Cadence Tutorial C: Simulating DC and Timing Characteristics Created for the MSU VLSI program by Professor A. Mason and the AMSaC lab group rev S06 (convert to spectre simulator) Document Contents Introduction Layout Extraction with Parasitic Capacitances Timing Analysis DC Analysis Introduction This document is the third of a three-part tutorial for using CADENCE Custom IC Design Tools (IC445) for a typical bottom-up circuit design flow with the AMI/C5N process technology and NCSU design kit. Tutorial A and B cover the use of the Virtuoso schematic entry tool, Affirma analog simulation tool and Virtuoso layout tool. This tutorial covers the timing analysis on the schematic and extracted view. It is assumed you have followed Tutorials A and B and have the schematic and layout views of a CMOS inverter. Layout Extraction with Parasitic Capacitances • Launch Cadence and open the layout view for the inverter cell. • In the Command Interpreter Window (CIW), set the capacitance-ignore-threshold by entering NCSU_parasiticCapIgnoreThreshold=1e-18 in the prompt at the bottom of the CIW and pressing Enter. Although this was not needed for functional simulations, it will make timing analysis more accurate. • In the Virtuoso Layout Editing window select Verify => Extract. A new window (below) with extraction options will appear. • Click on Set Switches and select Extract_parasitic_caps. Click on OK to extract parasitic elements from the layout with the new parasitic capacitance threshold. • After extraction, check the CIW to make sure there are no errors. • Load the extracted cellview from the Library Manager window. This will open up a layout should look similar to the one below. Press shift+f to see all the device parameters. Extract Window Extracted Layout View Cadence Tutorial C: Simulating DC and Timing Characteristics 1Timing Analysis STEP 1. Start Analog Environment • With the extracted view open, in the Virtuoso Layout Editing window select Tools => Analog Environment to open the Affirma Analog Circuit Design Environment window. 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". STEP 2. Setup Analog Simulator • In Affirma Analog Circuit Design Environment, click on Setup => Simulator/Directory/Host. • Choose spectre as the Simulator. • Enter a path for your simulations files and results. You may set this to any valid path, but you might find it useful to keep all simulations in one directory. If you don’t specify the whole path, the simulation files and results will be created under the directory you launched cadence. The following example saves the simulation file and results at /egr/courses/personal/ece410/<username>/simulation assuming you launched Cadence from your root 410 class directory If you need to run multiple simulations on the same cell, you can even use different paths for each simulation. Cadence Tutorial C: Simulating DC and Timing Characteristics 2STEP 3. Set up Analog Environment to use Extracted View • In Affirma Analog Circuit Design Environment, click on Setup => Environment. The Environment Options dialog box (below) will pop up. • In the Environment Options window, under the line Switch View List. type the word “extracted” before the word “schematic” (see below). Then click on OK. This entry is an ordered list of cell views which contain information that can be simulated. The simulator (in fact the netlister) will search until it finds one of these cellviews. The default entry does not contain an extracted cellview. As a result of this modification, the simulator will use the extracted cellview instead of the schematic cellview to include the effect of parasitic capacitance in the simulation. • Make sure to check the Y for Use SPICE Netlist Reader(spp), since the default transistor model files are written in SPICE syntax. STEP 4. Set up stimulus file • Create a new stimulus file by opening any text editor. Make sure to give the file a name either when you create it or when you save it, depending on your text editor (“stimulus.txt”, for example). • Enter the following text, save the file and exit the text editor. (Note: each circle bulleted item below represents one full line in the stimulus file; be careful if you cut and paste this text. You also need to be sure to hit Enter after the last line to insert a line return in the text file.) o simulator lang=spectre o Vdd (vdd! 0) vsource dc=3 o v1 (A 0) vsource type=pulse val0=0 val1=3 rise=0.05n fall=0.05n width=10n period=20n o c0 (0 Y) capacitor c=3f This assumes your input and output node are called ‘A’ and ‘Y’ and attaches a load capacitance to the output that simulates the input capacitance of gates attached to the output node. For additional information about stimulus files, including notes on creating files with multiple inputs, see the Guide to Writing Stimulus Files. Cadence Tutorial C: Simulating DC and Timing Characteristics 3• In the Affirma Analog Circuit Design Environment, include this new stimulus file by clicking Setup => Simulation Files and adding the name of the file in the “Stimulus File” box. STEP 5. Setup Analysis • 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: Set the Stop Time to “50n”. • Choose Enabled at the bottom of the screen and press OK. STEP 6. Setup output traces • In Affirma Analog Circuit Design Environment window, select Outputs => to be plotted => Select on Schematic. This will activate the Layout Editing window with the extracted view of the inverter, allowing you to pick which signals (nets/wires) you would like to have plotted during the simulation. • In the Layout Editing window select input gate poly and output metal1 (see example below). • This will complete the simulation setup. Now your Affirma Analog Circuit Design Environment window should look as follows (names may vary): STEP 7. Run Simulation • In the Affirma Analog Circuit Design Environment window select Simulation => Netlist and Run. • When the simulation is complete, the
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