate/Flip>Rotate>90 to turn the component ninety degrees. After placing all the components appropriately in the schematic window, they need to be connected by wires. Figure 1. Tone control amplifier Our next goal is to complete the connectivity of the schematic using wires (or nets). Click the LMB on the ADD WIRE icon in the Menu Palette(or use the F3 function key on your keyboard). A dialog box appears at the bottom of the window with the
corresponding command name. To draw the nets click the LMB at the desired starting point in the schematic window and drag the mouse till the destination and double click the LMB at the end point. You can draw as many different nets as you need since the DRAW NET command stays in effect until it is canceled by the connectivity as shown in the schematic in Fig. 1. The next step is to define input and output ports for our circuit. We can do this by selecting the PORTIN and PORTOUT symbols from the Generic Parts menu in the palette. Place the ports as desired and connect them to the circuit at the appropriate nodes. This completes the drawing of the schematic. Now, we need to alter the properties associated with the nets and instances. For example, you can see that both the input and output ports have the word NET written near them. This is the value of the property net associated with these wires(nets). All these are default values for the corresponding properties and need to be altered according to our requirements. Say, we want to name the input port as IN. To do this, place the cursor over the word NET near the input port. Then press Shift+F7. A dialog box appears at the bottom of the window prompting you for a new value for the property. Type IN and click the LMB on the OK button. The input port should now be named IN. Similarly, to change the output port press Shift+F7 and then enter out at the dialog box prompt before clicking on the OK button. We also need to change the resistor and capacitor values appropriately, using the same method as above. The last step in the schematic capture is the Design Check. From the menu select Check>Sheet>With Defaults This will start the design check. If all the above design steps have been correctly implemented, you should not get any warnings or errors in the report generated by the check. One common error is that a part overlaps. This means that the parts overlap in the layout. If this happens, adjust the parts to create more space between them. If you get any other errors, contact your TA. The above completes the schematic capture. From the File menu at the top of the window you can save the schematic by doing the following: File>Save sheet>default registration 2.2 Design Viewpoint A design viewpoint is needed to translate the schematic design into a form recognizable by the SPICE analog simulator. This can be created within Design Architect itself as follows. From the Libraries menu, choose Libraries>MGC Analog Libraries> Display Libraries Palette This brings up another menu on the right side. Click on Analog M/S Utilities, then Create Viewpoint in the palette menu. Change/enter the following values in the dialog box that opens up.
Design Viewpoint Name: default On pressing OK, the Accusim viewpoint will be created. This will take a couple of minutes depending upon your design and will be over when you see the following message at the bottom of the DA window: Viewpoint created successfully. You can now quit DA by clicking the left mouse key on the top left button of the DA window and keeping it pressed, select the Close option. If you list the files in your working class directory, you should find the files: tonecontrol.mgc_component.attr and the directory tonecontrol. Do not move or alter these files using Unix shell commands. To copy or move a design, use the MGC>Design Management menu from within the DA program. 2.3 Analog Simulation Analog simulations are performed in Accusim. You invoke it for this design with the command %accusim tonecontrol & The schematic sheet for the design will appear in a window. You can use the select functions (function keys F1 and F2) to select and unselect nets and instances in this window. Also you can use SHIFT F8 to view all the design( or VIEW ALL from the palette) and F8 to zoom in to a part of the window just as in Design Architect. You can also see another window which shows Server:stopped and also the type of Analysis, the Temperature at which the circuit is being simulated. This window shows you the status of the simulation whether it is running or finished and the type of analysis Accusim is performing on your design. On the right, in the palette menu, you can see the menu items DC MODE, FREQ MODE, TIME MODE. You can select the type of analysis you want to perform by clicking on the appropriate icon. Let us first do a Frequency mode analysis. Click on FREQ MODE in the palette menu. Now, observe that the type of the analysis has changed to AC in the status window. There is also other information like the frequency range and the points per decade (The default values will be shown and you will learn how to change them below). Click OK to continue. Follow the steps below to run the frequency domain simulation. 1. Click on the ADD FORCE button in the palette. You will be prompted for signal names to be forced. Enter IN in the Signal field. 2. Click on Frequency in the Mode field. Click on AC in the Force field. Enter the magnitude of the input signal, say 1(The default unit is Volt). Leave the Phase and DC Offset values at their default values of 0. Click OK. Before running the simulation, you need to set the simulator for the type of analysis. We are interested in AC analysis.
1. Click on the SETUP ANALYSIS button in the palette. 2. Click on AC in the pop up dialog box. Click on Decade in the Sweep type field. Enter a suitable number in the Points per decade field. More points lead to greater accuracy but at the cost of speed. A reasonable number would be 10 points per decade. Enter the range of frequencies within which the analysis needs to be done i.e., 100 100K (the default unit is Hz). Click OK when finished. 3. Click on RUN in the main pallet. Accusim will bring up another box for Add Keeps. Click on ALL (When you simulate bigger circuits, you would not want to retain the simulation results for all the intermediate points Accusim will die!). On clicking OK, the simulator will begin simulation. You can observe the progress of the simulation in the Status window. While the simulation is in progress you will notice that the cursor is the busy hourglass type. When you are re simulating with different forces, you may want to delete previous simulation keeps by pulling the Delete pull down menu. With the mouse, select the desired signals to be traced in the schematic window. Click on TRACE in the palette. You should now see a plot of the selected signal in the trace window. The scale of the plot will be (V, Hz) and the scale will be such that the plot will look different from the unexpected flat response. To see a Bode plot of the above simulation run, choose Results>Chart>Chart Results from the main menu. A dialog box will appear in the window. Enter OUT in the Signal A field and click on OK. A Bode plot of the signal OUT will appear on the screen which gives both the magnitude and phase information. The next step is to do the Transient analysis in the time domain. Click on the Time Mode button in the palette menu. Click OK if the Analysis Dialog box pops up. We now have to add a force and set up the simulator in a similar way as we did for the frequency mode analysis. Follow the steps below. 1. First, delete the force from the frequency simulation by selecting it with the mouse then using the delete key on your keyboard. 2. Click on the Add Force button in the palette menu. A pop up menu appears 3. Click on Voltage button under the Forcing field. 4. Enter IN in the Signal field. 5. Click the Time button in the Mode field. 6. Click on Pulse in the Force Type field. 7. Enter Initial = 400M, Pulse = 400M, Delay = 0, Rise = 20N, Width = 1M, Fall = 20N, and Period = 2M. This will set up a 500 Hz square wave with 400mV peak voltages, rise and fall times of 20ns and a 50% duty cycle. 8. Click on OK. To set up the simulator, follow the steps below.
1. Click on Setup Analysis button in the palette menu. A pop up menu will appear. 2. Click on the Transient button in the Analysis field. 3. Choose a 10U Time step and 10M Stop Time. 4. Click OK. 5. Click RUN on the main pallet. The simulator will start running. Note: For all transient or time mode analyses, typical time step to choose is one hundredth of the time period. Increasing the time step will increase the accuracy of your simulation at the expense of the time required to finish the simulation. At the same time, increasing the time step beyond a certain point will improve the accuracy very little and take a long time to finish the simulation. In general, one hundredth of the time period is a good starting point to verify the basic specs of your design in a reasonable amount of time and time step can be decreased if you want more accuracy. To view the results, select the OUT node in the schematic and click on the TRACE button in the palette menu. The graph is in (V, s). You can also plot the IN and OUT signals together by selecting both the nodes and clicking on the TRACE button in the palette menu. 2.4Zooming and Cursor Features 1) To zoom into a particular portion of the plot, in the Chart window, click on the RMB and a pop up menu will appear. Click on View>Area. A cross hair cursor will appear in the window. Click on the LMB and drag it over the area you want view. You can also use View>Zoom In and View>Zoom Out to zoom in and out of the plot. 2) When you add a cursor, sometimes, the cursor is not placed where you keep the cross hair in the chart window it gets placed at the front of the plot x=0. So, first place the cursor before you zoom in to a specific part of the chart. Then, move the cursor to wherever you want. 3) To move the cursor, click on it and it will get selected and appear green in color. Then, place the mouse arrow near the x reading or the y reading and you will see a special symbol showing four arrows in four directions. Now, just click on the LMB and keeping it pressed move the cursor to wherever you want to place it and release the LMB. Once that is done, zoom into the specific portion of the chart and then move the cursor to get the exact data point you want. 4) Everyone is strongly encouraged to make full use of the cursor feature to read off values and when you submit your plots, those cursors should be there on your printouts. It is easier for everyone to read your plots with the cursors on them. 2.5 Adding Text/Comments to a Chart To add comments to the chart, place the mouse in the schematic window and click on the RMB and follow Chart>Add Text:. A dialog box will appear at the bottom of the
window. Enter the comment you want in the Text field and click on the icon called Origin. Move the mouse to the schematic window where a cross hair will appear. Move the cross hair to the desire position where you want to place text and click the LMB to place the text. 3. Printing in Mentor Graphics You can print the various charts by following the steps listed below. Make sure that the window of the plot you want to print is selected. If not, click on that window and you can observe that the border of the window will become blue in color indicating that the window is selected. In Accusim, from the main menu, choose File > Print>Active window. Select Export Graphics to File then click OK. Next select the ps format from the list, then click select format. Enter the name and path of the file to generate. (e.g. I created a file in my home directory by using ~/temp.ps) After a few moments, you should see a message indicating that the print job is complete that the job is spooled to printer mentor. This means that the Mentor Graphics print program has created a postscript file (postscript is a file format which the printer understands) or the plot which can now be printed. This postscript file will be in the directory you chose. Go to a Unix shell prompt and change to the directory you created the file in. (e.g. You would type "% cd ~" from my example) If you now type "ls" you should see the file you created. To print this file, type the following at the Unix shell prompt % lpr P<printer name> <file name> For my example, you would use % lpr P2340eecs temp.ps From Design Architech, you would follow the same procedure. However you now select file>export Graphics from the top menu. A Note about Design Viewpoints For a any schematic you enter in DA, you need to create the viewpoint only once. If, after the viewpoint is created, you can change the schematic, you do not need to create another viewpoint. All you need to do is check and save the schematic and once you invoke accusim you can see that the schematic has been updated. A design viewpoint is a configuration file which along with the schematic sheet tells the analog simulator that this is an electronic design. It seems trivial now and you may be wondering what the use of this file is since you are simulating all the schematics you are creating. But, this may not be the case when you are building a huge circuit with various smaller schematics as subcircuits.