Cadence Tutorial
School of Electrical Engineering and Computer Science Washington State University, Pullman, WA-99163 This
tutorial is an introduction to Cadence tool for circuit design and simulations.
It deals with the schematic entry of the circuits and their simulations using
Cadence tools
Composer
is the tool for schematic entry of the circuit designed. Once the schematic is
ready, the simulations can be done using the Analog Artist Environment. Logging onto
the SUN systems
At
the SUN systems log in screen enter your EECS log in ID, change the environment
to Common Desktop Environment (CDE) in the options menu followed by pressing OK
and then enter your password followed by ENTER. Creating a working
directory
Ones
logged in successfully, open a terminal window. To do this
Now
at the prompt “gto%”, type mkdir Cadwork to create a working
directory named Cadwork. Then enter the directory by typing cd Cadwork at the prompt. Now you are in the directory Cadwork. All the work done will be
stored in this directory. Starting Cadence To start cadence, type runcad &
at the command prompt. This starts Cadence
in the background and you should get a window with the icfb Command
Interpreter Window (CIW) as below. The
Library manager window, which looks like this, also opens. You
will also get a "What's New" window which you can read and then close
or minimize. To view the help manuals, click the Help
button on the icfb window. Creating a library The fist task is to create a design library. To create a library,
go to ®New®New File®New®Library. The following window opens. Enter the name of the your library say
tutorial in the text box. If you want to store the library at a different path, enter the
name in the path text box. Next click on the radio button Attach to an existing tech library. This displays a drop down menu box.
From the menu box choose the technology you want to use. In this tutorial we
shall use TSMC 0.4u CMOS035 (4M, sblock, HVFET) technology, which is the CMOS
0.35mm technology. Then click the OK button. This will create a library with name tutorial, attached to TSMC 0.35mm technology. Look at the Library manager window. This new library created
should be an entry in it under the column named Library. Creating a cell-view for schematic capture Once
you have created a design library, you can start to put your design into it. In
this tutorial we shall create an inverter. Now
to create a cell-view go to File®New®Cell-view From the drop down menu box of Library name, choose the library
you want the current cell-view to be in. In the present case, it is the tutorial library. Enter the name of the your cell-view say inverter in
the text box. Then choose the Composer Schematic from the drop down menu called Tool. Composer Schematic is the schematic entry tool (this is
the default option). Clicking the OK button should open the Composer Schematic editor,
which looks as follows: Get acquainted with the Composer Schematic editor window. On the left side are various shortcuts to commonly
used commands such as: check & save, save, zooming in and out, stretch,
copy, move, delete, place component instances, drawing wires, placing ports,
etc. These commands and many more can also be accessed from the menu. Design InverterNow lets create an inverter in the editor. To create an inverter
we need a PMOS transistor, a NMOS transistor, dc voltage source and ground. Getting the
required components
®New Click on the instance button on the left or go to Add®Instance. The following windows open.
Choose the library as NCSU_Analog_Parts. Click on the list item P_Transistors. The list now displays the
contents Click on the list item pmos and bring the mouse pointer to the
editor. You will see a PMOS transistor. Left-click the mouse on the editor at
the position you want to place the pmos transistor. Notice that the mouse
pointer continues to carry an instance of pmos transistor. If you want more
pmos transistor you can place them on your editor. To deselect the instance
press Esc. Now we need a nmos transistor. Click on the instance button on the
left or go to Add®Instance.
Click on the list item N_Transistors. The list now displays the contents Click on the list item nmos and place it on the editor as shown Now we need the dc voltage source (Vdd) and ground (gnd). The instances of these components can be found in Supply_Nets. Repeat the procedure described for the transistors and finish the inverter as shown below. Notice, when the mouse pointer is taken over the component
instance, a rectangular dotted outline appears. If you wish to rotate or flip
this component it can be done now. To rotate or flip the component, click the
right mouse button or go to Edit®Rotate (use the shortcut key ‘r’). The following window appears.
Depending on the requirement the component view can be modified. Let’s now connect the gate of both nmos and pmos transistor.
Select the wire button from the left of go to Add®Wire (Narrow). Hide the window, which
appears by pressing Hide. Next click on the gate of the pmos and take the mouse pointer to
the gate of nmos. Notice the wire being dragged along with the mouse pointer.
Terminate the wire at the gate of NMOS by clicking on it. Then click on the
overlapping terminals of the transistors and drag the wire to the right.
Terminate the wire after some length by double clicking the left mouse
button. Next we need to define the input and outputs of the inverter. Click the Pin button from left or go to Add®Pin. This window pops up. ®New®New Enter the input pin name as IN in the
text box, select the input from Direction menu box and press Hide. Place
as shown the following window. Again click the Pin button from left or go to Add®Pin. Enter the output pin name as OUT in the text box, select the output from Direction menu box and press Hide. Place as
shown in the following window. Click Ecs to deselect the pin option. Changing the
properties of instance Now lets make the PMOS transistor width 4 times that of the NMOS
transistor. Click on the pmos transistor. Notice the appearance of a solid
rectangle around it. Now strike the key ‘q’ or go to Edit®Properties®Object. The following window opens. Now the default width of the transistor is its minimum width of
600nm in 0.35mm technology. Lets make the transistor
bigger by 4 times. So the new width is 2400nm. Change the width in the text box
as shown and then press Apply, for the changes to take effect. Then
finish by pressing OK. Saving the schematic Click on the Check
& Save button or go
to Design® Check and Save or use the shortcut
key X to check the schematic created and to
save it. Check the icfb window to see
if it has reported any errors or warnings, if there are any you have to go back
and fix them. Assuming
the schematic is free of errors and warnings we proceed with the simulation of
the inverter. Simulation For
the simulations of the designs we use the Analog Artist Environment tool. Start
by going to Tools®Analog
Environment. This will open the Affirma Artist Circuit Design Environment
simulation window, which is as The
design should be set to the right Library, Cell and View. Now
go to Setup®Simulator/Directory/Host. From the window that opens up select the simulation type as spectreS from the drop down menu. Finish by clicking OK. Then go to Setup®Stimulus®Edit Analog. From window that opens up
choose the radio button graphical and click OK The window shown opens. This is the window in which the stimulus to the inverter is defined. Observe that the input defined in out schematic is the entry in this window. You can also find the global input Vdd if you click the Global Sources radio button. Click on OFF
IN/gnd! Voltage dc, in the text box. Change the Function to a pulse. And
enter the pulse parameters as shown and click the change button. Observe the
input IN change from off to
on. Now
go to the global sources, enable the Vdd input, give it a dc value of 3.3V
and click the change button. Observe Vdd change from off to on. Finish by clicking OK. Next
we have to give the type of analysis to be performed. Let us perform a
transient analysis for 20ns. So go to Analysis®Choose or click the second button on the right of the window. Click
the transient analysis radio button and enter the stop time as 20n. Finish by clicking OK. Now
we have to choose the outputs to be plotted in the waveform viewer. Go
to Outputs®To be plotted®Select
on schematic. Then
the editor window automatically comes into focus. Click on the wires connected to the input and output pins as shown. Notice that these
wires are highlighted. Go back to the Analog Artist Environment window and now
there are two entries in the output section of the window. These are the
signals that will be displayed. To start the simulations click on the button
showing the traffic light (with green). After simulations are done the waveform
window opens and displays the signals IN and OUT as shown. Click
on this button Switch Axis
mode in the waveform viewer to
display the waveforms separately. You can use the crosshair to make measurements on the waveforms. They can be obtained by
clicking the buttons Crosshair
marker A, Crosshair marker B or using
the hot keys a and b. The
netlist generated by the simulator can be viewed by going to Simulation®Netlist®Display
Final Netlist It
is a good idea to save the state of simulation, if you want to redo any of the
simulations without having to re-enter everything from scratch. To save the
current state of the simulation, in the Analog Artist Environment window go to
Session®Save state. In the window that opens, enter the
name for the state of simulation to be saved. To
redo a saved simulation, follow this procedure. When the Analog Artist Environment window opens, go to
Setup®Simulation. From
the window that opens select the simulation type as SpectreS from the drop down
menu. Finish by clicking OK. Then go to Session®Load state. A window as follows opens. From
this select the state you want to load and finish by clicking OK.
The previously saved state of simulation is loaded and you can make any changes
if needed and continue with simulation. Creating a Symbol To
create a symbol for the schematic go to Design®Create Cellview®From cellview… The
following window appears. Verify
the name of the library and the cell name for which you are creating the symbol
and proceed by clicking OK. A
new window appears, with a symbol for your schematic. In the present case it’s
the symbol for the inverter. Now
this symbol can be used as a component in any other schematic. If
any changes are made to the schematic, the symbol must be recreated (modified),
following the same procedure. |