CDS LAB1/en

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.