cadence_tutorial - 6.012 Microelectronics Devices and...

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6.012 Microelectronics Devices and Circuits Fall 2005 1 Cadence Tutorial (Part One) By Kerwin Johnson Version: 10/24/05 (based on 6.776 setup by Mike Perrott) Table of Contents Introduction .......................................................... ............................. 2 Shorthand Conventions .................................................................. 4 Getting Help within Cadence ......................................................... 4 Troubleshooting ............................................................................. 4 General Setup ...................................................... .............................. 5 Setting up Cadence on the MIT Server ......................................... 5 Create a Library ............................................... .............................. 6 Schematics and Symbols ..................................................... ............... 7 Create the Inverter Schematic ....................................................... 7 Create the Inverter Symbol .......................................................... 11 Create the Inverter Test Bench .................................................... 13 Create the Device Models ................................................................ 15 Simulation ....................................................................................... 19 Configure the Simulator ........................................ ....................... 19 Run the Simulation ...................................................................... 25 Analysis ........................................................................................... 26 Plot Results ...................................... ............................................ 26 Calculate Values .......................................................................... 27
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6.012 Microelectronics Devices and Circuits Fall 2005 2 Introduction This tutorial will introduce the use of Cadence for simulating circuits in 6.012. Cadence is a suite of tools for IC design. It allows for schematic capture, simulation, layout and post-layout verification of analog and digital designs. We will be using a portion of the analog design flow, which can handle up to 200,000 devices. We will use Composer to enter/capture the schematics of circuits that we will simulate and the test benches with which we will simulate them. We will use Analog Design Environment (ADE) to configure a simulator, in this case Spectre to simulate our circuit with device models that represent the transistors in our circuit. This tutorial will explain how to set Cadence up on the MIT Server. An overview of the work flow in Cadence is shown in Figure 1. Starting from near the top of the figure we will use the Command Interface Window (CIW) to start the schematic composer. We will capture a schematic of an inverter. We will create a symbol for the inverter. We will create a test bench to test the inverter. We will create device models for our FETs in a text editor. We will open ADE and configure and run our simulation in spectre. Then we will plot our results. A circuit simulator is a tool. In order to design circuits efficiently you need to use this new tool effectively. The simulator is good at solving thousands of operating points. A good way to use the simulator is to first understand the circuit and then sweep the simulation over the entire area where the solution lies. We know that the DC transfer function of a well behaved inverter always has a cross over point VM where vin =vout, which we want to find. We will simulate the DC transfer function of the inverter by simulating every point on the curve. Then we can select VM by inspection. Because we understand the inverter's behaviour we know that if we sweep the input from 0 to VDD we will find the VM if it exists. It is easy to use the simulator ineffectively. The simulator is not your brain; it can't understand anything about the circuit. If you are just collecting data and not thinking about it, then you are wasting time. The simulator isn't a pen and paper calculation.
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