CDS LAB1/en

Eesti keelne leht The objective of the tutorial is to design a digital circuit and simulate it using Cadence

Setting up the work environment

All the actions in this and the followin paragprah will be happening in terminal

• create a new directory lab1
• move to the new directory
• insert 'cad' and from the menu choose '1' (initializes environmental variables for Cadence)
• insert setenv EDITOR SciTE for making SciTE as the default text editor

Starting Cadence

According to Cadence 2016 EDA ver.

• If it is the first time launching Cadence, then using terminal:

ams_cds -64 -tech c18a6 -add CORELIB -add IOLIB_6AM -add GATES_ANA

here we choose AMS 0.18μm with 6 metal layers as the technology and add three gate libraries

• For the subsequent launches of Cadence use the command:

ams_cds

The actions from this point on will be executed using the Graphical User Interface of Cadence

Several windows will open. On a dialog window asking about designkit simply press OK.

Cadence main window will be also opening, which is located by default near the left bottom corner.

Cadence will log messages in this window, including error messages.

Also Cadence Library Manager window will open

It is recommended to check the checkbox Show Categories

Creating a new library

In the Library Manager window choose from menu File->New->Library

A new window will open, where a name can be inserted. In here labor_1 will be used. After inserting the name, press OK

Another window will appear with couple of choices. Choose Attach to an existing technology library and press OK

In the next window choose TECH_C18A6 and press OK

Creating new circuit into our library

Choose the newly created library labor_1 in the Library Manager window. In the menu choose File->New->Cell View

In the window opened fill the field Cell by giving your new circuit a name, e.g. half_adder. On the images below poolsummaator is used as the name.

Choose Applications->Schematics XL and select checkbox Always use this application for this type of file and press OK. As a result the schematic editor window will open.

Editing the half-adder schematic

As a result of the previous actions we should have a schematic editor window open.

Overview of some of the schematic shortcuts

• 'i' - instance , adding new instance of a cell to the schematic
• 'w' - wire , adding wire to the schematic for connecting elements
• 'l' - label , inserting a label on a connection
• 'p' - pin , creating a new input/output pin
• 'f' - fit to view , changes to zoom to fit the whole schematic to the view
• 'u' - undo , undo the last action
• 'X' - Check and Save' , checks the schematic and saves it

Zooming in can be done by holding down the right mouse button and selecting an area. To leave the zoom quickly, press f

From every insertion mode you can quickly exit by pressing ESC

When in the wire mode, you can press s for snap functionality.

NB! Undo works until last save. Changes made before the save can not be taken back.

Creating a sheet border and title

Press the key i and in the opening window press Browse

Library Browser will open where you should navigate to Library->BORDERS->Cell->A5.

We can close the Library Manager window by pressing Close

Also we can hide the Add Instance window temporarily by pressing Hide

After that the border can be placed by moving mouse in the black area of schematic editor and pressing left mouse button.

When the border is placed, you can press f to fit the view.

Adding schematic elemenets of half-adder

Press the key i and choose Browse

In the Library Browser window that opened, navigate to Library->CORELIB where the schematic elements can be found.

First of all we need to place some AND gates.

To do that, navigate to Category->AND->Cell->AND2X1

The numbers after the gate name show how many inputs does it have and its fan-out (number of gate inputs it can feed or connect to). In this case we have a two input AND gate with 1 fan-out.

For a half-adder we need 3 AND gates, 2 NOT gates (inverters) and 1 OR gate

Adding inputs and outputs for the half-adder

To add the inputs and output pins, press p

For the first pin name choose A with direction input

Placing pins works the same way as placing instances of cells

For a half-adder we need two inputs (A and B) and two outputs (Sum and Carry)

Adding connections

Press w to choose wire and by using snap, connect the elements as following:

For a better readability of the schematic you can add labels by pressing the key l.

Saving the half-adder schematic

For saving the schematic press x

In the Cadence main window you can check whether the saving was successful and if not, what were the errors.

Creating a symbol for the half-adder

In order to use the newly created schematic as a component in other schematics, a symbol view must be created.

To create a symbol, navigate from menu to Create->Cellview->From Cellview

A window will pop up, where you can simply press OK as the default options are fine at this point.

In the following popup window you can choose the pin specifications.

Finally continue by pressing OK

Next, Cadence Symbol Editor will open

The symbol created by default is a rectable. By using the toolbar, it can be designed as desired.

Full adder from two half-adders

By using the finished half-adder component, we can create a full adder

This can be made into a symbol as well, so it can be used in other schematics as a component.

The component can be also be opened in schematic view. To do that:

• Double click on the component symbol
• Use the command Descend Edit or its shortcut Shift+E

To return back to last view Return command can be used or its shortcut Ctrl+E

This can be useful for example when we need to change the fanout of the component.

NB! when changing the component, it will change everywhere, where it has been used.

4-bit adder

Now we should have all the components necessary for a 4-bit adder: one half-adder and three full-adders.

Carry chain can be done serially in this schematic with Ripple Carry Adder, which is suitable for a small 4-bit adder. For bigger adders parallel carry would be recommended, e.g. Lynch-Swartzlander Spanning Tree Carry Lookahead Adder.

For this schematic we will be using 4-bit bus.

To make the buses more visible, we will use bold line for the connections.

For bringing out individual bits from a bus and connecting them, labelling must be used.

4-bit adder simulation

Now that we have finished, checked and saved the schematic design, we will proceed with simulation.

Let us open the tool NC-Verilog

From Cadence main window choose from menu Tools->NC-Verilog

A new window will open where a few things should be configured

• Run Directory - press on the button with three dots (...) and choose a working directory. (In this case we should create the working directory, for example lab1_simu)

• Choose design by pressing on Browse, selecting your schematic and schematic view.

NC-Verilog initialization and settings

Press on the first icon in the left Initialize Design

A window will pop up where OK should be chosen.

Configure a few parameters from Setup->Netlist

A menu will open

Choose the following settings

• Netlist These Views->schematic symbol
• choose Single Netlist File
• Global Sim Time set as 1 with unit ps
• Global Sim Precision set as 1 with unit fs

Next, choose Setup->Record Signals

Choose All

Creating NC-Verilog Netlist

As a next step we should generate netlist for our schematic. For that press on the NC-Verilog main window a button Generate Netlist

On the Cadence main window you can check whether the netlist was generated successfully without errors.

NC-Verilog simulation input data

For the simulator to be able to simulate anything, we need input data

For this purpose choose from NC-Verilog main window Commands->Edit Test Fixture

In the opened window there are two panes: TestBench and Stimulus

For a simple simulation, we are only interested in Stimulus section. Press there on the Edit button.

A text editor should open with a file testfixture.verilog

Here you can see code generated by default that initializes the simulation.

Now we can add our own values. 4 to 5 input number pairs, every change simulate for 10 simulation units.

After editing, it is a good idea to check for errors. This can be done from Edit Test Fixture window with a button Check Syntax

If everything is good, a message will be displayed

NC-Verilog Simulation

Start the simulation by clikcing in the NC-Verilog main window, on the left side on a button Simulate

As a result SimVision will start and two new windows will open: Console - SimVision and Design Browser = SimVision

Simulation results, SimVision

Viewing simulation results

In the opened windows, let us focus on the Design Browser

Here, on the left side of the window, choose -simulator-test+top

and on the right side of the window choose inputs-outputs for which values we are interested of seeing on the waveform graph. Finally press on Send to Waveform.

A new window will appear: Waveform - SimVision

Here, press on a triangle symbol by the Run button, so that we could edit time field.

In the time field insert a number of simulation units, 60.

and press Run button.

So that we could see the whole view, the waveform should be zoomed out. Press = button on the right border of the toolbar.

If we have multiple outputs that are related with each other, in this case Carry signal and sum output, it is reasonable to view them together for a better readabilty.

In order to do that simply copy these two signals - with left mouse button click on the signals while holding SHIFT key and then by clicking right mouse button, choose Copy from the menu.

Finally paste the signals down below, by clicking again with the right mouse button on the black background and choose Paste

Afterwards choose the copied signals and by pressing again on the right mouse button choose Create->Bus from the menu.

Simulation results

Now we can easily follow the waveform and check the results of the adder